The Physiology of Circulation

Circulating blood might be compared to a train that travels around the country, picking up and delivering passengers at each stop on its route. For example, as blood flows through capillaries surrounding the air sacs in the lungs, it picks up oxygen and unloads carbon dioxide. Later, when this oxygenated blood is pumped to capillaries in other parts of the body, it unloads the oxygen and picks up carbon dioxide and other substances generated by the cells (Fig. 11-11). The microscopic capillaries are of fundamental importance in these activities. It is only through and between the cells of these thin-walled vessels that the necessary exchanges can occur. All living cells are immersed in a slightly salty liquid called tissue fluid, or interstitial fluid. Looking again at Figure 11-11, one can see how this fluid serves as “middleman” between the capillary membrane and the neighboring cells. As water, oxygen, and other necessary cellular materials pass through the capillary walls, they enter the tissue fluid. Then, these substances make their way by diffusion to the cells. At the same time, carbon dioxide and other end products of metabolism leave the cells and move in the opposite direction. These substances enter the capillaries and are carried away in the bloodstream for processing in other organs or elimination from the body.
Figure 11-11 Connection between small blood vessels through capillaries. The blood delivers oxygen (O2) to the tissues and picks up carbon dioxide (CO2) for transport to the lungs. Note the lymphatic capillaries, which aid in tissue drainage.
Connection between small blood vessels through capillaries

Capillary Exchange

Diffusion is the main process by which substances move between the cells and the capillary blood. Recall that diffusion is the movement of a substance from an area where it is in higher concentration to an area where it is in lower concentration. Diffusion does not require transporters or cellular energy. An additional force that moves materials from the blood into the tissues is the pressure of the blood as it flows through the capillaries. Blood pressure is the force that filters, or “pushes” water and dissolved materials out of the capillary into the tissue fluid. Fluid is drawn back into the capillary by osmotic pressure, the “pulling force” of substances dissolved and suspended in the blood. Osmotic pressure is maintained by plasma proteins (mainly albumin), which are too large to go through the capillary wall. These processes result in the constant exchange of fluids across the capillary wall. The movement of blood through the capillaries is relatively slow, owing to the much larger cross-sectional area of the capillaries compared with that of the vessels from which they branch. This slow progress through the capillaries allows time for exchanges to occur. Note that even when the capillary exchange process is most efficient, some water is left behind in the tissues. Also, some proteins escape from the capillaries into the tissues. The lymphatic system, collects this extra fluid and protein and returns them to the circulation (see Fig. 11-11).

The Dynamics of Blood Flow
Blood flow is carefully regulated to supply tissue needs without unnecessary burden on the heart. Some organs, such as the brain, liver, and kidneys, require large quantities of blood even at rest.
The requirements of some tissues, such as the skeletal muscles and digestive organs, increase greatly during periods of activity. For example, the blood flow in muscle can increase 25 times during exercise. The volume of blood flowing to a particular organ can be regulated by changing the size of the blood vessels supplying that organ. An increase in a blood vessel’s diameter is called vasodilation. This change allows for the delivery of more blood to an area. Vasoconstriction is a decrease in a blood vessel’s diameter, causing a decrease in blood flow. These vasomotor activities result from the contraction or relaxation of smooth muscle in the walls of the blood vessels, mainly the arterioles. A vasomotor center in the medulla of the brain stem regulates vasomotor activities, sending its messages through the autonomic nervous system. Blood flow into an individual capillary is regulated by a precapillary sphincter of smooth muscle that encircles the entrance to the capillary (see Fig. 11-7). This sphincter widens to allow more blood to enter when tissues need more oxygen.

Return of Blood to the Heart Blood leaving the capillary networks returns in the venous system to the heart, and even picks up some speed along the way, despite factors that work against its return. Blood flows in a closed system and must continually move forward as the heart contracts. However, by the time blood arrives in the veins, little force remains from the heart’s pumping action. Also, because the veins expand easily under pressure, blood tends to pool in the veins. Considerable amounts of blood are normally stored in these vessels. Finally, the force of gravity works against upward flow from regions below the heart. Several mechanisms help to overcome these forces and promote blood’s return to the heart in the venous system. These are:
* Contraction of skeletal muscles. As skeletal muscles contract, they compress the veins and squeeze blood forward (Fig. 11-12).
* Valves in the veins prevent back flow and keep blood flowing toward the heart.
* Breathing. Pressure changes in the abdominal and thoracic cavities during breathing also promote blood return in the venous system. During inhalation, the diaphragm flattens and puts pressure on the large abdominal veins. At the same time, chest expansion causes pressure to drop in the thorax. Together, these actions serve to both push and pull blood through these cavities and return it to the heart.
     As evidence of these effects, if a person stands completely motionless, especially on a hot day when the superficial vessels dilate, enough blood can accumulate in the lower extremities to cause fainting from insufficient oxygen to the brain.
Role of skeletal muscles and valves in blood return
Figure 11-12 Role of skeletal muscles and valves in blood return. (A) Contracting skeletal muscle compresses the vein and drives blood forward, opening the proximal valve, while the distal valve closes to prevent backflow of blood. (B) When the muscle relaxes again, the distal valve opens, and the proximal valve closes until blood moving in the vein forces it open again.
The Pulse
The ventricles regularly pump blood into the arteries about 70 to 80 times a minute. The force of ventricular contraction starts a wave of increased pressure that begins at the heart and travels along the arteries. This wave, called the pulse, can be felt in any artery that is relatively close to the surface, particularly if the vessel can be pressed down against a bone.
At the wrist, the radial artery passes over the bone on the forearm’s thumb side, and the pulse is most commonly obtained here. Other vessels sometimes used for taking the pulse are the carotid artery in the neck and the dorsalis pedis on the top of the foot. Normally, the pulse rate is the same as the heart rate, but if a heartbeat is abnormally weak, or if the artery is obstructed, the beat may not be detected as a pulse. In checking another person’s pulse, it is important to use your second or third finger. If you use your thumb, you may find that you are getting your own pulse. When taking a pulse, it is important to gauge the strength as well as the regularity and rate.
Pulse Rate Various factors may influence the pulse rate:
* The pulse is somewhat faster in small people than in large people and usually is slightly faster in women than in men.
* In a newborn infant, the rate may be from 120 to 140 beats/minute. As the child grows, the rate tends to become slower.
* Muscular activity influences the pulse rate. During sleep, the pulse may slow down to 60 beats/minute, whereas during strenuous exercise, the rate may go up to well over 100 beats/minute. For a person in good condition, the pulse does not go up as rapidly as it does in an inactive person, and it returns to a resting rate more quickly after exercise.
* Emotional disturbances may increase the pulse rate.
* In many infections, the pulse rate increases with the increase in temperature.
* An excessive amount of secretion from the thyroid gland may cause a rapid pulse.
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