Summary of Important Concepts
The circulatory system is more elaborate in vertebrates than in other animals (Table 1). This is a direct consequence of body size. The simple mechanical demand of moving greater quantities of blood over greater distances has required the evolution of a more complex and stronger heart with multiple chambers. Larger masses of tissues reduce the effectiveness of diffusion and have required the development of closed circulation within an extensive system of blood vessels and capillaries. The efficiency of the blood in its ability to carry and deliver oxygen has been finely adjusted by the controls on blood pressure and the molecular mechanisms of oxygen transport.
The vertebrate system was able to make suitable adjustments for the greater demands of the high metabolic levels of mammals, including their need for an even higher rate of oxygen delivery and the new requirement of finely tuned thermoregulation. Many of the details of oxygen transport and that of other gases and nutrients are more suited to a physiology text. The following chapter will explore the contributions that anatomical design can make to circulatory function.
TABLE 1. CHARACTERISTICS OF THE HUMAN CIRCULATORY SYSTEM
The blood vessels in the body form a closed circulation, in which blood does not leave the vessels. Although some white blood cells do actively roam outside of the capillaries and plasma routinely escapes from them, the erythrocytes and the bulk of the plasma continues to follow an endless circuit away from and back to the heart. Many invertebrates possess open circulation, in which blood is pumped initially through an aorta but then flows freely among the body cells before finding its way back into the heart. Closed circulation is a verebrate characteristic.
Arteries and veins are defined according to the direction of blood flow relative to the heart. Arteries carry blood away from the heart. Arterial blood is usually richer in oxygen than venous blood, but this does not hold true for the pulmonary vessels or for the vessels in a fetus. Because arterial blood is under pressure, the walls of arteries have to be reasonably sturdy. As the vessels course farther from the heart, they branch into smaller and smaller vessels. The smallest branches of arteries are calledarterioles. Arterioles have thinner walls and distribute blood among small divisions of tissues.
Veins carry blood from body tissues back to the heart. This blood is under very little pressure and the walls are relatively thinner. The smallest branches of veins are called venules. Blood carried by venules and veins converges into larger vessels in its route to the heart.
Arterioles and venules are joined by microscopically small vessels called capillaries that pass beside or close to nearly all the cells of the body. Passages through capillaries are commonly smaller than the diameter of red blood cells, and only the elasticity of their cell walls permits these blood cells to pass through. The small diameter provides a resistance to blood flow that reduces pressure nearly to zero. The slow movement of blood in capillaries facilitates the exchange of gases and nutrients with surrounding tissues.
Structure of Blood Vessels
Arteries and veins have a similar structure. The walls have three layers. The inner most layer is the tunica intima, orendothelium. This is a simple squamous epithelium whose function is larger vessels is simply to provide minimal resistance to the passage of blood. The tunica intima also includes a layer of elastic connective tissue that gives it greater strength.
The middle layer of vessels, or tunica media, contains smooth muscle and another layer of elastic fibers. This muscle is responsible for manipulating the diameter of the passage within the vessel. Narrowing the passage increases the resistance to blood flow and reduces its volume. If major vessels throughout the body are constricted in a systemic response, blood pressure will be elevated. The muscle and elastic tissues of an artery cause its wall to maintain a round shape and an open passage within. After the artery is squeezed, it immediately resumes its shape.
The outer layer of the vessel wall is a supporting layer of connective tissue called the tunica externa (or adventitia). It helps to anchor the blood vessel within the body.
Veins differ structurally from arteries primarily in the thickness of the muscular and elastic layers. Because venous blood is under considerably less pressure, venous walls are correspondingly weaker. An empty vein usually collapses and remains in that state until it is filled with fluid. Many veins also contain simple valves to maintain direction of the blood flow. The valves consist of a pair of flaps that are passively opened by the movement of blood and equally passively closed by any backflow in the opposite direction.
Smaller blood vessels depart from this basic structure by reducing or eliminating the outer layers. The smaller arterioles and venules have lost the outer layer and reduce the muscle of the tunica media to a single layer of fibers. Capillaries dispense with all but the epithelial cells of the endothelium. The thin walls and the flat shape of the squamous cells encourage the distribution of oxygen and other small molecules through them. The endothelial cells of the endothelium are also more loosely joined to facilitate the escape of fluids and white blood cells to escape into adjacent tissues.
