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The peripheral vascular system includes all of the circulatory system outside of the heart. It is divided into several subsystems:
The structure of each part of the peripheral system changes as the location and function change. Other than capillaries, blood vessels consist of three layers: the outer layer, or tunica adventitia, is made up of connective tissue giving support and shape to the vessel; the middle layer, or tunica media, is composed of elastic and muscular tissue regulating the vessel's diameter; and the inner layer, or tunica intimae, is an endothelial layer providing a smooth pathway for blood flow. Contained within the layers are varying amounts of collagen and muscle fibers.
The major responsibility of the arteries is blood transport to nourish tissue. This is carried out in a high pressure system. The walls of major arteries, such as the pulmonary artery and the aorta, are thick and composed of a great deal of elastic tissue and less of smooth muscle. The large arteries are called elastic vessels because they can increase their diameters to accept blood volume and contract to normal size, pushing blood forward. Branches of the elastic arteries are called nutrient arteries. They supply nutrients and oxygen to tissues and organs. These smaller arteries are composed of more smooth muscle and less elastic tissue.
Elasticity is needed in the aorta to handle the blood forced from the left ventricle during systole. In diastole, blood is forced forward through the circulatory system by the rebound action of the elastic walls. The diameter of arterioles, veins, and arteries is controlled by the autonomic nervous system. Blood flow is partially controlled by the blood vessel diameter.
Velocity of blood flow through the arteries is reduced peripherally as the system divides into smaller branches. About 15% of total blood volume is contained in the arterial bed at any one time, making it a low volume/high pressure system.
Blood flow to tissues continues from the small arteries to the arterioles. The arterioles' function is to control the flow of blood to the capillary bed. The walls of the arterioles are composed principally of smooth muscle. Contraction and dilatation of this smooth muscle wall is regulated by enervation of the autonomic nervous system, which controls the diameter of the lumen of the arteriole, responding to tissue needs and blood volume. Additional control is supplied by alterations in the diameter of the precapillary sphincters that constrict and relax with autonomic stimulation or with changes in oxygen level, pH, or temperature.
Systemic vascular resistance (SVR), the opposing force to blood flow in vessels, is created primarily by the diameter of the vessels into which blood is being forced. Because of the small amount of elastic tissue in arterioles, they become very important in the determination of systemic vascular resistance. Small changes in the lumen diameter vastly influence tissue perfusion. Decreased arteriolar diameter creates a rise in SVR and a drop in blood flow. Arterioles are quite small in length, with diameters normally varying beween 8 and 50 um. The arterioles divide into metarterioles to perfuse the capillaries.
Capillary walls are thin, semipermeable membranes composed of a single endothelial layer. Capillaries respond to the condition of the tissues. Nutrient and metabolite exchange occurs through diffusion across the membrane. Oxygen and nutrients enter the interstitial fluid and cells as carbon dioxide, and metabolic waste products exit. The arteriolar lumen regulates the blood flow through the capillaries. Tissue needs are met by channels between capillaries, metarterioles, and venules. Microcirculation (the capillary unit) is composed of microscopic arterioles, precapillaries, true capillaries, venules, and small veins.5, 24
The smallest veins, the venules, receive blood from the capillaries. The function of these smallest parts of the circulatory system is to act as collecting tubules. The thin walls of the venules make possible the exchange of some nutrients and oxygen for waste products. Capillaries and venules are called exchange vessels. Postcapillary sphincters are situated at the juncture between the capillary and the venule. The venule walls are composed of weak muscle.
Venules join together to form veins. Like arteries, the veins are composed of three layers. Pressure in the arterial system far exceeds that in the venous system. This permits a relatively thin-walled, less elastic system to transport blood from the capillary bed to the right atrium. The venous system is referred to as a capacitance system because it can accommodate a large volume of blood with relatively low pressure. Approximately 75% of circulating blood volume is contained in the venous system at any time. Veins are structured with one-way valves for unidirectional forward blood flow. There are actually two sets of valves: The first carries blood from the superficial veins to the more effective pumping action of the deep veins, and the second set in the deep veins maintains the unidirectional flow during muscular pumping. This musculovenous pump is augmented by another pressure gradient for venous return—the abdominal-thoracic pump. Respiration changes the pressure gradients of thorax and abdomen, aiding the return of blood from the abdomen into the thorax and into the right heart. The gradient is more pronounced with deep inspiration but continues throughout the respiratory cycle.
The role of the cardiovascular system is maintenance of homeostasis at the cellular level. The distrition of cardiac output to each part of the body is carefully regulated to meet demand. The pattern of peripheral blood flow is an extremely important element in this process. The vascular bed determines volume and distribution of the cardiac output. The precapillary sphincters and metarterioles have the property of vasomotion. Vasomotion, an alternating vasodilatation and vasoconstriction, is thought to be related to intrinsic factors that influence local blood distribution. The capillary vascular bed is autoregulated in the peripheral vasculature. This means that local capillary flow is constant, regardless of varying perfusion pressures. The pressures are termed hydrostatic and oncotic.4, 5, 24 Blood entering the capillaries comes in when hydrostatic pressure is generated by the heart; it is higher at the arteriolar end, lower at the venule end. The hydrostatic pressure is basically the blood pressure. It pushes blood away. This is called outward force, and it moves fluid to the interstitial spaces. Oncotic pressure (colloidal osmotic pressure) draws fluid. The plasma proteins, the primary one being albumin, generate the oncotic pressure. This is termed inward force; it draws water toward it. Oxygen and nutrients are extracted for the tissues, and waste products are picked up in the capillaries. The filtration process is dependent on the capillary dynamics of hydrostatic and oncotic pressures. Filtration pressure is equal to hydrostatic pressure minus oncotic pressure. Normally, filtration pressure is positive. It means that the nutrient-carrying fluid is pushed out of the vascular space into the interstitial space at the capillaries while fluid containing waste products is pulled into the vascular space from the interstitial space. Edema results when this filtration process is not in proper working order.
