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CHAPTER

34

Circulation Through

Special Regions

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, and how these are regulated.

■

Describe how cerebrospinal fluid (CSF) is formed and reabsorbed, and its role in

protecting the brain from injury.

■

Understand how the blood–brain barrier impedes the entry of specific substances

into the brain.

■

Delineate how the oxygen needs of the contracting myocardium are met by the

coronary arteries and the 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 for a transition to independent life after birth.

INTRODUCTION

The distribution of the cardiac output to various parts of the

body at rest in a normal man is shown in Table 34–1. The

general principles described in preceding chapters apply to

the circulation of all these regions, but the vascular supplies of

many organs have additional special features that are impor-

tant to their physiology. The portal circulation of the anterior

pituitary is discussed in Chapter 24, the pulmonary circula-

tion in Chapter 35, the renal circulation in Chapter 38, and

the circulation of the splanchnic area, particularly the intes-

tines and liver, in Chapters 26 and 29. This chapter is con-

cerned with the special circulations of the brain, the heart,

and the skin, as well as the placenta and fetus.

CEREBRAL CIRCULATION:

ANATOMIC CONSIDERATIONS

VESSELS

The principal arterial inflow to the brain in humans is via four
arteries: two internal carotids and two vertebrals. In humans,
the carotid arteries are quantitatively the most significant. The
vertebral arteries unite to form the basilar artery, and the basi-
lar 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 ce-

rebral hemisphere on that side. Normally no crossing over oc-

curs, probably because the pressure is equal on both sides.

Even when it is not, the anastomotic channels in the circle do

not permit a very large flow. Occlusion of one carotid artery,

particularly in older patients, often causes serious symptoms

of cerebral ischemia. There are precapillary anastomoses be-

tween the cerebral vessels, but flow through these channels is

generally insufficient to maintain the circulation and prevent

infarction when a cerebral artery is occluded.

Venous drainage from the brain by way of the deep veins

and dural sinuses empties principally into the internal jugular

veins in humans, although a small amount of venous blood