B. Cellular Elements

38

CIRCULATION AND

GAS EXCHANGE

OUTLINE

I.II.

Transport systems functionally connect body cells with the organs of exchange: anoverview

Most invertebrates have a gastrovascular cavity or a circulatory system for internaltransport

A.B.

Gastrovascular Cavities

Open and Closed Circulatory Systems

III.IV.

Diverse adaptations of a cardiovascular system have evolved in vertebrates

Rhythmic pumping of the mammalian heart drives blood through pulmonary and systemiccircuits

A.B.C.D.E.F.G.

The Heart: General Form and FunctionControl of the HeartBlood Vessel StructureBlood Flow VelocityBlood Pressure

Blood Flow Through Capillary BedsCapillary Exchange

V.VI.

The lymphatic system returns fluid to the blood and aids in body defenseBlood is a connective tissue with cells suspended in plasma

A.B.C.D.

Plasma

Cellular Elements

The Formation of Blood Cells and PlateletsBlood Clotting

VII.

Cardiovascular diseases are the leading cause of death in the United States and many otherdeveloped nations

Circulation and Gas Exchange 683

VIII.

Gas exchange supplies oxygen for cellular respiration and disposes of carbon dioxide: anoverview

A.B.

Gas Exchange and the Respiratory MediumRespiratory Surfaces: Form and Function

IX.X.XI.

Gills are respiratory adaptations of most aquatic animalsTracheae are respiratory adaptations of insects

Lungs are the respiratory adaptation of most terrestrial vertebrates

A.B.C.D.E.F.G.

Form and Function of Mammalian Respiratory SystemsVentilating the LungsThe Control of Breathing

Loading and Unloading of Oxygen and Carbon DioxideRespiratory Pigments and Oxygen TransportCarbon Dioxide Transport

Adaptations of Diving Mammals

OBJECTIVES

After reading this chapter and attending lecture, the student should be able to:

1.List major animal phyla with gastrovascular cavities and explain why they do not need a circulatory

system.

2.Distinguish between open and closed circulatory systems.

3.Using an arthropod as an example, describe the circulation of hemolymph.4.Explain how hemolymph differs from blood.

5.Using an earthworm as an example, describe circulation of blood and explain how it exchanges

materials with interstitial fluid.

6.List the components of a vertebrate cardiovascular system.7.Distinguish between an artery and a vein.

8.Using diagrams, compare and contrast the circulatory schemes of birds, amphibians and mammals.9.Distinguish between pulmonary and systemic circuits and explain the function of each.10.Explain the advantage of double circulation over a single circuit.

11.Trace a drop of blood through the human heart, listing the structures it passes through en route.12.List the four heart valves, describe their location and explain their function.13.Distinguish between systole and diastole.

14.Describe the events of the cardiac cycle and explain what causes the first and second heart sounds.15.Define heart murmur and explain its cause.

16.Define pulse and describe the relationship between size and pulse rate among different mammals.17.Define cardiac output and explain how it is affected by a change in heart rate or stroke volume.

18.Define myogenic and describe some unique properties of cardiac muscle which allow it to contract in

a coordinated manner.

19.Define pacemaker and describe the location of two patches of nodal tissue in the human heart.

20.Describe the origin and pathway of the action potential (cardiac impulse) in the normal human heart.21.Explain why it is important that the cardiac impulse be delayed at the AV node and describe the

function and importance of the Purkinje fibers.

684 Circulation and Gas Exchange

22.Explain how the pace of the SA node can be modulated by sympathetic and parasympathetic nerves,

changes in temperature, physical conditioning and exercise.

23.Compare the structures of arteries and veins and explain how differences in their structures are

related to differences in their functions.

24.Describe how capillary structure differs from other vessels and explain how this structure relates to

its function.

25.Recall the law of continuity and explain why blood flow through capillaries is substantially slower

than it is through arteries and veins.

26.Define blood pressure and describe how it is measured.

27.Explain how peripheral resistance and cardiac output affect blood pressure.

28.Explain how blood returns to the heart, even though it must travel from the lower extremities against

gravity.

29.Define microcirculation and explain how blood flow through capillary beds is regulated.

30.Explain how osmotic pressure and hydrostatic pressure regulate the exchange of fluid and solutes

across capillaries.

31.Describe the composition of lymph and explain how the lymphatic system helps the normal

functioning of the circulatory system.

32.Explain why protein deficiency can cause edema.

33.Explain how the lymphatic system helps defend the body against infection.34.Explain why vertebrate blood is classified as connective tissue.35.List the components of blood and describe a function for each.

36.Outline the formation of erythrocytes from stem cells to destruction by phagocytic cells.

37.Outline the sequence of events that occur during blood clotting and explain what prevents

spontaneous clotting in the absence of injury.38.Explain how atherosclerosis affects the arteries.

39.Distinguish between thrombus and embolus; atherosclerosis and arteriosclerosis; low-density

lipoproteins (LDLs) and high-density lipoproteins (HDLs).

40.List the factors that have been correlated with an increased risk of cardiovascular disease.

41.Describe the general requirements for a respiratory surface and list the variety of respiratory organs

adapted for this purpose.

42.Describe respiratory adaptations of aquatic animals.

43.Describe countercurrent exchange and explain why it is more efficient than concurrent flow of water

and blood.

44.Describe the advantages and disadvantages of air as a respiratory medium and explain how insect

tracheal systems are adapted for efficient gas exchange in a terrestrial environment.

45.For the human respiratory system, describe the movement of air through air passageways to the

alveolus, listing the structures it must pass through on the journey.

