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OBJECTIVES
After studying this chapter, you should be able to:
Define the special features of the circulation in the brain, coronary vessels, skin, and fetus.
Describe how cerebrospinal fluid (CSF) is formed and reabsorbed, and its role in protecting the brain from injury.
Understand the function of the blood–brain barrier.
Delineate how the oxygen needs of the contracting myocardium are met by the coronary arteries, and consequences of their occlusion.
List the vascular reactions of the skin and the reflexes that mediate them.
Understand how the fetus is supplied with oxygen and nutrients in utero, and the circulatory events required at birth.
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The vascular supplies of many organs have special features that are important to their physiology. The portal circulation of the anterior pituitary is discussed in Chapter 18; the pulmonary circulation in Chapter 34; the renal circulation in Chapter 37; and the circulation of the splanchnic area, particularly the intestines and liver, in Chapters 25 and 28. This chapter is concerned with the special circulations of the brain, the heart, and the skin, as well as the placenta and fetus.
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CEREBRAL CIRCULATION: ANATOMIC CONSIDERATIONS
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The principal arterial inflow to the brain in humans is via two internal carotid arteries and two vertebral arteries, with the carotid arteries quantitatively the most significant. The vertebral arteries unite to form the basilar artery, and the basilar artery and the carotids form the circle of Willis below the hypothalamus. The circle of Willis is the origin of the six large vessels supplying the cerebral cortex. Substances injected into one carotid artery are distributed almost exclusively to the cerebral hemisphere on that side. Occlusion of one carotid artery, particularly in older patients, often causes serious symptoms of cerebral ischemia. Venous drainage from the brain by way of the deep veins and dural sinuses empties principally into the internal jugular veins.
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The cerebral vessels have a number of unique anatomic features. In the choroid plexuses, there are gaps between the capillary endothelial cells, but the choroid epithelial cells that separate them from the cerebrospinal fluid (CSF) are connected to one another by tight junctions. The capillaries in the brain substance resemble nonfenestrated capillaries in muscle, but tight junctions between the endothelial cells limit the passage of substances via the paracellular route. In addition, there is little vesicular transport. However, multiple transport systems are present. The brain capillaries are also surrounded by the endfeet of astrocytes that are closely applied to the basal lamina of the capillaries, but with gaps of about 20 nm between endfeet.
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Three systems of nerves innervate the cerebral blood vessels. Postganglionic sympathetic neurons have their cell bodies in the superior cervical ganglia, and their endings contain norepinephrine. Many also contain neuropeptide Y. Cholinergic ...