An anastomosis is a direct connection between two blood vessels that are larger than capillaries and that permits blood to flow from one to the other. Anastomoses may occur between arteries, between veins, or between an artery and a vein. Each of these pairings serves a different function.
Arterial anastomoses provide alternate pathways, or collateral circulation, for blood to reach a given tissue. Under ideal circumstances, we expect the arterial blood advancing into the anastomosis from opposite directions to slow to a stop at a point where the blood pressure is zero. The location of the zero point depends on the relative pressures in the two arteries that have been joined. Realistically, however, those pressures change constantly from one moment to the next and the zero point also changes constantly.
Consider, for example, the brachial artery crossing the elbow and dividing into the ulnar and radial arteries. Any contraction of the skeletal muscles adjacent to one of these arteries will inhibit blood flow and decrease pressure downstream of the contraction. Flexion of the joint will also compress the brachial artery and interfere with blood flow because the artery passes on the anterior aspect (inside the flexure) of the elbow. Collateral circulation enables blood to travel through smaller vessels that branch off the brachial artery above the elbow and join the radial and ulnar arteries in the forearm below the compression. The anastomoses therefore permit immediate compensation for the effects of muscular and skeletal activity on the circulation.
Arterial anastomoses are commonly found where such interference is likely, at the major joints in the limbs; at the ends of long vessels where blood pressure is likely to be low, as on the thoracic wall; and in vital organs, such as the brain and heart, where a temporary interruption of blood flow could damage tissues.
Venous anastomoses serve a similar purpose in providing alternative pathways by which blood may return to the heart. Because venous blood has very low pressure, its flow is easily blocked by the activity of body tissues or by external pressure. There is a much greater development of collateral circulation among veins than arteries. Consequently, injuries or surgical procedures that damage or block a smaller vein are much less serious than disruptions of arteries.
Arteriovenous anastomoses occur at the level of arterioles. The circulatory system needs to be able to supply more blood to a given tissue than it needs in a resting state. It can reduce overall blood pressure by shunting that surplus flow directly into the venules. When the tissues are very active and require more oxygen, the shunts can be constricted by muscular sphincters to direct an increased flow into the capillary beds. Actively manipulating these passages therefore permits the body to adjust the flow of blood into or away from tissues according to their need.
Blood in the arteries must be put into motion to circulate throughout the body. In the digestive system, the body uses peristaltic contractions to move food along the alimentary canal. A rhythmic peristalsis might work for arteries, but probably could not generate the force necessary to overcome the resistance presented by the smallest capillaries. Thus a larger pump, the heart, is needed. The simplest heart is a muscular chamber that can contract and push blood along the artery. The introduction of valves controls the direction in which the blood will move. The complexity of the heart is increased by its division into chambers that initially served to increase the effective force of contraction.
It is simple to imagine a heart created by strengthening the muscle already in the walls of the vessels. However, the muscle of the heart has diverged into a unique tissue, different from both skeletal and visceral muscle. Cardiac muscle is striated &emdash; that is, its contractile proteins are aligned in bands within the fibers. Individual cells are stacked end to end to form long muscle fibers. Cardiac cells do not required individual innervation to stimulate contraction. Instead, excitation is initiated rhythmically by a small cluster of specialized cardiac cells and then spreads directly from one cell to another.
In its striation and formation of long fibers, cardiac tissue resembles skeletal muscle. By its composition of individual cells and its ability to spread rhythmic waves of excitation across the tissue, it more resembles smooth muscle. Clearly this specialized tissue has adopted the desirable characteristics of both of its relatives.
Valves maintain the direction of the blood flow. It is important for the integrity and efficiency of the circulatory system that the valves operate passively. Each set of valves, whether in the veins or in the heart, is opened by the pressure of blood flowing in the proper direction. Each set of valves is closed by any tendency of the blood beyond it to flow backwards.
Valves separate the chambers of the heart and have specialized forms corresponding to the size of the passages and the blood pressure generated by the different chambers.
The Chambered Heart
The force with which a muscular chamber can contract is proportional to the initial stretch of its walls. If sufficient blood enters a chamber of the heart under pressure, it will distend the walls and permit the fibers to generate a stronger contraction. This relationship of stretch and force of contraction has been formalized as Starling's Law.
How does the body create an initial pressure to fill the heart? The solution is to create a series of chambers. The first chamber fills with blood returning from the veins that has very little pressure. When it contracts, it fills a second chamber with blood under a greater pressure. The second chamber is thus able to produce a stronger force. The third chamber contracts more strongly yet, and so on with each successive chamber.