46.Define negative pressure breathing and explain how respiratory movements in humans ventilate the

lungs.

47.Define the following lung volumes and give a normal range of capacities for the human male:

a.Tidal volumeb.Vital capacityc.Residual volume

48.Explain how breathing is controlled.

49.List three barriers oxygen must cross from the alveolus into the capillaries and explain the advantage

of having millions of alveoli in the lungs.

50.Describe how oxygen moves from the alveolus into the capillary and explain why a pressure gradient

is necessary.

Circulation and Gas Exchange 685

51.Distinguish between hemocyanin and hemoglobin.

52.Describe the structure of hemoglobin, explain the result of cooperative binding and state how many

oxygen molecules a saturated hemoglobin molecule can carry.

53.Draw the Hb-oxygen dissociation curve, explain the significance of its shape and explain how the

affinity of hemoglobin for oxygen changes with oxygen concentration.

54.Describe the Bohr effect and explain how the oxygen dissociation curve shifts with changes in carbon

dioxide concentration and changes in pH.55.Explain the advantage of the Bohr shift.

56.Describe how carbon dioxide is picked up at the tissues and deposited in the lungs, describe the role

of carbonic anhydrase and state the form most of the carbon dioxide is in as it is transported.57.Explain how hemoglobin acts as a buffer.

58.Describe respiratory adaptations of diving mammals including the role of myoglobin.

KEY TERMS

open circulatorysystemsinuses

closed circulatorysystemsheartatriaventriclesarteriescapillariesvenulesveins

pulmonary circuitsystemic circuitdouble circulationheart cyclesystolediastole

atrioventricularvalves

semilunar valvesheart murmurheart ratecardiac outputstroke volume

sinoatrial (SA) node

atrioventricular(AV) nodeblood pressurehydrostatic forceperipheral resistancemicrocirculation

precapillary sphincterscapillary bedlymphatic systemlymph

lymph nodeslymph capillarieselectrolytesplasma proteinslipid escortsimmunoglobinsfibrinogenserythrocyteshemoglobinerythropoietinleukocytesstem cellsbasophilseosinophilsneutrophilslymphocytes

monocytesplatelets

clotting factorsfibrinogenfibrin

hemophiliathrombusembolusarrhythmiaartherosclerosisplaques

low-density lipo-proteins (LDLs)high-density lipo-proteins (HDLs)arteriosclerosisangina pectorishypertension

respiratory mediumrespiratory surfacegillslungstracheaeventilationcountercurrentexchange

spiraclespharynxlarynxtracheabronchibronchiolesalveoli

negative-pressurebreathingdiaphragmtidal volumeresidual volumevital capacityparabronchibreathing centersmedulla oblongatapons

partial pressurerespiratory pigmentshemocyaninhemoglobincooperativitydissociation curveBohr shift

carbonic anhydrase

686 Circulation and Gas ExchangeLECTURE NOTES

The exchange of materials (whether nutrients, gases, or waste products) between an organism and itsenvironment must take place across a moist cell membrane.

• The molecules must be dissolved in water in order to diffuse or be transported across the membrane.• In protozoans, the entire external surface may be used for this exchange.

• The simple, multicellular animals (sponges, cnidarians) have body structures such that each cell isexposed to the surrounding waters.Three-dimensional animals face the problem that some of their cells are isolated from the surroundingenvironment.

• These animals have specialized organs, where exchange with the environment occurs, coupled withspecial systems for the internal transport through body fluids to the cells.• The association of specialized organs with an internal transport system not only reduces the distanceover which molecules must diffuse to enter or leave a cell, but permits regulation of internal fluidcomposition.

I.

Transport systems functionally connect body cells with the organs of exchange: anoverview

Diffusion is too slow of a process to transport chemicals through the body of an animal.• Time of diffusion is proportional to the square of the distance the chemical must travel.• If a glucose molecule takes 1 second to diffuse 1 µm, it will take 100 seconds to diffuse 1mm.The presence of a circulatory system reduces the distance a substance must diffuse to enter or leavea cell.

• The distance is reduced because the circulatory system connects the aqueous environment ofthe cell with organs specialized for exchange.• Molecules will diffuse or be transported into the blood which carries these molecules to thecells or to the exchange surface depending on whether the molecules are used (nutrients) orproduced (wastes) by the cells.

⇒ Oxygen diffuses from air in the lungs across the thin lung epithelium into the blood.⇒ This oxygenated blood is carried by the circulatory system to all parts of the body.

⇒ As the blood passes through capillaries in the tissues, oxygen diffuses from the blood

into the cells across the cells’ plasma membranes.

⇒ Carbon dioxide is produced by the cells and moves in the opposite direction through the

same system.The circulatory system does more than move gases, it is a critical component to maintaininghomeostasis of the body.

• The chemical and physical properties of the immediate surroundings of the cells can becontrolled using the carrying capacity of the blood.

⇒ The blood passes from the cells through organs (liver, kidneys) which regulate the

nutrient and waste content of the blood.

Circulation and Gas Exchange 687

II.

Most invertebrates have a gastrovascular cavity or a circulatory system for internaltransport

A.

Gastrovascular Cavities

The cnidarian body plan does not require a specialized internal transport system. The bodywall is only two cells thick and encloses a central gastrovascular cavity.

• The water inside the gastrovascular cavity is continuous with the surrounding water.• The gastrovascular cavity functions in digestion and distribution of nutrients.• Gastrodermal cells lining the cavity have direct access to the nutrients produced bydigestion, however, the structure of the body is such that nutrients only have a shortdistance to diffuse to the outer cell layer.Planarians and other flatworms also have a gastrovascular cavity.

