When the left ventricle contracts, blood is sent out into the aorta under pressure.
Pulse
The surge of blood entering the arteries causes their elastic walls to swell, but then they almost immediately recoil. This alternate expanding and recoiling of an arterial wall can be felt as a pulse in any artery that runs close to the surface. The pulse is often checked by placing several fingers on the radial artery, which lies near the outer border of the palm side of the wrist near the thumb. The pulse can also be obtained at other locations. The pulse rate indicates the heartbeat rate and also gives information about the strength and rhythm of the heartbeat.
The pulse rate indicates the heartbeat rate.
Blood Pressure
Blood pressure is the force of blood against a blood vessel wall. Two aspects of blood pressure are considered: (1) how blood pressure is maintained in the arteries and arterioles, and (2) how blood pressure varies in other parts of the circulatory system.
Maintaining Blood Pressure
The two factors that affect blood pressure are cardiac output and peripheral resistance. The following factors can cause cardiac output and peripheral resistance to rise:
^ Cardiac output ^ Peripheral resistance
Heart rate increase Arterial Constriction
Blood volume increase
The pressoreceptors mentioned earlier regulate the heartbeat rate and also blood pressure. For example, when a person stands up quickly and blood pressure falls, the pressoreceptors signal a cardiac control center and a vasomotor control center in the medulla oblongata. Impulses conducted along sympathetic nerve fibers then cause heartbeat rate to increase and the arterioles to constrict to a greater degree. The end result is a rise in blood pressure.
Certain hormones also cause blood pressure to rise. Epinephrine and norepinephrine increase the heart rate. The renin-angiotensinaldosterone mechanism constricts arterioles and leads to an increase in blood volume when sodium and water are reabsorbed. ADH (antidiuretic hormone) released by the posterior pituitary also causes a rise in blood volume and therefore blood pressure.
What factors lead to a reduction in blood pressure? If blood pressure rises above normaL the pressoreceptors signal the cardiac control center and vasomotor control center in the medulla oblongata. Subsequently, the heart rate decreases and the arterioles dilate. In addition, recall that atrial natriuretic factor opposes the actions of aldosterone and ADH by decreasing blood pressure. Therefore, blood pressure is under the control of both the nervous system and the endocrine system.
Blood pressure is dependent on cardiac output and peripheral resistance. These are regulated by the activation of autonomic nerve impulses and by the release of certain hormones.
Blood Flovv in Arteries, Arterioles, and Capillaries
Blood pressure accounts for the movement of blood in the arteries and arterioles. Blood pressure decreases with distance from the left ventricle of the heart. Blood pressure is higher in the arteries than in the arterioles, and there is a sharp' drop in pressure when blood reaches the capillaries. This decrease in pressure may be correlated with the increase in the total cross-sectional area of the vessels; there are more arterioles than arteries, and many more capillaries than arterioles. Also, blood moves much slower in the capillaries than it does in the aorta, to allow time for the exchange of molecules between the blood and the tissues.
Blood Flovv in Veins and Venules Movement of blood in the veins is not due to blood pressure, but to skeletal muscle contraction. When skeletal muscles contract, they press against the weak walls of the veins, causing the blood to move past a valve. Once past the valve, blood is prevented from flowing backward. Blood flow gradually increases in the venous system, due to a progressive reduction in the cross-sectional area, as small venules join to form veins. Because blood pressure in the veins is low, the return of venous blood to the heart depends on several adaptations. As already mentioned, the squeezing action produced by skeletal muscle contraction and the presence of one-way valves move blood in veins. These adaptations, along with pressure differences between the thoracic and abdominal cavities, help return venous blood to the heart. As noted in chapter 14, when a person inhales, the diaphragm contracts, creating a partial vacuum in the thoracic cavity. Because pressure is higher in the abdominal cavity, the pressure difference causes blood to move from the abdominal to the thoracic veins.
Blood pressure steadily decreases vvith distance from the heart's left ventricle. Blood pressure causes the flovv of blood in the arteries and arterioles. Skeletal muscle contraction causes the flovv of blood in the venules and veins, and valves prevent backflovv of blood.
Hypertension
Blood pressure is usually measured in the brachial artery with a sphygmomanometer, an instrument that records changes in terms of millimeters of mercury. Normal resting blood pressure for a young adult is 120/80. The higher number is the systolic pressure, the pressure recorded in an artery when the left ventricle contracts, and the lower number is the diastolic pressure, the pressure recorded in an artery when the left ventricle relaxes. About 20% of all Americans are estimated to have hypertension (hi "perten'shun), or high blood pressure. Reasons for the development of hypertension are varied. One possible reason is associated with the kidney's secretion of renin. The renin-angiotensin-aldosterone system leads to absorption of sodium and high blood pressure. The same effect can be brought about directly by excess salt in the diet.
