Peripheral circulation arterial system

Animal Physiology for Msc And BS Zoology students.

1. THE PERIPHERAL CIRCULATION Arterial System Abuzar Tabassum

2. Peripheral Circulatory System Systemic vessels Transport blood through most all body parts from left ventricle and back to right atrium. Pulmonary vessels Transport blood from right ventricle through lungs and back to left atrium.

3. THE PERIPHERAL CIRCULATION Blood pumped from the left ventricle of the mammalian heart carries oxygenated blood via the arterial system to capillary beds in the tissues, where the oxygen is exchanged for carbon dioxide. The venous system returns the deoxygenated blood to the right atrium. Although all blood vessels share some structural features, the vessels in various parts of the peripheral circulation are adapted for the functions they serve.

4. S tructure of Blood Vessels A layer of endothelial cells, called the endothelium, lines the lumen of all blood vessels. In larger vessels, the endothelium is surrounded by a layer of elastic and collagenous fibers. the walls of capillaries consist of a single layer of endothelial cells. Circular and longitudinal smooth muscle fibers may intermingle with or surround the elastic and collagenous fibers.

5. structure of Blood Vessels The walls of larger blood vessels comprise three layers : Tunica adventitia: the limiting fibrous outer coat Tunica media: middle layer consisting of circular and longitudinal muscle . Tunica intima: inner layer, closest to the lumen, composed of endothelial cells and elastic fibers .

6. structure of Blood Vessels The boundary between the tunica intima and the tunica media is not well defined ; the tissues blend into one another. Owing to increased muscularization , arteries have a thickened tunica media T he larger arteries close to the heart are more elastic , with a wide tunica intima. The thick walls of larger blood vessels require their own capillary circulation , termed the vasa vasorum . In general, arteries have thicker walls and much more smooth muscle than veins of similar outside diameter. In some veins, muscular tissue is absent.

7. Arterial System The arterial system consists of a series of branching vessels with walls that are thick, elastic, and muscular-well suited to deliver blood from the heart to the fine capillaries that carry blood through the tissues.

8. Functions of Arteries Arteries serve four main functions: Act as a channel for blood between the heart and capillaries Act as a pressure reservoir for forcing blood into the small-diameter arterioles Dampen oscillations in pressure and flow generated by the heart and produce a more even flow of blood into the capillaries Control distribution of blood to different capillary networks via selective constriction of the terminal branches of the arterial tree

9. Arterial blood pressure is determined by: the volume of blood in the arterial system and the properties of its walls . If either is altered, the pressure will change. The volume of blood in the arteries is determined by the rate of filling via cardiac contractions and of emptying via arterioles into capillaries. If cardiac output increases, arterial blood pressure will rise. I f capillary flow increases, arterial blood pressure will fall. Normally, however, arterial blood pressure varies little, because the rates of filling and emptying are evenly matched (i.e., cardiac output and capillary flow are evenly matched).

10. Blood flow through the capillaries is proportional to the pressure difference between the arterial and venous systems. Because venous pressure is low and changes little, arterial pressure exerts primary control over the rate of capillary blood flow and is responsible for maintaining adequate perfusion of the tissues. Arterial pressure varies among species, generally ranging from 50 to 150 mm Hg. Pressure differences are small along large arteries (less than 1 mm Hg). B ut pressure drops considerably along small arteries and arterioles because of increasing resistance to flow with decreasing vessel diameters.

11. The oscillations in blood pressure and flow generated by contractions of the heart are dampened in the arterial system, because of the elasticity of arterial walls. As blood is ejected into the arterial system, pressure rises and the vessels expand. Although elastic, arteries become progressively stiffer with increasing distension. As a result, they are easily distended at low pressures, but then resist further expansion at high pressures.

12. Elastic vessels are unstable and tend to balloon; that is, since they cannot develop high wall tension as pressure increases, they tend to bulge out. In blood vessels, this instability is prevented by a collagen sheath that limits their expansion. Ballooning of a blood vessel (aneurism) can occur, however, if the collagen sheath breaks down.

13. Blood pressure Blood pressures reported for the arterial system are usually transmural pressures (i.e., the difference in pressure between the inside and outside, across the wall of the blood vessel). The pressure outside vessels is usually close to ambient, but changes in the extracellular pressure of tissues can have a marked effect on transmural pressure and therefore on vessel diameter and consequently blood flow. For example, contractions of the heart raise pressure around coronary vessels and result in a marked reduction in coronary flow during systole.

14. During a heartbeat cycle, the maximum arterial pressure is referred to as systolic pressure and the minimum as diastolic pressure; the difference is the pressure pulse . The pressure pulse travels at a velocity of 3-5 m.s -l . The velocity of the pressure pulse increases with decrease in artery diameter and increasing stiffness of the arterial wall. In the mammalian aorta, the pressure pulse travels at 3-5 m.s -1 and reaches 15-35 m.s-1 in small arteries. Transmural pressures are typically given in millimeters of mercury B oth the systolic and diastolic pressures generally are indicated with a slash between them (e.g., 120/80 mmHg).

15. Effect of gravity and body position on pressure and flow When a person is lying down, the heart is at the same level as the feet and head, and pressures will be similar in arteries in the head, chest, and limbs. Once a person moves to a sitting or standing position, the relationship between the head, heart, and limbs changes with respect to gravity, and the heart is now a meter above the lower limbs. The result is an increase in arterial pressure in the lower limbs and a decrease in arterial pressure in the head. The height of the column of blood simply results in a higher blood pressure due to gravity.

16. Gravity has little effect on capillary flow, which is determined by the arterial-venous pressure difference . That is, gravity raises arterial and venous pressure by the same amount and therefore does not greatly affect the pressure gradient across a capillary bed .

17. Velocity of arterial blood flow Blood flow and the oscillations in flow with each heartbeat are greatest at the exit to the ventricle, decreasing with increasing distance from the heart. At the base of the aorta flow is turbulent and reverses during diastole as closure of the aortic valves creates whirlpools in the blood ejected into the aorta during systole. In most other parts of the circulation, flow is laminar and oscillations in velocity are damped by the compliance of the aorta and proximal arteries.

18. Mean velocity in the aorta, the point of maximal blood velocity, is calculated as about 33 cm/sec in humans (based on a cross-sectional area of about 2.5 cm2 and cardiac output of about 5 L/min). If we assume that maximal velocity in a vessel is twice the mean velocity then the maximal velocity of blood flow in the human aorta would be 66 cm/s. If cardiac output is increased by a factor of 6 during heavy exercise, maximal velocity is raised to 3.96 m/s. In contrast, the pressure pulse associated with each heartbeat travels through the circulation at 3-35 m/s; thus, the pressure pulse travels faster than the flow pulse .