• The highly ramified structure of the cavity and the flattened body shape ensure all cellsare exposed to cavity contents.A gastrovascular cavity cannot perform the necessary internal transport required by morecomplex animals, especially if they are terrestrial.• These animals have some type of circulatory system.

B.

Open and Closed Circulatory Systems

Open and closed circulatory systems are alternative solutions to moving materials efficientlythrough the bodies of animals.

• They usually function in combination with other organ systems which increases theefficiency of the overall body.Insects, other arthropods, and mollusks have an open circulatory system.

Open circulatory system = Circulatory system in which hemolymph bathes internal organsdirectly while moving through sinuses. (See Campbell, Figure 38.2a)• Hemolymph is a body fluid which acts as both blood and interstitial fluid.

• Circulation results from contractions of the dorsal vessel (heart) and body movements.• Relaxation of the \"heart\" draws blood through the ostia (pores) into the vessel.• Chemical exchange between the hemolymph and cells occurs in the sinuses which arean interconnected system of spaces surrounding the organs.A closed circulatory system is found in annelids, squids, octopuses, and vertebrates.Closed circulatory system = Circulatory system in which blood is confined to vessels and adistinct interstitial fluid is present.• The heart (or hearts) pumps blood into large vessels.

• The major vessels branch into smaller vessels which supply blood to organs. (SeeCampbell, Figure 38.2b)• Blood exchanges materials with the interstitial fluids which come into direct contactwith the cells.

688 Circulation and Gas Exchange

III.

Diverse adaptations of a cardiovascular system have evolved in vertebrates

A cardiovascular system consists of a heart, blood vessels and blood.

• The heart has of one or two atria, chambers that receive blood, and one or two ventricles,chambers that pump blood out.• Arteries carry blood away from the heart to organs where they branch into smaller arteriolesthat give rise to microscopic capillaries (the site of chemical exchange between blood andinterstitial fluid).⇒ Capillaries have thin, porous walls and are usually arranged into networks called

capillary beds that infiltrate each tissue.• Capillaries rejoin to form venules which converge to form the veins that return blood to theheart.An examination of vertebrate circulatory systems shows various adaptations have evolved withinthis taxon.

Fish have a 2-chambered heart with one atrium and one ventricle. (See Campbell, Figure 38.3a)• Blood pumped from the ventricle goes to the gills; here oxygen diffuses into the gillcapillaries and CO2 diffuses out; gill capillaries converge into arteries that carry blood tocapillary beds in other organs; blood from the organs travel through veins to the atrium ofthe heart, then into the ventricle.• Blood flows through two capillary beds during each complete circuit: one in the gills and asecond in the organ systems (systemic capillaries).⇒ As blood flows through a capillary bed, blood pressure drops substantially.• Blood flow to the tissues and back to the heart is aided by swimming motions.

Amphibians have a three-chambered heart with two atria and one ventricle. (See Campbell, Figure38.3b)

• Blood flows through a pulmonary circuit (to the lungs and skin) and a systemic circuit (toall other organs) in a scheme called double circulation.• Blood flow pattern: ventricle → lungs and skin to become oxygenated → left atrium →ventricle → all other organs → right atrium.⇒ The second passage through the ventricle ensures sufficient blood pressure for the

systemic circulation.• There is some mixing of oxygen-rich and oxygen-poor blood in the single ventricle althougha ridge present in the ventricle diverts most of the oxygenated blood to the systemic circuitand most of the deoxygenated blood to the pulmonary circuit.Most reptiles (excluding crocodilians) have a three chambered heart although the ventricle ispartially divided.

• This reduces mixing of oxygenated and deoxygenated blood.

Circulation and Gas Exchange 6

Birds and mammals have a four-chambered heart with two atria and two ventricles. (Please notethat crocodiles also have a four-chambered heart.)

• Double circulation is similar to that of amphibians except that oxygenated and deoxygenatedblood does not mix due to the presence of two separate ventricles.• The complete separation of oxygenated and deoxygenated blood increases the efficiency ofoxygen delivery to the cells.

IV.

Rhythmic pumping of the mammalian heart drives blood through pulmonary andsystemic circuits

A.

The Heart: General Form and FunctionThe human heart is:

• Located beneath the sternum.

• Cone-shaped and about the size of a clenched fist.• Surrounded by a sac with a two-layered wall.• Comprised mostly of cardiac muscle tissue.

• The two atria have thin walls and function as collection chambers for blood returningto the heart.• The ventricles have thick, powerful walls that pump blood to the organs.The heart chambers alternately contract and relax in a rhythmic cycle.• A complete sequence of contraction and relaxation is the cardiac cycle.• During systole, heart muscle contracts and the chambers pump blood.• During diastole, the heart muscles relax and the chambers fill with blood.

There are four valves in the heart which prevent backflow of blood during systole: (SeeCampbell, Figure 38.4b)

• The valves consist of flaps of connective tissue.

• Atrioventricular valves are found between each atrium and ventricle and keep bloodfrom flowing back into the atria during ventricular contraction.• Semilunar valves are located where the aorta leaves the left ventricle and where thepulmonary artery leaves the right ventricle; these prevent blood from flowing back intoventricles when they relax.• A heart murmur is a defect in one or more of the valves that allows backflow of blood. Serious defects are usually corrected by surgical replacement of the valve.Heart rate = The number of heartbeats per minute.• Usually measured by taking the pulse.

