Cutaneous Blood Flow

The skin is the outer covering of the body and as such serves as the first line of defense against invasion by disease-causing organisms. The skin, as the interface between the internal and external environments, also helps to maintain a constant deep-body temperature despite changes in the ambient (external) temperature—a process called thermoregulation. The thinness and large area of the skin (1.0 to 1.5 mm thick; 1.7 to

Test Yourself Before You Continue

1. Describe blood flow and oxygen delivery to the myocardium during systole and diastole.

2. State how blood flow to the heart is affected by exercise. Explain how blood flow to the heart is regulated at rest and during exercise.

3. Describe the mechanisms that produce vasodilation of the arterioles in skeletal muscles during exercise. Give two other reasons for the increased blood flow to muscles during exercise.

4. Explain how the stroke volume can increase during exercise despite the fact that the filling times are reduced at high cardiac rates.

428 Chapter Fourteen

428 Chapter Fourteen

Figure 14.21 Changing patterns of blood flow in the brain. A computerized picture of blood-flow distribution in the brain after injecting the carotid artery with a radioactive isotope. In (a), on the left, the subject followed a moving object with his eyes. High activity is seen over the occipital lobe of the brain. In (a), on the right, the subject listened to spoken words. Notice that the high activity is seen over the temporal lobe (the auditory cortex). In (b), on the left, the subject moved his fingers on the side of the body opposite to the cerebral hemisphere being studied. In (b), on the right, the subject counted to 20. High activity is seen over the mouth area of the motor cortex, the supplementary motor area, and the auditory cortex.

Figure 14.21 Changing patterns of blood flow in the brain. A computerized picture of blood-flow distribution in the brain after injecting the carotid artery with a radioactive isotope. In (a), on the left, the subject followed a moving object with his eyes. High activity is seen over the occipital lobe of the brain. In (a), on the right, the subject listened to spoken words. Notice that the high activity is seen over the temporal lobe (the auditory cortex). In (b), on the left, the subject moved his fingers on the side of the body opposite to the cerebral hemisphere being studied. In (b), on the right, the subject counted to 20. High activity is seen over the mouth area of the motor cortex, the supplementary motor area, and the auditory cortex.

1.8 square meters in surface area) make it an effective radiator of heat when the body temperature rises above the ambient temperature. The transfer of heat from the body to the external environment is aided by the flow of warm blood through capillary loops near the surface of the skin.

Blood flow through the skin is adjusted to maintain deep-body temperature at about 37° C (98.6° F). These adjustments are made by variations in the degree of constriction or dilation of ordinary arterioles and of unique arteriovenous anastomoses (fig. 14.22). These latter vessels, found predominantly in the fingertips, palms of the hands, toes, soles of the feet, ears, nose, and lips, shunt (divert) blood directly from arterioles to deep venules, thus bypassing superficial capillary loops. Both the ordinary arterioles and the arteriovenous anastomoses are innervated by sympathetic nerve fibers. When the ambient temperature is low, sympathetic nerves stimulate cutaneous vasoconstriction; cutaneous blood flow is thus decreased, so that less heat will be lost from the body. Since the arteriovenous anastomoses also constrict, the skin may appear rosy because the blood is diverted to the superficial capillary loops. In spite of this rosy appearance, however, the total cutaneous blood flow and rate of heat loss is lower than under usual conditions.

Skin can tolerate an extremely low blood flow in cold weather because its metabolic rate decreases when the ambient temperature decreases. In cold weather, therefore, the skin requires less blood. As a result of exposure to extreme cold, however, blood flow to the skin can be so severely restricted that

Epidermis

Epidermis

■ Figure 14.22 Circulation in the skin showing arteriovenous anastomoses. These vessels function as shunts, allowing blood to be diverted directly from the arteriole to the venule, and thus to bypass superficial capillary loops.

Cardiac Output, Blood Flow, and Blood Pressure the tissue dies—a condition known as frostbite. Blood flow to the skin can vary from less than 20 ml per minute at maximal vasoconstriction to as much as 3 to 4 L per minute at maximal vasodilation.

As the temperature warms, cutaneous arterioles in the hands and feet dilate as a result of decreased sympathetic nerve activity. Continued warming causes dilation of arterioles in other areas of the skin. If the resulting increase in cutaneous blood flow is not sufficient to cool the body, sweat gland secretion may be stimulated. Perspiration helps to cool the body as it evaporates from the surface of the skin. The sweat gland also secrete bradykinin, a polypeptide that stimulates vasodilation.

Under the usual conditions of ambient temperature, the cutaneous vascular resistance is high and the blood flow is low when a person is not exercising. In the pre-exercise state of fight or flight, sympathetic nerve activity reduces cutaneous blood flow still further. During exercise, however, the need to maintain a deep-body temperature takes precedence over the need to maintain an adequate systemic blood pressure. As the body temperature rises during exercise, vasodilation in cutaneous vessels occurs together with vasodilation in the exercising muscles. This can produce an even greater lowering of total peripheral resistance. If exercise is performed in hot and humid weather, and if restrictive clothing increases skin temperature and cutaneous va-sodilation, a dangerously low blood pressure may be produced after exercise has ceased and the cardiac output has declined. People have lost consciousness and have even died as a result.

Changes in cutaneous blood flow occur as a result of changes in sympathetic nerve activity. Since the activity of the sympathetic nervous system is controlled by the brain, emotional states, acting through control centers in the medulla ob-longata, can affect sympathetic activity and cutaneous blood flow. During fear reactions, for example, vasoconstriction in the skin, along with activation of the sweat glands, can produce a pallor and a "cold sweat." Other emotions may cause vasodila-tion and blushing.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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