Primitive vertebrates, if we are to make inferences from fish, had hearts with four successive chambers, the sinus venosus, atrium, ventricle, and conus arteriosus. The sinus venosus is simply the confluence of the major veins of the body. It pumps blood through a valve into the atrium. Pressure builds as the blood successively passes through the other chambers.
STRUCTURE AND FUNCTION OF THE HUMAN HEART
The human (and, more generally, mammalian) heart is an elaboration of this primitive vertebrate pattern. It has been reduced to two pumping chambers, atrium and ventricle, but each of these has been divided into right and left chambers so that there are two independent streams of blood passing through the heart at all times.
Blood from the veins of the body is collected into the superior and inferior venae cavae, which empty into the right atrium. The right atrium pumps into the right ventricle, enabling the right ventricle to pump blood more forcefully out of the heart. From the right ventricle, blood travels through the pulmonary trunk and arteries to the lungs to be oxygenated. Blood returns from the lungs via the pulmonary veins into the left atrium. From the left atrium it passes through the left ventricle and out the aorta to be distributed to body tissues.
These eight large arteries and veins entering and leaving the heart &emdash; superior and inferior venae cavae, four pulmonary veins, pulmonary trunk, and aorta &emdash; are termed the great vessels of the heart.
The heart is surrounded by a serous membrane derived from the peritoneum and called the pericardium. The pericardium forms its own closed, double-walled sac. It has a visceral layer, the epicardium, that adheres to the heart, and a parietal layer that anchors to adjacent organs. The two layers come together at the bases of the great vessels. Between them is a potential space, the pericardial cavity. A small amount of serous fluid lubricates the cavity to permit the heart to move as it beats.
The parietal layer of the pericardium is fused with the central tendon of the diaphragm below the heart around the inferior vena cava as it rises through its hiatus. This anchors the heart in place. On its right and left sides, the pericardium adheres to the pleural membranes of the lungs. Anteriorly it faces the sternum and ribcage; posteriorly the esophagus and aorta.
The wall of the heart consists of three tissue layers. The external layer, the epicardium, is the visceral layer of pericardium. The middle layer, the myocardium, consists of the cardiac muscle. The inner layer, or endocardium, is an epithelium that is continuous with the endothelium lining the blood vessels. Like the endothelium, it is smooth and tends to repel blood cells.
There is normally a layer of lipid between the epicardium and myocardium. This lipid is liquid at body temperature, but becomes solidified fat at room temperature. It is concentrated along the pathways of the coronary arteries and probably enhances their ability to stretch and contract.
The ventricles have much thicker and stronger walls than the atrium. The left ventricle is thicker and more rounded than the right. These proportions indicate the force of contraction needed to move the blood to its immediate destination. The left ventricle must work harder to propel blood throughout the body than the right does to move it to the lungs. Nonetheless, all of the chambers must move the same volume of blood in the same number of strokes.Thus their capacities are of comparable size, expelling approximately 70-90 ml. (two or three ounces) of blood with each contraction.
The fibers of the myocardium encircle the chambers of the heart so as to compress them. A single fiber will weave around both right and left chambers. This ensures that the two will contract simultaneously.
The atria are formed embryologically by the merging of the sinus venosus and the atrium. These two parts are distinguishable in the adult heart by the structure of their walls. The walls derived from the sinus are smooth and regular, resembling the lining of large veins. The true atrial walls have conspicuous muscular bands, called the pectinate muscles. The abrupt line along which the two parts fuse is called the crista terminalis.
The septum that divides the atria into right and left chambers formed later in the fetal period by growth inward from all sides of the chamber. The point of the last communication between the two sides is marked by a small depression, the fossa ovalis (see below).
The right atrium receives the superior and inferior venae cavae. These are large vessels, vertically aligned with one another and blending into the right wall of the chamber. A smaller opening into the atrium permits the flow of blood from the coronary sinus. The right atrium empties inferiorly into the right ventricle by way of the tricuspid valve.
The left atrium receives four pulmonary veins, two from each of the lungs. These enter the back of the heart running horizontally. Blood from the left atrium passes through the bicuspid valve into the left ventricle.
The ventricles have thicker and more muscular walls than the atria. The muscle fibers that form them encircle the chambers and terminate in pronounced ridges on the inner walls. These ridges are called trabeculae carnae and give the ventricles a much different appearance than the atria.