• In humans, the average resting heart rate of a young adult is 60 beats per minute; ratesvary depending on the individual’s activity level.• It is not uncommon for heart rates to change during the day.

690 Circulation and Gas Exchange

• There is an inverse relationship between animal body size and pulse; elephants have arate of 25 beats per minute while some shrews have 600 beats per minute.

Circulation and Gas Exchange 691

Cardiac output = The volume of blood per minute that the left ventricle pumps into thesystemic circuit; depends on heart rate and stroke volume.• Stroke volume is the amount of blood pumped by the left ventricle each time itcontracts. The average human stroke volume is about 75 ml per beat.• The cardiac output of an average human is 5.25 L per minute.

B.

Control of the Heart

Cardiac muscle cells are myogenic (self-excitable) and can contract without input from thenervous system. The tempo of contraction is controlled by the sinoatrial (SA) node, aspecialized region of the heart, sometimes called the pacemaker.

• Located in the right atrium wall near the entrance of the superior vena cava.

• The SA node is composed of specialized muscle tissue which has characteristics ofboth muscle and nerve tissue.

• Contraction of the SA node initiates a wave of excitation that spreads rapidly from thenode and causes the two atria to contract in unison.

• This wave of contraction will pass down the atria until it reaches the atrioventricular(AV) node; a second mass of specialized muscle tissue located near the base of thewall separating the atria.

• The impulse is delayed at the AV node for 0.1 second to ensure the atria arecompletely empty before the ventricles contract.

• The impulse is then carried by a mass of specialized muscle fibers to the tip of theventricles; the impulse then travels through the Purkinje fibers upward through theventricular walls.

• The impulses produce electrical currents as they pass through the cardiac muscle.

• An electrocardiogram (EKG) is the detection of these currents which are conductedthrough the body fluids to the body’s surface.Although the SA node controls the rate of heartbeat, it is influenced by several factors:

• Two antagonistic sets of nerves influence the heart rate; one speeds up contractions inthe SA node, the other slows contractions.

• Hormones influence the SA node with some like epinephrine increasing the rate.

• Others factors, including body temperature changes and exercise, also have aninfluence.

⇒ Exercise creates a greater demand for oxygen in the muscles and an increase in

heart rate is an adaptation to meet this demand.C.

Blood Vessel Structure

The walls of arteries and veins have 3 layers:

• An outer layer of connective tissue with elastic fibers that permits stretching and recoilof the vessel.

• A middle layer of smooth muscle and elastic fibers.

• An inner endothelium of simple squamous epithelium. (See Campbell, Figure 38.7)• The middle and outer layers of arteries are thicker than those in veins.

692 Circulation and Gas Exchange

• Capillaries are comprised only of the endothelial lining which permits the exchange ofchemicals with the interstitial fluids.

Circulation and Gas Exchange 693

D.Blood Flow Velocity

There is a great difference in the speed at which blood flows through the various parts of thecirculatory system. Blood travels about 30 cm/sec in the aorta and about 0.026 cm/sec incapillaries.

• The velocity decreases in accordance with the law of continuity which states that afluid will flow faster through narrow portions of a pipe than wider portions if thevolume of flow remains constant.• An artery gives rise to so many arterioles and then capillaries that the total diameter ofvessels is much greater in capillary beds than in the artery, thus blood flows moreslowly in capillaries.• Resistance to blood flow is greater in the smaller vessels since the blood contacts moreepithelial surface area.• Blood flows faster as it enters the venules and veins since the cross-sectional area isdecreased.

E.Blood Pressure

Fluids are driven through pipes by hydrostatic pressure which is the force exerted by fluidsagainst the surfaces they contact.

Blood pressure = The hydrostatic force that blood exerts against a vessel wall.• Pressure is greater in arteries than in veins and greatest during ventricular systole.⇒ This is the main force propelling blood from the heart through the vessels.• Peripheral resistance results from impedance by arterioles; blood enters the arteriesfaster than it can leave.⇒ Thus, there is a pressure even during diastole, driving blood into capillaries

continuously.• Determined by cardiac output and degree of peripheral resistance.

⇒ Stress may trigger neural and hormonal responses that cause the smooth muscles

of vessel walls to contract, constricting blood vessels and increasing resistance.• In veins pressure is near zero; blood returns to the heart by the action of skeletalmuscles around the veins.⇒ Veins have valves that allow blood to flow only towards the heart.

• Breathing also helps return blood to the heart since the pressure change in the thoraciity during inhalation cause the vena cavae and large veins near the heart to expandand fill.

F.Blood Flow Through Capillary Beds

All tissues and organs receive a sufficient supply of blood even though only 5-10% of thecapillaries are carrying blood at any one time; the supply is adequate due to the vast numberof capillaries present in each tissue.

• Capillaries in the brain, heart, kidneys, and liver usually carry a full load of blood.

694 Circulation and Gas Exchange

• Two mechanisms regulate the distribution of blood in capillary beds. Both arecontrolled by nerve signals and hormones.• One mechanism involves the contraction and relaxation of the smooth muscle layer inthe walls of arterioles.⇒ Contraction of the muscle layer constricts the arteriole and reduces the blood flow

from it into the capillary bed.⇒ Relaxation of the muscle layer dilates the arteriole and increases the blood flow

into the capillary bed.• The second mechanism involves the contraction and relaxation of precapillarysphincters, rings of smooth muscle located at the entrance to capillary beds.⇒ Contraction of these sphincters reduces blood flow into the capillary bed;

relaxation increases blood flow into the capillary bed.The diversion of blood from one area of the body to another changes the blood supply tocapillary beds.

