Blood Flow and Vascular Distensibility Chapter 15

Vascular Distensibility and Functions of the Arterial and Venous Systems

• Vascular distensibility allows arteries to accommodate the pulsatile output of the heart and average out pressure pulsations, providing smooth, continuous flow of blood through small blood vessels of tissues.
• Veins are more distensible than arteries, allowing them to store large quantities of blood that can be called into use when needed.
• Vascular distensibility is normally expressed as Increase in volume / (Increase in pressure × Original volume).
• Vascular compliance (capacitance) is the total quantity of blood that can be stored in a given part of the circulation for each millimeter of mercury of pressure.

Factors Affecting Vascular Distensibility and Compliance

• The compliance of a vein in the systemic circulation is about 24 times as great as its corresponding artery because it is about eight times as distensible and has a volume that is three times as great.
• Sympathetic stimulation decreases vascular capacitance.
• The two most important factors that can increase pulse pressure are increased stroke volume and decreased arterial compliance.
• Decreased arterial compliance can result when the arteries "harden" with aging (arteriosclerosis).

Abnormal Pressure Pulse Contours

• With aortic valve stenosis, the aortic pulse pressure is greatly decreased because of diminished blood flow through the stenotic aortic valve.
• With patent ductus arteriosus, some of the blood pumped into the aorta flows immediately through the open ductus arteriosus into the pulmonary artery, allowing the diastolic pressure to fall very low before the next heartbeat, thereby increasing pulse pressure.
• With aortic regurgitation, the aortic valve is absent or functions poorly.

The Veins Function as Blood Reservoirs

• More than 60 percent of the blood in the circulatory system is usually contained in the veins.
• The venous system can serve as a blood reservoir for the circulation.
• When blood is lost from the body, activation of the sympathetic nervous system causes the veins to constrict, which takes up much of the "slack" of the circulatory system caused by the lost blood.

Mechanisms for Blood Flow Control in Specific Tissues

• The myogenic theory of autoregulation suggests that sudden stretch of small blood vessels causes the smooth muscles in the vessel walls to contract automatically.
• In the kidneys, blood flow is regulated, in part, via a mechanism called tubuloglomerular feedback.
• In the brain, the concentrations of carbon dioxide and hydrogen play prominent roles in local blood flow control.

Autonomic Control of the Circulation

• Sympathetic stimulation of the small arteries and arterioles increases the vascular resistance and decreases the rate of blood flow through the tissues.
• Innervation of large vessels, especially the veins, makes it possible for sympathetic stimulation to decrease the volume of the vessels.
• Parasympathetic stimulation decreases heart rate markedly and slightly decreases heart muscle contractility.

Control of the Sympathetic Vasoconstrictor System

• The sympathetic nerves carry large numbers of vasoconstrictor nerve fibers and only a few vasodilator fibers.
• The vasoconstrictor fibers are distributed to almost all segments of the circulation.
• Vasomotor centers of the brain control the sympathetic vasoconstrictor system.