CHAPTER 9 SUMMARY
•Circulatory systems have evolved in most animal groups because diffusion is too slow for distribution of O2, nutrients, wastes, and hormones. These systems can also transmit pressure that can move flexible body parts, and provide the driving force for filtration in renal organs.
•Circulation consists of up to three components: fluid, pump(s), and vessels.
•Open circulation (in most molluscs, arthropods, etc.) has a fluid called hemolymph that moves freely among organs, and which may be pumped by cilia, body movements or heart. There may be vessels, but they open up into body cavities.
•Closed circulation (in cephalopods, vertebrates, etc.) has a fluid called blood that is pumped by a heart and stays entirely within blood vessels, with tissue exchange occurring in tiny vessels called capillaries.
CIRCULATORY FLUIDS
•Fluids consist of liquid plasma and cellular elements. The percentage of whole-blood volume occupied by blood cells is known as the hematocrit.
•Plasma is a complex liquid that serves as a transport medium. All plasma constituents are freely diffusible across the capillary walls (in closed systems) except the plasma proteins, which remain in the plasma and perform a variety of functions: creating colloidal osmotic pressure, buffering pH changes, binding and transporting various molecules such as cholesterol, iron and oxygen, participating in clotting and immune processes.
•Cellular elements in vertebrates are erythrocytes, leukocytes, and clotting cells.
•Erythrocytes (red blood cells) are specialized for O2 transport in the blood. In vertebrates, they are full of hemoglobin, an iron-containing protein that loosely, reversibly binds with O2. This is useful because O2 is poorly soluble in blood. Erythrocytes usually have a short life span. Undifferentiated pluripotent stem cells in the red bone marrow give rise to all cellular elements of the blood. Erythrocyte maturation is stimulated by erythropoietin, a hormone secreted by the kidneys in response to reduced O2 delivery.
•Leukocytes (white blood cells) are the defense corps of the body. Leukocytes are present in the blood only while in transit from their site of production and storage in the bone marrow (and also in the lymphoid tissues in the case of the lymphocytes) to their site of action in the tissues. All leukocytes have a limited life span and must be replenished by ongoing differentiation and proliferation of stem cells.
•Clotting cells are thrombocytes in most vertebrates, and platelets in mammals; the latter are actually cell fragments derived from large megakaryocytes in the bone marrow. Clotting cells play an important role in hemostasis, the arrest of bleeding from an injured vessel. Hemostasis steps are (1) vascular spasm, (2) platelet plugging, and (3) clot formation. Spasm reduces blood flow through an injured vessel, whereas aggregation of platelets at the site of vessel injury quickly plugs the defect. Platelets start to aggregate upon contact with ex-posed collagen in the damaged vessel wall.
•Clot formation reinforces the platelet plug and converts blood near a vessel injury into a gel. Most factors necessary for clotting are always present in the plasma in inactive forms. When a vessel is damaged, exposed collagen initiates a cascade of reactions involving successive activation of these clotting factors, ultimately converting fibrinogen into fibrin. Fibrin, an insoluble threadlike molecule, is laid down as the meshwork of the clot; the meshwork in turn entangles blood cells to complete clot formation. When no longer needed, clots are dissolved by plasmin, a factor also activated by exposed collagen.
•Insects contain similar clotting factors, though detailed reactions are not known.
CIRCULATORY PUMPS
Anatomical and Evolutionary Considerations
•Pumps come in a variety of forms: (1) flagella, (2) extrinsic muscle or skeletal pumps (in which musculoskeletal movements push the circulatory fluid); (3) peristaltic pumps (in which vessel walls contract in a wave); and (4) chamber muscle pumps (muscular chambers that squeeze fluid through one-way valves).
•Many animals have primary or systemic hearts aided by auxiliary pumps, which usually move fluid from distant parts of the body.
•Arthropods hearts draw in hemolymph through valved ostia and pump out into arteries tha empty into a hemocoel.
•Vertebrate hearts began with two chambers: an atrium to collect returning blood, and a ventricle to pump fluid to the body. This evolved in mammals into a four-chambered (two atria, two ventricles), dual pump that moves fluid through two separate vessel systems, the pulmonary (to the lungs) and systemic (to the rest of the body). One-way valves direct the blood in the proper direction and prevent it from flowing in the reverse.
•Contraction in a mammalian heart of the spirally arranged cardiac muscle fibers produces a wringing effect for efficient pumping. Also important for efficient pumping is the fact that the vertebrate muscle fibers in each chamber act as a functional syncytium, contracting as a coordinated unit.
Electrical Activity of Hearts
•Myogenic hearts are capable of initiating electrical signals on their own, usually starting as special pacemaker cells. In most vertebrates, the cardiac impulse originates at the SA node, the pacemaker of the heart, which has the fastest rate of spontaneous depolarization to threshold.