• After ingesting food, the digestive tract receives a large supply of blood due to dilationof arterioles and the opening of precapillary sphincters associated with the system.• During exercise, blood is diverted to the skeletal muscles.

G.

Capillary Exchange

The exchange of materials between the blood and interstitial fluids that are in direct contactwith the cells occurs across the thin walls of capillaries.

• The capillary wall is a single, \"leaky\" layer of flattened endothelial cells that overlap attheir edges.• Materials may cross these cells in vesicles (via endocytosis and exocytosis), bydiffusion through the cell, or by bulk flow between the cells due to hydrostaticpressure.Direction of fluid movement at any point along a capillary depends on the relative forces ofhydrostatic pressure and osmotic pressure. (See Campbell, Figure 38.11)

• Fluid flows out of a capillary at the upstream end near an arteriole and into a capillaryat the downstream end near a venule.• About 85% of the fluid which leaves the blood at the arteriole end of the capillary, re-enters from the interstitial fluid at the venous end.⇒ The remaining 15% of the fluid is eventually returned by the lymphatic system.

V.The lymphatic system returns fluid to the blood and aids in body defense

Capillary walls leak fluid and some blood proteins which return to the blood via the lymphaticsystem. This is the 15% (4 L/day) of fluid that does not re-enter the capillaries.• The fluid, lymph, is similar in composition to interstitial fluid.

• The lymph enters the system by diffusing into lymph capillaries which intermingle with theblood capillaries.

Circulation and Gas Exchange 695

• The lymphatic system drains into the circulatory system at two locations near the shoulders.• Lymph vessels have valves that prevent backflow and depend mainly on movement ofskeletal muscles to squeeze the fluid along.• Rhythmic contractions of vessel walls also help draw fluid into the lymphatic capillaries.• Lymph nodes are specialized swellings along the system that filter the lymph and attackviruses and bacteria.

⇒ This defense is conducted by specialized white blood cells inhabiting the lymph nodes.⇒ The nodes become swollen and tender during an infection due to rapid multiplication of

the white blood cells present.• Lymph capillaries penetrate small intestine villi and absorb fats, thus transporting them fromthe digestive system to the circulatory system.

VI.Blood is a connective tissue with cells suspended in plasma

Vertebrate blood is connective tissue with several cell types suspended in a liquid matrix calledplasma.

• The average human has 4 to 6 liters of whole blood (plasma + cellular elements).

• Cellular elements are the cells and cell fragments of the blood and represent about 45% ofthe blood volume.A.

Plasma

Water accounts for 90% of plasma which also contains electrolytes and plasma proteins.Electrolytes = Inorganic salts in the form of dissolved ions that help maintain osmoticbalance of the blood; some also help buffer the blood.• Electrolyte balance in the blood is maintained by the kidneys.

Plasma proteins help buffer blood, help maintain the osmotic balance between blood andinterstitial fluids, and contribute to its viscosity.

• Some escort lipids through blood, some are immunoglobins, some (fibrinogens) areclotting factors.

• Serum is blood plasma that has had the clotting factors removed.Plasma also contains substances in transit through the body such as nutrients, metabolicwastes, respiratory gases, and hormones.B.

Cellular Elements

Erythrocytes = Red blood cells; biconcave discs that function in transport of oxygen.• Each cubic millimeter of human blood contains 5-6 million erythrocytes.

• Lack nuclei and mitochondria; generate ATP exclusively by anaerobic metabolism.• Contain hemoglobin, an iron-containing protein that reversibly binds oxygen. About250 million molecules of hemoglobin are in each erythrocyte.

696 Circulation and Gas Exchange

• Erythrocyte production is controlled by a negative-feedback mechanism; if tissues arenot receiving enough oxygen, the kidneys convert a plasma protein to the hormoneerythropoietin, which stimulates production of erythrocytes in the bone marrow.

Circulation and Gas Exchange 697

Leukocytes = White blood cells that function in defense and immunity.• There are 5 types of leukocytes: basophils, eosinophils, neutrophils, lymphocytes andmonocytes.• There are usually 5,000-10,000 leukocytes per cubic millimeter of blood although thisnumber increases during an infection.• Actually spend most of their time outside the circulatory system in the interstitial fluidand lymphatic system; large numbers are found in the lymph nodes.• Lymphocytes become specialized during an infection and produce the body’s immuneresponse.Platelets = Fragments of cells 2 to 3 µm in diameter.

• Originate as pinched-off cytoplasmic fragments of large cells in the bone marrow.• Lack nuclei.

• Function in blood clotting.

C.

The formation of Blood Cells and Platelets

The cellular elements of the blood must be replaced as they wear out.

• The average erythrocyte circulates in the blood for 3 – 4 months before beingdestroyed by phagocytic cells in the liver and spleen.• The components are usually recycled with new molecules being constructed from themacromolecule components of the old cells.The multipotent stem cells give rise to all three of the cellular elements.

• These are found in the red marrow of bones, especially the ribs, vertebrae, breastbone,and pelvis.• Form in the early embryo and are renewed by mitosis.

• Produce a number of new blood cells equivalent to the number of dying cells.Using DNA technology to correct genetic defects in multipotent stem cells may provide atreatment for such diseases as leukemia and sickle-cell anemia.D.

