Circulation

• Veins have thinner walls and contain valves to prevent the backflow of blood as it travels back to the heart.
• The circulation serves the needs of the body tissues by transporting nutrients to the tissues, removing waste products, transporting hormones, and maintaining an appropriate environment in all tissue fluids for survival and optimal function of the cells.
• In some organs, such as the kidneys, the circulation serves additional functions.
• Capillaries facilitate the exchange of materials between the blood and tissues.
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• Vessel diameter plays the greatest role of all factors in determining the rate of blood flow through a vessel.
• It may seem paradoxical that adding more blood vessels to a circuit reduces the total vascular resistance.
• Arterioles are the last small branches of the arterial system, acting as control conduits through which blood is released into the capillaries, with strong muscular walls that can close the arterioles completely or dilate the vessels severalfold.
• For blood vessels arranged in parallel, the total resistance to blood flow is expressed as follows: It is obvious that for a given pressure gradient, far greater amounts of blood will flow through this parallel system than through any of the individual blood vessels.
• About two thirds of the total systemic resistance to blood flow is resistance in the small arterioles.
• Ranges of blood flow of more than 100- fold in separate tissue areas have been recorded between the limits of maximum arteriolar constriction and maximum arteriolar dilation.
• The fourth power law makes it possible for the arterioles, responding with only small changes in diameter to nervous signals or local tissue chemical signals, either to turn off the blood flow to the tissue almost completely or, at the other extreme, to cause a vast increase in flow.
• Increasing the resistance of any of the blood vessels increases the total vascular resistance.
• Blood vessels branch extensively to form parallel circuits that supply blood to the many organs and tissues of the body.
• The total resistance is far less than the resistance of any single blood vessel.
• Blood pumped by the heart flows from the high- pressure part of the systemic circulation (i.e., aorta) to the low- pressure side (i.e., vena cava) through many miles of blood vessels arranged in series and in parallel.
• The parallel arrangement permits each tissue to regulate its own blood flow, to a great extent, independently of flow to other tissues.
• The internal diameters of the arterioles range from as little as 4 micrometers to as much as 25 micrometers.
• Flow through each of the parallel vessels in Figure 14-9 B is determined by the pressure gradient and its own resistance, not the resistance of the other parallel blood vessels.
• Blood flow through many tissues is controlled mainly in response to their need for nutrients and removal of waste products of metabolism.
• The right atrium receives deoxygenated blood from the body through the superior vena cava (SVC) and inferior vena cava (IVC).
• Blood flow to the kidney is far in excess of its metabolic requirements and is related to its excretory function, which requires that a large volume of blood be filtered each minute.
• The circulation is divided into the systemic circulation and the pulmonary circulation.
• The function of the arteries is to transport blood under high pressure to the tissues, with strong vascular walls and high velocity blood flow.
• Delayed compliance means that a vessel exposed to increased volume at first exhibits a large increase in pressure, but progressive delayed stretching of smooth muscle in the vessel wall allows the pressure to return toward normal over a period of minutes to hours.
• With each beat of the heart, a new surge of blood fills the arteries, but were it not for distensibility of the arterial system, all this new blood would have to flow through the peripheral blood vessels almost instantaneously, only during cardiac systole, and no flow would occur during diastole.
• The compliance of the arterial tree normally reduces the pressure pulsations to almost no pulsations by the time the blood reaches the capillaries, therefore, tissue blood flow is mainly continuous with very little pulsation.
• Sympathetic control of vascular capacitance is important during hemorrhage.
• At the lowest point of each pulse, called the diastolic pressure, it is about 80 mm Hg.
• Two major factors affect the pulse pressure: (1) the stroke volume output of the heart; and (2) the compliance (total distensibility) of the arterial tree.
• Vascular Distensibility and Functions of the Arterial and Venous Systems are discussed in Unit IV of The Circulation.