Blood pressure is the force needed to propel blood through the arterial system. It is usually measured by its ability to displace mercury to a given height in millimeters (mm/Hg). A systolic pressure of 100 means that the pressure in the artery would force a column of mercury to a height of 100 mm. Cardiac output and peripheral resistance are the mechanisms for determining systolic and diastolic pressures. Systolic pressure occurs near the end of the contractile phase of the cardiac cycle. It is the maximum arterial pressure. Systolic pressure is governed by the elasticity of the aorta, the speed with which blood is ejected, and the amount of blood ejected. The systolic pressure will rise if there is a very rapid ejection of a large amount of blood into an aorta that is rigid due to atherosclerotic disease. Arterial pressure rises in the aorta during systole and varies with location in the system. Normally, systolic pressure in the leg (femoral artery) is equal to or greater than the pressure in the arm (brachial). There are problems that can reverse this reading. Hypertension resulting from coarctation of the aorta produces a systolic leg pressure less than that of the arm.
Diastolic pressure occurs at the end of the relaxation phase of the cardiac cycle. It is the minimum arterial pressure. Diastolic pressure is a reflection of the condition of the arteries. Blood is forced into the aorta during systole, creating stretch. Recoil follows, forcing blood toward the periphery. The elasticity of the peripheral arterial system is measured by the diastolic pressure. Diastolic pressure determinates are heart rate and peripheral resistance.
Pulse pressure is the difference between systolic and diastolic pressure. It is a reflection of stroke volume because diastolic pressure remains relatively consistent. Hypovolemic shock, with a decreased pulse pressure, demonstrates this. The increase in peripheral resistance keeps diastolic pressure constant as systolic pressure falls due to a reduced stroke volume. One factor influencing pulse pressure is a slowed heart rate. This rate allows time for blood to flow from the aorta to the periphery before the next beat occurs. Diastolic pressure is influenced by this added time. Stroke volume influences the systolic pressure. Any decline in stroke volume, such as occurs in hypovolemia, reduces systolic pressure. When peripheral resistance is high, more blood remains in the aorta, raising both systolic and diastolic pressures. Any of these problems may influence pulse pressure.16
Blood pressure is measured with cuff and stethoscope by collapsing an artery (brachial) in the arm and then slowly reducing pressure to the occluded artery while listening for sound. When pressure in the artery is higher than in surrounding tissue, which is still compressed, blood flow begins, and sounds are produced. Called Korotkoff sounds, the first Korotkoff sound is the systolic pressure. The sounds change as the cuff is deflated until turbulent flow is no longer produced, and sound disappears. The disappearance is the diastolic pressure. Occasionally a muffled or changed sound is heard just before the sound disappears. If all three sounds are heard, they should be recorded as 130/70/60. Muffled sound may indicate aortic insufficiency.
Mean arterial blood pressure is the average pressure required to pump blood through the arterial system. Mean arterial pressure determines tissue blood flow. It is a better indicator of the state of peripheral vasculature health than is the diastolic measurement. The formula for determining mean arterial pressure (MAP):24
MAP = diastolic pressure = pulse pressure/3 or
MAP = D = ⅓ systolic minus diastolic.
The controlling influence for blood pressure is a negative feedback system that acts to keep the pressure at a constant level. Nerve tissues sensitive to stretch are found both in the walls of internal carotid arteries near the bifurcation from the common carotids and in the wall of the arch of the ascending aorta. These receptors are sensitive to mechanical stretch or pressure and are referred to as aortic and carotid mechanoreceptors, pressoreceptors, and baroreceptors. Increases in nerve activity, responding to stretch or pressure elevation, transmit messages via the glossopharyngeal (9th cranial) nerve from the carotid or via the vagus (10th cranial) nerve from the aorta to cardiovascular areas of the brain stem. Response is with increased parasympathetic activity and decreased sympathetic activity, which decreases heart rate and vaso- and venoconstriction. Blood pressure returns to normal as cardiac output drops and SVR is decreased. Conversely, when the baroreceptors sense falling blood pressure, there is increased sympathetic activity to increase heart rate and vaso- and venoconstriction.
Blood pressure regulation through the renin-angiotensin-aldosterone system responds to changes in the volume of body fluid. Rerun, an enzyme, is produced by the kidneys (see Fig. 3.4) in response to a decrease in renal blood flow. Renin combines with a plasma protein, antiotensinogen, to form angiotensin I. In the lung, angiotensin I is activated to become angiotensin II, a potent vasoconstrictor and an agent for feedback regulation to decrease the manufacture of renin. The increase in sodium levels is thought to enhance the activity of angiotensin II. Aldosterone release from the adrenal cortex is stimulated by angiotensin II. Sodium reabsorption is increased by aldosterone, which in turn increases water retention and vascular volume.
The ultime control of blood pressure in response to physiologic or psychologic stress is the autonomic nervous system. The parasympathetic system responds with vagus nerve slowing of heart rate. The sympathetic system is more diverse, controlling the diameter of the lumen of arterioles, constricting the large veins, and increasing force and rate of ventricular contractions. These activities in turn increase venous return and cardiac output. The metabolic needs of each organ dictate the distribution of blood flow. As perfusion needs change, blood flow is altered. With increased metabolism, blood flow is increased to deliver oxygen and nutrition and remove waste products. The peripheral vascular system is uniquely equipped to carry out this function.