In the right ventricle, the three flaps of the tricuspid valve open into the ventricle. Each of the flaps is extended by a few fibers called the chordae tendiniae, which in turn are anchored to the wall or floor of the ventricle by muscular processes called thepapillary muscles. The papillary muscles are extensions of bundles of muscle fibers that make up the ventricular walls. As the ventricle contracts, so do the papillary muscles. At first glance, it might appear that they function to draw open the flaps of the valve. This is not the case, and it may be emphasized that opening and closing the valves is a passive operation. Because the muscles contract as the ventricle is contracting, they are working just as rising blood pressure in the ventricle is closing the valve. Their critical role is to provide tension on the chordae tendiniae to stabilize the flaps of the valve and prevent them from being swept upwards into the atrium.
In the left ventricle, a similar arrangement ensues. The atrioventricular valve on the left has two flaps. Hence it is called thebicuspid valve (or mitral valve after its fancied resemblance to a bishop's hat, or mitre). The flaps of the bicuspid valve also have chordae tendiniae and papillary muscles.
The passages out of the ventricles utilize a different set of valves. The base of the pulmonary trunk, emerging from the right ventricle, contains a pulmonary semilunar valve. This also has three flaps. Because the pressure experienced in the arteries between heart beats is so much less than that emerging from the heart, it is not necessary to strengthen them with papillary muscles. On the left side, the aorta is separated from the left ventricle by an aortic semilunar valve of similar construction.
One additional feature is unique to the right ventricle. The septomarginal band (formerly called the "moderator band") spans the chamber from its septum to its floor. This band appears to be one of the trabeculae of the wall, but it contains a highly specialized bundle of cardiac fibers conducting impulses to initiate contraction of the walls (see below).
The Intrinsic Conduction System
It is essential to human life that the heart contract in a regular rhythm, about 70 times per minute. The heart does not rely on the brain for this rhythm, but is self stimulating. Specialized cardiac muscle fibers have assumed the role of initiating, propagating, and synchronizing excitation across the walls of the heart.
The "pacemaker" of the heart where a heartbeat is initiated, is the sinuatrial node, a small clump of cells in the wall of the right atrium. These cells are self-exciting, the frequency of heartbeat determined by their intrinsic properties. The membrane activity they generate spreads from one cardiac cell to the next across the walls of the atria.
As the atria are contracting, the impulse is prevented from engulfing the ventricles by an insulating layer of connective tissue. Instead, the excitation is detected by the atrioventricular node, near the junction of the four chambers. After a calculated delay, the signal is passed on to the ventricular walls, dispersing on the right and left atrioventricular bundles. The delay ensures that the contractions of the atria and ventricles are sequential, rather than overlapping. The arrangement of cardiac fibers, coiling about both ventricles at once, assures that the right and left sides contract synchronously.
The rhythm of sinuatrial stimulation can be altered by the autonomic nervous system. Sympathetic fibers terminating in the wall of the heart and releasing the neurotransmitter norepinephrine increase the rate and force of contraction. The hormones norepinephrine and epinephrine secreted into the bloodstream by the suprarenal medulla during sympathetic arousal enhance this effect. Acetylcholine, released by parasympathetic fibers terminating in the heart, slows the heart rate.
EVOLUTION OF VERTEBRATE CIRCULATION
The evolutionary transition from the primitive vertebrate four-chambered heart to the modern mammalian four-chambered heart is elegantly described by comparative anatomy and reiterated in human embryology. So lucid is this model that we must remind ourselves that living vertebrates do not themselves form an evolutionary sequence, but represent only the end products of many evolutionary lineages.
Circulation through Gills
The four chambered heart of a fish described above may be understood as a primitive vertebrate form. Blood leaving the conus arteriosus enters a ventral aorta and runs anteriorly to the branchial arches. These arches are specialized for respiration as internal gills in modern fishes. The branchial arches contain arteries known as the aortic arches. Each of the aortic arches produces a rich capillary bed from which gases are exchanged with the passing water. The aortic arches are reformed and the oxygenated blood is collected into a dorsal aorta that runs posteriorly. Branches of the dorsal aorta supply the body and its visceral organs.
This circulatory system loses most of its blood pressure at the capillaries of the aortic arches. Blood runs through other body arteries because of fluid pressure and skeletal muscular contractions. Obviously the oxygen supply must be adequate for the needs of the fish, but the flow of arterial blood must be described as sluggish by mammalian standards. The evolution of the lung as a respiratory organ provided an opportunity to improve on this pattern.