Blood Clotting

A clot forms when platelets clump together to form a temporary plug and release clottingfactors (some are also released from damaged cells) that initiate a complex reaction resultingin conversion of inactive fibrinogen to active fibrin. (See Campbell, Figure 38.14)• Fibrin aggregates into threads that form the clot.

• Anticlotting factors normally prevent spontaneous clotting in the absence of injury.

698 Circulation and Gas Exchange

VII.

Cardiovascular diseases are the leading cause of death in the United States andmany other developed nations

Diseases of the heart and blood vessels are referred to as cardiovascular diseases.• Account for more than 50% of all deaths in the United States.• May culminate in a heart attack or stroke.

Heart attack = Death of the cardiac muscle resulting from prolonged blockage of one ormore coronary arteries.Stroke = Death of nervous tissue in the brain often resulting from blockage of arteries in thebrain.A thrombus is often associated with a heart attack or stroke.Thrombus = A blood clot that blocks a key blood vessel.• If it blocks the coronary arteries, a heart attack occurs.• An embolus is a moving clot.

• If the thrombus or embolus blocks an artery in the brain, a stroke results.Arteries may gradually become impaired by atherosclerosis.

Atherosclerosis = Chronic cardiovascular disease characterized by plaques that develop on theinner walls of arteries and narrow the bore of the vessels.• Decreases blood flow through the vessels.• Increase the risk of clot formation and heart attack.

Arteriosclerosis = Degenerative condition of arteries (form of atherosclerosis) in which plaquesbecome hardened by calcium deposits.Angina pectoris = Chest pains that occur when the heart receives insufficient oxygen due to thebuild up of plaques in the arteries.• An indicator that the heart is not receiving sufficient oxygen.Hypertension = High blood pressure; promotes atherosclerosis.• Increases risk of heart attack and stroke.

Smoking, lack of exercise and a diet rich in animal fats correlate with increased risk ofcardiovascular disease.

Abnormally high concentrations of LDL's (low density lipoproteins) in the blood correlate withatherosclerosis; HDL's (high density lipoproteins) actually reduce deposition of cholesterol inarterial plaques.

• Exercise tends to increase HDL concentration.• Smoking increases the LDL to HDL ratio.

Circulation and Gas Exchange 699

VIII.Gas exchange supplies oxygen for cellular respiration and disposes of carbon

dioxide: an overview

Circulatory systems transport oxygen and carbon dioxide between respiratory organs and otherparts of the body.A.

Gas Exchange and the Respiratory Medium

Gas exchange = The movement of O2 and CO2 between the animal and its environment.• Supports cellular respiration by supplying O2 and removing CO2.

The respiratory medium (source of oxygen) for terrestrial animals is the air, while it is waterfor aquatic animals.

• Air is 21% oxygen while the amount of dissolved oxygen in water varies due totemperature, solute concentrations, and other factors.Respiratory surface = Portion of the animal surface where gas exchange with the respiratorymedium occurs. Oxygen diffuses in; carbon dioxide diffuses out.• O2 and CO2 can only diffuse through membranes if they are first dissolved in the waterthat coats the respiratory surface.• This surface must be large enough to provide O2 and expel CO2 for the entire body.

B.

Respiratory Surfaces: Form and Function

A variety of respiratory surfaces have evolved that are adaptive for organism size andenvironment.

• Protozoa and other unicellular organisms exchange gases over their entire surfacearea.• Animals such as sponges, cnidarians and flatworms have body structures such that theplasma membrane of each body cell contacts the outside environment and can functionin exchange.• In animals with a more three-dimensional body plan, most of the body cells areisolated from the environment and the respiratory surfaces are generally thin, moistepithelium with a rich blood supply.⇒ Usually only a single cell layer separates the respiratory medium from the blood or

capillaries.Animals that are relatively small or have a shape (long, thin) that results in a high surface tovolume ratio may use their outer skin as a respiratory organ.

• Earthworms have moist skin which overlays a dense network of capillaries.

• Gases diffuse across the entire surface (O2 in; CO2 out) and into the circulatorysystem.• Earthworms and other animals (some amphibians) that use their skin for gas exchangemust live in water or damp places in order to keep the exchange surface moist.

700 Circulation and Gas Exchange

Most other animals lack sufficient body surface area to exchange gases for the entire body.These animals possess a region of the body surface that is extensively branched or folded,thus providing a large enough respiratory surface area for gas exchange.

• In most aquatic animals, external gills are present and are in direct contact with thewater.• Terrestrial animals have internal respiratory surfaces that open to the atmospherethrough narrow tubes.⇒ Air lacks the supportive density and moisturizing qualities of water.⇒ Lungs and insect tracheae are two variations.

IX.Gills are respiratory adaptations of most aquatic animals

Gills are outfoldings of the body surface specialized for gas exchange. (See Campbell, Figure38.17)

• In some invertebrates (i.e. echinoderms), gills have simple shapes and are distributed overthe entire body.• In other invertebrates (i.e. annelids), gills may be flap-like and extend from each bodysegment or be clustered at one end and be long and feathery.• Other animals (mollusks, fishes) have gills that are localized on a body region where thesurface is finely subdivided to provide a large amount of surface area.• The gills must be efficient since although water keeps the respiratory surface wet, it has alower oxygen concentration than air.Ventilation = Any method of increasing the flow of the respiratory medium over the respiratorysurface; brings in a fresh supply of O2 and removes CO2.Due to the density and low oxygen concentration of water, fish must expend a large amount ofenergy to ventilate water.