•Once initiated, the action potential spreads in a mammalian heart throughout the right and left atria, only by cell-to-cell spread of the impulse through gap junctions. The impulse passes from the atria into the ventricles through the AV node, the only point of electrical contact between these chambers. The action potential is delayed briefly at the AV node, ensuring that atrial contraction precedes ventricular contraction to allow complete ventricular filling. The impulse then travels rapidly down the interventricular septum via the bundle of His and is rapidly dispersed through the myocardium via Purkinje fibers and gap junctions.
•The action potentials of contractile cardiac muscle fibers exhibit a prolonged positive phase, or plateau, accompanied by a prolonged period of contraction, which ensures adequate ejection time. This plateau is primarily due to activation of slow Ca++ channels. Because a long refractory period occurs in conjunction with this prolonged plateau phase, summation and tetanus of cardiac muscle are impossible, thereby ensuring the alternate periods of contraction and relaxation essential for pumping of blood.
•The spread of electrical activity throughout the heart can be recorded from the surface of the body. This record, the ECG, can provide useful information about the status of the heart.
Mechanical Events of the Cardiac Cycle
•The cardiac cycle consists of three important events: (1) The generation of electrical activity as the heart autorhythmically depolarizes and repolarizes. (2) Mechanical activity consisting of alternate periods of systole (contraction and emptying) and diastole (relaxation and filling), which are initiated by the rhythmical electrical cycle. (3) Directional flow of blood through the heart chambers, guided by valve opening and closing induced by pressure changes that are generated by mechanical activity.
•The atrial pressure curve remains low throughout the entire cardiac cycle, with only minor fluctuations. The aortic pressure curve remains high the entire time, with moderate fluctuations (in humans, typically varying between a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg). The ventricular pressure curve fluctuates dramatically because ventricular pressure must be below the low atrial pressure during diastole to allow the AV valve to open so filling can take place, and it must be above the high aortic pressure during systole to force the aortic valve open to allow emptying to occur.
Cardiac Output and Its Control
•Cardiac output, the volume of blood ejected by a ventricle per minute, is determined by the heart rate times the stroke volume. This is the crucial physiological factor.
•Heart rate in vertebrates is varied by altering the balance of parasympathetic and sympathetic influence on the SA node. Parasympathetic stimulation slows the heart rate, and sympathetic stimulation speeds it up.
•Stroke volume depends on (1) the extent of ventricular filling, with an increased end-diastolic volume resulting in a larger stroke volume by means of the length-tension relationship (intrinsic control), and (2) the extent of sympathetic stimulation, with increased sympathetic stimulation resulting in increased contractility of the heart, that is, increased strength of contraction and increased stroke volume at a given end-diastolic volume (extrinsic control).
Nourishing the Heart Muscle
•Cardiac muscle is supplied with oxygen and nutrients by blood delivered to it by the coronary circulation, not by blood within the heart chambers. Most coronary blood flow occurs during diastole, because the coronary vessels are compressed by the contracting heart muscle during systole. Coronary blood flow is normally varied to keep pace with cardiac oxygen needs.
CIRCULATORY PATHWAYS AND VESSELS
Anatomical and Evolutionary Considerations
•Hemolymph and blood transport materials and transmit force as they are pumped under pressure.
•Hemolymph may travel in arteries initially, but then emerges to bathe tissues directly.
•Blood flows in a closed loop between the heart and the tissues. Closed vascular systems provide more precise control of fluid delivery to organs according to their needs.
•Pressure can be used as a force to extend limbs (e.g., in arachnids), filter blood, and create erections.
Flow Regulation and Hemodynamics
•Blood flow can be selectively directed to different organs as needed. The flow rate (F) of blood through a vessel is directly proportional to the pressure gradient (P) and inversely proportional to the resistance (R): F = P/R, or C.O. =∆P/R for a cardiovascular system.
•The higher pressure at the beginning of a vessel is established by the pressure imparted to the blood by cardiac contraction. The lower pressure at the end is due to frictional losses as flowing blood rubs against the vessel wall.
•Resistance, the hindrance to blood flow through a vessel, is influenced most by the vessel’s radius. Resistance is inversely proportional to the fourth power of the radius, so small changes in radius profoundly influence flow.
•Gravity creates a backpressure against which the heart must work, an important factor in tall animals.
Open Circulation (Arthropods)
•In large crustacea, arteries may branch into fine capillary-like vessels in many organs, and hemolymph flows through gills before returning to the heart. Insect circulation has fewer arteries and no capillary-like vessels.
Closed Circulation (Vertebrates)
•The vascular system evolved from one circuit to two separate circuits in vertebrates:
•The primitive vertebrate flow pattern is: Heartgillsother organsheart. This is the pattern in water-breathing fish, though agnathans have auxiliary hearts in some veins.
•In lungfish, a separate circuit through lungs evolved, necessitating partial separation of flow output from the heart. In many adult amphibians, flow goes first to lung and skin, returns to the heart, then proceeds to the rest of the body.