Circulation through Lungs
The lung/swim bladder organ system that appears in bony fish is, like the branchial arches themselves, a derivative of the pharynx. Its blood supply, the pulmonary artery, is a branch of the sixth aortic arch. Blood draining from the primitive lung joins other venous blood in returning to the heart. As the lungs became increasingly important in respiration for fishes and amphibians, the pulmonary veins became more important as carriers of oxygen. Any alterations of circulation or heart structure that might keep this oxygenated blood from being diluted before it reaches other body tissues would increase the efficiency of oxygen supply to those tissues and would be evolutionarily favored. Changes in this direction appear in modern lungfish and amphibians.
Air-breathing terrestrial vertebrates should be able to by-pass the capillary beds of the aortic arches with much of their blood flow. This is accomplished in amphibians and all derived tetrapods by directing that flow primarily through the fourth aortic arch, as a direct link between the ventral and dorsal aortas. This shunt thus elevates the pressure in the dorsal aorta.
At the same time, lower tetrapods display modifications of the heart that prevent blending of oxygenated blood from the pulmonary veins with deoxygenated blood from the other veins of the body. Incomplete septa within the atria and ventricle and a flap of tissue within the conus arteriosus called the spiral fold direct parallel streams of venous blood through these chambers. Mixing of the streams is incomplete. More importantly, the two streams of blood are targeted separately. The more oxygenated blood is directed to the more anterior aortic arches, supplying the head and body via carotid arteries and dorsal aorta. Less oxygenated blood enters the fifth and sixth aortic arches to the lungs.
The design, as described, is clearly a successful evolutionary strategy, as it has been maintained for hundreds of millions of years. It has the advantage, among others, of varying the proportion of blood entering the lungs or passing directly to the body. For a fish that can carry on respiration alternately with gills or lungs, or for an aquatic amphibian or vertebrate that ceases to breathe for periods of time, this option is essential.
Complete separation of the two circuits occurs in birds and mammals. The mammalian pattern, of which humans are typical, reduces the aortic arches to one (the left fourth arch) and converts the sixth arch into the pulmonary arteries. The septa divide the atria and ventricles each into two chambers. The spiral fold of the conus arteriosus divides that into the entwined bases of the two great vessels, the pulmonary trunk and the ascending aorta. Birds follow an almost identical strategy except that the right fourth aortic arch is retained and the left one lost.
Human Ontogeny Recapitulating Phylogeny
The evolutionary sequence inferred from comparative anatomy is replayed in every human fetus. In the human embryo six pairs of branchial and aortic arches are created. The fate of each one can be traced into the adult form. The first two arches are lost. The termination of the ventral aorta persists as the internal carotid artery to the brain, while the third arch becomes the external carotid artery to the neck and face. Of the two fourth arches, the right one is lost (except as the base of the right subclavian artery) while the left arch persists. The fifth aortic arch is lost and the sixth becomes the pulmonary artery. The connection of the sixth arch with the dorsal aorta is retained until birth.
The Fetal Circulation Pattern
This developmental sequence accomodates the two different patterns of circulation that must serve mammals before and after birth. A fetus obtains its oxygen from the maternal bloodstream at the placenta. Its lungs are non-functional. Worse, the lungs are filled with fluid, which applies a pressure to pulmonary vessels and greatly increases the resistance to blood flow there. If the right side of the heart were strong enough to pump a full stream of blood through the lungs before birth, it would be hypertrophied and critically overpowered for its role after birth.
Fetal circulation resolves these problems with several structural modifications from the adult pattern. Oxygenated blood from the placenta enters the body via the umbilical vein. It passes through the liver via a shunt called the ductus venosus and enters the inferior vena cava. The inferior vena cava and superior vena cava both drain into the right atrium of the heart. However, the two streams do not completely mix. The parts of the atrial septum do not fuse and leave a passage, the foramen ovale, between the right and left atria. Oxygenated blood from the inferior vena cava is preferentially directed through the foramen ovale, while deoxygenated blood from the superior vena cava preferentially passes into the right ventricle. The transfer of blood from the right to the left atrium reduces the volume and pressure of blood pumped by the right ventricle.