• Fish have a unique arrangement of blood vessels in their gills, which maximizes O2 uptakefrom H2O.• Called countercurrent exchange, blood flows opposite to the direction in which water passesover gills, maintaining a constant concentration gradient for oxygen between the blood andthe water passing over the gill surface. (See Campbell, Figures 38.18 and 35.19)

X.Tracheae are respiratory adaptations of insects

Air has several advantages over water as a respiratory medium:• A higher oxygen concentration.

• Oxygen and carbon dioxide diffuse faster through air than water.• Respiratory surfaces do not have to be ventilated as thoroughly.

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The major disadvantage is that respiratory surfaces are continually desiccated.

• The evolution of respiratory surfaces within the body (tracheae, lungs) helped solve thisproblem.Tracheae are tiny air tubes that branch throughout the insect body; air enters the system throughpores called spiracles and diffuses through the small branches which extend to the surface ofnearly every cell.

• Some small insects rely on diffusion alone to move O2 into the system and CO2 out; othersuse rhythmic body movements for ventilation.• Cells are exposed directly to the respiratory medium so insects do not use their circulatorysystems to transport O2 and CO2.⇒ This is a major reason why the open circulatory system works so well in insects.

XI.Lungs are the respiratory adaptation of most terrestrial vertebrates

Lungs are highly vascularized invaginations of the body surface that are restricted to one location.• The circulatory system must transport oxygen from the lungs to the rest of the body.• Land snails use an internal mantle as a lung.• Spiders possess booklungs.

• Various degrees of lung development are found in terrestrial vertebrates: frogs have simpleballoonlike lungs with limited surface area; mammals have highly subdivided lungs with alarge surface area.A.

Form and Function of Mammalian Respiratory Systems

Located in the thoracic cavity, mammalian lungs are enclosed in a sac consisting of twolayers held together by the surface tension of fluid between the layers.• Air entering the nostrils is filtered by hairs, warmed and moistened.

• The air then travels through the pharynx, then through the glottis, and into larynx(which possesses vocal cords and functions as a voice box).• The flow then enters the cartilage-lined trachea that forks into two bronchi whichfurther branch into finer bronchioles that dead-end in alveoli.• Alveoli are lined with a thin layer of epithelium which serves as the respiratorysurface.• Oxygen dissolves in the moist film covering the epithelium and diffuses across to thecapillaries covering each alveolus; carbon dioxide moves in the opposite direction bydiffusion.

B.

Ventilating the Lungs

Vertebrates ventilate lungs by breathing (alternate inhalation and exhalation of air).• Maintains a maximum O2 concentration and minimum CO2 concentration in thealveoli.

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Frogs ventilate the lungs by positive pressure breathing. Air is pushed down the windpipeinto the lungs.

• Air is pulled into the mouth by lowering the floor region; this enlarges the oral cavity.• The nostrils and mouth are closed and the floor of the mouth is raised, forcing airdown the trachea.• Air is exhaled by elastic recoil of lungs and by compression of the lungs by themuscular body wall.Mammals ventilate their lungs by negative pressure breathing. During inhalation, air ispulled into the lungs by the negative pressure created as the thoracic cavity enlarges by twopossible mechanisms:

• When a mammal is at rest, most of the shallow inhalation results from contraction ofthe diaphragm.⇒ The diaphragm is a dome-shaped, thin sheet of muscle that forms the bottom wall

of the thoracic cavity. When it contracts, it pushes downward towards theabdomen, enlarging the thoracic cavity.⇒ This lowers the air pressure in the lungs below atmospheric pressure and causes

inhalation.• Action of the rib muscles in increasing lung volume is important during vigorousexercise.⇒ Contraction of the rib muscles pulls the ribs upwards, which expands the rib cage.⇒ As the thoracic cavity enlarges, the lungs also expand, since the surface tension of

the fluid between the layers of the lung sac causes the lungs to follow.⇒ Lung volume increases, resulting in negative pressure within the alveoli, causing

air to rush in.Exhalation occurs when the diaphragm and/or the rib muscles relax, decreasing the volumeof the thoracic cavity.

The amount of air inhaled and exhaled depends upon size, activity level and state of health.• Tidal volume is the volume of air an animal inhales and exhales with each breathduring normal quiet breathing. Averages about 500 ml in humans.• Vital capacity is the maximum air volume that can be inhaled and exhaled duringforced breathing. Averages 3400 – 4800 ml in college-age females and males,respectively.• Residual volume is the amount of air that remains in the lungs even after forcedexhalation.Birds have a more complex process for ventilation:

• Besides lungs, birds have 8 or 9 air sacs in their abdomen, neck and wings that serveto trim the density of the body and act as sinks for the heat dissipation by metabolismof flight muscles.• The air sacs do not exchange gases, they serve as bellows to keep the air moving.

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• The lungs also have tiny channels called parabronchi; air flows through the entiresystem, lungs and air sacs, in only one direction regardless of whether it is inhaling orexhaling.• The continuous flow of air through the parabronchi provides a constant supply ofoxygen to the blood whether the bird is inhaling or exhaling.

C.

The Control of Breathing

Breathing is an automatic action, we inhale when nerves in the breathing control centers ofthe medulla oblongata and pons send impulses to the rib muscles or diaphragm, stimulatingthe muscles to contract.