•In most reptiles, the single ventricle has 3 subchambers; flow normally goes from the heart to the lungs to the heart and then to the body, but flow can bypass the lungs completely during diving. Crocodiles have similar basic flow, but with two separate ventricles; and the foramen of Panizza allows blood to flow from the initial lung circulation to the body, again bypassing the lung during diving.
•Birds and mammals have four-chambered hearts with completely separate flows to the lungs (pulmonary) and body (systemic) vessels.
Vertebrate circulatory vessels have separate distinct functions:
•Arteries are large-radius, low-resistance passageways from the heart to the tissues; they also serve as a pressure reservoir. Because of their elasticity, arteries expand to accommodate the extra volume of blood pumped into them by cardiac contraction and then recoil to continue driving the blood forward when the heart is relaxing. Systolic pressure is the peak pressure exerted by the ejected blood against the vessel walls during cardiac systole. Diastolic pressure is the minimum pressure in the arteries when blood is draining off into the vessels downstream during cardiac diastole.
•Arterioles are the major resistance vessels, and control flow to individual organs. Their high resistance produces a large drop in mean pressure between the arteries and capillaries. Tone, a baseline of contractile activity, is maintained in arterioles at all times. Arteriolar vasodilation, an expansion of arteriolar caliber above tonic level, decreases resistance and increases blood flow through the vessel, whereas vasoconstriction, a narrowing of the vessel, increases resistance and decreases flow. Arteriolar caliber is subject to two types of controls:
•Local (intrinsic) controls involve local chemical changes associated with changes in the level of metabolic activity; these act directly on the arteriolar smooth muscle to induce changes in the caliber of the local arterioles. This local control mechanism adjusts blood flow to the tissue to match momentary metabolic needs.
•Extrinsic control is accomplished primarily by sympathetic nerve influence and to a lesser extent by hormones. Extrinsic controls are important in maintaining mean arterial blood pressure. Arterioles are richly supplied with sympathetic nerve fibers, whose increased activity produces generalized vasoconstriction and a subsequent increase in mean arterial pressure. Decreased sympathetic activity produces generalized arteriolar vasodilation, which lowers mean arterial pressure. These extrinsically controlled adjustments of arteriolar caliber help maintain the appropriate pressure head for driving blood forward to the tissues.
•Capillaries are thin-walled, small-radius, extensively branched vessels ideally suited to serve as sites of exchange between the blood and surrounding tissues.
•The surface area for exchange is maximized and diffusion distance is minimized.
•Because of their large total cross-sectional area, the velocity of blood flow through capillaries is relatively slow, providing adequate time for exchanges to take place.
•Two types of passive exchanges—diffusion and bulk flow—take place across capillary walls. Individual solutes are exchanged primarily by diffusion down concentration gradients. Lipid-soluble substances pass directly through the single layer of endothelial cells lining a capillary, whereas water-soluble substances pass through water-filled pores between the endothelial cells. Plasma proteins generally do not escape.
•Imbalances in physical pressures acting across capillary walls are responsible for bulk flow of fluid through the pores back and forth between the plasma and interstitial fluid. Fluid is forced out of the first portion of the capillary (ultrafiltration), where outward pressures (mainly capillary blood pressure) exceed inward pressures (mainly plasma-colloid osmotic pressure). Fluid is returned to the capillary along its last half, when outward pressures fall below inward pressures. The reason for the shift in balance down the length of the capillary is the continuous decline in capillary blood pressure while the plasmacolloid osmotic pressure remains constant. Bulk flow is responsible for the distribution of extracellular fluid between the plasma and the interstitial fluid.
•Lymph vessels pick up leftover fluids. Normally, slightly more fluid is filtered than is reabsorbed. The extra fluid, any leaked proteins, and tissue contaminants such as bacteria are picked up by the lymphatic system. In amphibians and reptiles, lymph hearts aid flow back to the blood circulation
•Veins are large-radius, low-resistance passageways for return of blood from the tissues to the heart. Additionally, they can accommodate variable volumes of blood and therefore act as a blood reservoir.
•The capacity of veins to hold blood can change markedly with little change in venous pressure. Veins are thin-walled, highly distensible vessels that can passively stretch to store a larger volume of blood.
•The primary force responsible for venous flow is the pressure gradient between the veins and atrium (that is, what remains of the driving pressure from the heart).
•Venous return is enhanced by sympathetically induced venous vasoconstriction, by external compression of the veins resulting from contraction of surrounding skeletal muscles, and by other auxiliary pumps. One-way venous valves ensure that blood is driven toward the heart and prevented from flowing back toward the tissues. Venous return is also enhanced by respiratory activity, which produces a less-than-atmospheric pressure in the chest cavity that encourages flow from the lower veins that are exposed to atmospheric pressure to the chest veins, and by slightly negative pressures created within the atria during ventricular systole and within the ventricles during ventricular diastole exert a suctioning effect on venous return.
INTEGRATED CARDIOVASCULAR FUNCTION
•The cardiovascular system has two main goals: regulating gas transport as needed by the body, and maintaining blood pressure in a viable range.