The fetal pulmonary trunk remains connected to the aortic arch by the ductus arteriosus. This passage represents the primitive connection of the sixth aortic arch with the dorsal aorta. Because the lungs still have a high resistance to blood flow, the ductus arteriosus functions as a relief valve permitting even more blood to be diverted away from the lungs and into the aorta. Relatively little blood trickles through the lungs and returns to the left atrium; thus blood in the left side of the heart is predominantly that which was received from the inferior vena cava through the foramen ovale.
Note that the most highly oxygenated blood flows to the coronary arteries (to the heart tissue) and the carotid arteries (to the head), while other parts of the body are supplied with blood that has been diluted by the ductus arteriosus. Since other visceral systems of the fetus are not performing vital functions and the skeletal muscles do little work in the nearly weightless environment of the womb, the heart and brain are the only tissues that demand a large supply of oxygen.
Changes at Birth
At birth, the newborn circulation must immediately switch to an adult circulation pattern. The groundwork for this transformation has already been laid, and only the stimulus of birth itself is required to trigger the changes. The following changes happen almost simultaneously.
The vessels of the umbilical cord are emptied of their contents and collapse. The umbilical arteries constrict, permanently shutting off the flow of blood out of the fetus to the placenta. The now unneeded umbilical vein also collapses. Its remnants persist as the ligamentum teres, a strand of connective tissue running from the umbilicus to the liver on the inside of the abdominal wall.
Ductus venosus in the liver is constricted and closed. Its remnants persist as the ligamentum venosum between the right and left lobes of the liver. Before birth, the ductus venosus carried blood from both the umbilical vein and the hepatic portal vein to the inferior vena cava. The hepatic portal vein drained the non-functioning digestive tract. After birth, the portal vein carries newly absorbed nutrients. When the ductus closes, these are forced to pass through sinusoids of the liver for reprocessing and storage.
The lungs empty of fluid and resistance to blood flow drops. Ductus arteriosus constricts and all of the pulmonary blood is now directed into the lungs. Ductus arteriosus persists as a non-functioning connection between the pulmonary trunk and the aortic arch, the ligamentum arteriosum.
The increased flow of blood through the lungs causes an increase in the return of blood to the left atrium. As pressure rises in the left atrium, it equals the pressure in the right. The overlapping flaps of the septum are now pushed together to effectively close the foramen ovale. The final shunt of blood away from the lungs is now eliminated and the full volume of blood passes through the right ventricle and pulmonary trunk. A depression in the wall of the septum, the fossa ovalis, marks the site of the foramen ovale. In a substantial number of individuals, the parts of the septum never fuse completely, but are held together by the balanced pressure.
REGULATION OF THE CIRCULATORY SYSTEM
The circulatory system constantly moves blood about the body, but the needs of the body and of individual tissues are constantly changing. It is necessary for the body to be able to monitor its effectiveness and to adjust the flow of blood both for the body as a whole and for individual tissues.
Regulation of Pressure
Blood pressure is a measure of the effectiveness of the heart. Although absolute pressure differs in various parts of the body, pressure may be considered a property of the system as a whole because changes in pressure will register in all open blood vessels. Three factors are important in determining blood pressure: the force of the heartbeat, the volume of the blood, and the volume of the blood vessels. Each of these is subject to regulation.
Pressure of Ventricular Contraction
The heart is able to regulate the force of its contraction. This is effected by the autonomic nervous system. Sympathetic arousal and its associated hormones epinephrine and norepinephrine increase both the rate and force of contraction. Parasympathetic stimulation slows the heart and lessens its force.
Blood pressure rises and falls in a cyclic manner following the action of the heart. As the ventricle contracts, pressure leaving the blood peaks. This is called systolic pressure. The pressure wave advances through the arteries, but this takes time. Thus different arteries may experience systole (the moment of maximum pressure) at different times, depending on their distance from the heart. Diastolic pressure is a minimum pressure corresponding to the interval between ventricular contraction.
The pressure wave traveling through the arteries is dissipated by a number of factors that absorb its energy. The friction between blood and the walls of the arteries produces a resistance to flow that is very sensitive to vessel size and length. Resistance is proportionate to the inverse of the fourth power of the diameter. Thus resistance experiences a sudden increase whenever vessels diverge or when blood enters a narrow branch of an arterial trunk. The change in resistance is enough to reflect a small wave of pressure upstream, back to the heart. The elasticity of arterial walls also absorbs energy from the pressure wave. Vessel walls stretch at systole, momemtarily reducing the pressure. This energy is partly returned to the bloodstream during diastole as the walls rebound.