• This occurs about 10 – 14 times per minute and the degree of lung expansion iscontrolled by a negative-feedback mechanism involving stretch receptors in the lungs.• The medulla’s control center also monitors blood and cerebrospinal fluid pH, whichdrops as blood CO2 concentrations increase. When it senses a drop in pH (increasedCO2 level), the tempo and depth of breathing are increased and the excess CO2 isremoved in exhaled air.• O2 concentration in the blood only effects the breathing control centers when itbecomes severely low.⇒ O2 sensors in the aorta and carotids send signals to the breathing control centers

and the centers respond by increasing the breathing rate.The breathing control centers thus respond to a variety of neural and chemical signals.• The response is an adjustment in the rate and depth of breathing to meet the demandsof the body.• The control is effective since it is coordinated with control of the circulatory system.

D.

Loading and Unloading of Oxygen and Carbon Dioxide

Whether a gas enters or leaves the blood depends on the concentration gradient of gaseswhich is measured as differences in partial pressure (e.g. PO2 = partial pressure of oxygen). Oxygen comprises about 21% of the atmosphere and carbon dioxide about 0.03%.

• The partial pressure of a gas is the proportion of the total atmospheric pressure (760mm) contributed by the gas; PO2 = 160 mm Hg (0.21 × 760) and PCO2 = 0.23 mm Hg.• Gases always diffuse from areas of high partial pressure to those of low partialpressure.• Blood arriving at the lungs from the systemic circulation has a lower PO2 and a higherPCO2 than air in the alveoli.⇒ Thus, the blood exchanges gases with air in the alveoli and the PO2 of the blood

increases while the PCO2 decreases.• In systemic capillaries gradients of partial pressure favor diffusion of oxygen out ofthe blood and diffusion of CO2 into it, since cellular respiration rapidly depletesinterstitial fluid of O2 and adds CO2. (See Campbell, Figure 38.25)

E.

Respiratory Pigments and Oxygen Transport

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Oxygen is carried by respiratory pigments in the blood of most animals since oxygen is notvery soluble in water.

• The pigments are proteins which contain metal atoms; the metal atoms are responsiblefor the color of the protein.In arthropods and mollusks, hemocyanin is the O2 carrying pigment.• The oxygen-binding component is copper and results in a blue color.• It is dissolved directly in plasma, not confined to cells.

Hemoglobin is the oxygen-transporting pigment in almost all vertebrates.

• Consists of 4 subunits, each containing a heme group that bonds O2; an iron atom is atthe center of each heme group and actually binds to oxygen.• The binding of oxygen to hemoglobin is reversible. (Binding occurs in the lungs;release occurs in the tissues.)• Binding of O2 to one subunit induces a shape change that increases the affinity of theother 3 subunits for oxygen (cooperativity).• The unloading of oxygen from one heme group results in a conformational change thatstimulates unloading from the other three.• The cooperative nature of this mechanism is evident in the dissociation curve depictedin Figure 38.26.• Bohr shift is the lowering of hemoglobins' affinity for oxygen upon a drop in pH. (SeeCampbell, Figure 38.26b) This occurs in active tissues due to the entrance of CO2into the blood.

F.

Carbon Dioxide Transport

Hemoglobin not only transports oxygen but also helps the blood transport carbon dioxideand assists in buffering the blood against harmful pH changes.Carbon dioxide is transported by the blood in three forms:• Dissolved CO2 in the plasma (7%).

• Bound to the amino groups of hemoglobin (23%).• As bicarbonate ions in the blood (70%).

Carbon dioxide from cells diffuses into the blood plasma and then into erythrocytes; in theerythrocytes, carbonic anhydrase catalyzes a reversible reaction wherein CO2 is convertedinto bicarbonate.

• The CO2 reacts with water to form carbonic acid.

• The carbonic acid quickly dissociates to bicarbonate and hydrogen ions.• Bicarbonate then diffuses out of the erythrocyte and into the blood plasma.

• The hydrogen ions attach to hemoglobin and other proteins which results in only aslight change in the pH.• The process is reversed in the lungs.

G.

Adaptations of Diving Mammals

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Diving mammals such as seals, dolphins, and whales have special adaptations which allowthem to make long underwater dives.

Weddell seals, which make dives to 200-500 m depths for 20 minutes or more, have beenextensively studied.

• These mammals can store large amounts of oxygen, about twice as much per kilogrambody weight as humans.⇒ 70% of the oxygen load is found in the blood (51% in humans) and 5% in the

lungs (36% in humans).⇒ They also possess a higher myoglobin (oxygen storing pigment) concentration in

their muscles.• The high oxygen load in the blood may be due to the higher blood volume in theseseals (twice the amount of blood per kilogram of body weight as humans).• The seals also have a very large spleen which can store about 24 L of blood.

⇒ The spleen probably contracts after a dive and forces more erythrocytes loaded

with oxygen into the blood.• The high concentration of myoglobin permits these seals to store about 25% of theiroxygen in muscles (compared to 13% in humans).These adaptations provide diving mammals with a large oxygen reservoir at the beginning ofa dive, but they also have several adaptations to conserve oxygen during the dive.

• A diving reflex slows the pulse, and oxygen consumption declines as the cardiacoutput slows.• Most of the blood is routed to the brain, spinal cord, eyes, adrenal glands, and placentadue to regulatory mechanisms that reduce blood flow to the muscles.• The reduced blood flow to the muscles results in a depletion of the myoglobin oxygenreserves after about 20 minutes, the muscles then shift to anaerobic production ofATP.The special adaptations found in diving mammals emphasizes that animals can respond toenvironmental pressures in the short term through physiological adaptation as well as in thelong term by natural selection.

REFERENCES

Campbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.

Marieb, E.N. Human Anatomy and Physiology. 3rd ed. Redwood City, California: Benjamin/Cummings, 1995.