These factors reduce blood pressure to nearly zero in the capillaries and veins. The slow passage of blood in the capillaries is adaptive in that it facilitates diffusion of gases and nutrients. Movement of blood in the veins is accomplished primarily from external sources, such as the contraction of skeletal muscles in the limbs and wall of the trunk.
The elasticity of vessel walls provides an immediate feedback mechanism for resistance. When high pressure forces the vessel to stretch and increase its diameter, the small increase in diameter leads to a much greater drop in resistance and pressure and an increase in flow.
Blood Volume and Viscosity
The volume of blood is regulated primarily by long term mechanisms. Water retention in the body increases fluid volume. This may be adjusted by the mechanisms of thirst and urine production. Adjustments in the proportion of body sodium and potassium by the kidneys shifts the balance of fluid between intracellular and extracellular spaces. Albumin in the plasma can be used to regulate osmotic pressure and to draw fluid specifically into the blood vessels. These mechanisms may be regulated through hormones secreted by a number of endocrine glands, including the pituitary and suprarenal glands. The heart itself secretes a hormone, atrial natriuretic factor, to fine-tune volume and pressure.
The viscosity of the blood corresponds to its resistance to flow. A high viscosity requires more pressure to move blood. Changes in the albumin content or the hematocrit affect viscosity. Red blood cell count can be changed quickly by the release of more cells from storage in the spleen or more slowly by adjusting the rate of their manufacture.
Blood Vessel Volume
The volume of the blood vessels relative to the volume of blood helps to determine pressure. Blood pressure can be manipulated in the short term by constricting or dilating the walls of the arteries. These mechanisms depend on sensors that report blood pressure to the brain. Specialized pressure sensors called baroreceptors are strategically located in the walls of the atria and the aorta, and in the carotid bodies at the base of the internal carotid artery. These report to the vasomotor center in the medulla. The vasomotor center responds to low pressure by stimulating vasoconstriction along sympathetic pathways. High pressure in the atria cause a dilation of the arteries by an inhibition of their walls. The center is also sensitive to blood chemistry and may elevate pressure when carbon dioxide levels rise or oxygen levels drop.
Veins also have the capacity to adjust their volume. When their walls are relaxed, they may serve as reservoir for blood. Venous constriction takes up the slack in blood vessel volume, speeds up the passage time of blood in the venous system, and effectively places the sluggish venous blood back into circulation.
Longer term changes in vessel volume may be accomplished through hormones. A sudden drop in blood pressure, as from a loss of blood, results in the secretion of vasopressin (= anti-diuretic hormone) by the neurohypophysis. In addition to causing water retention in the kidneys, vasopressin increases the tone of the smooth muscle in vessel walls and elevates blood pressure. Another hormone, angiotensin released by the kidneys, is also a vasoconstrictor.
Additional changes in blood vessel volume may represent pathological changes, such as the clogging of arteries with plaque.
Regulation of Blood Flow
The ability to shunt blood to the tissues that need it most enables the circulatory system to provide for the body's requirements while conserving on the bulk of blood and the energy needed to move it through the tissues. More importantly, perhaps, the dynamic changes in circulatory patterns enables the body to respond in the shorter moments of extreme demand. This capacity requires the ability of the nervous system to anticipate changing demands or a mechanism by which the tissues communicate their need to the circulatory system. Both systems of controls exist.
The competition between the parasympathetic and sympathetic nervous systems to direct energy either to storage or to skeletal muscle is also played out in the circulatory system. During times of relaxation, the visceral systems are most active in maintaining the state of the body and in processing new energy stores. The parasympathetic dominates. The blood flow to the visceral organs is relatively abundant. Sympathetic arousal changes the priority of distributing energy in favor of skeletal muscle. Blood vessels to the viscera are constricted and an increased proportion of blood flows to the body wall and limbs.
Sympathetic arousal is a body-wide response and cannot identify the specific muscles that are being used. It is therefore desirable that individual tissues have the ability to signal their own needs for increased blood flow. The messenger of this signal is the molecule nitric oxide (NO). NO is synthesized by body tissues in need of oxygen and diffuses across cell membranes. It causes the smooth muscle of the arteriole walls and precapillary sphincters to relax and the vessels to dilate. As oxygen supply increases or need drops, the rate of synthesis of NO should decrease. This interplay of oxygen and NO represents a negative feedback system that can be specific to individual capillary beds and small areas of tissue.