Chapter 20

### The Systemic Circulation

The systemic circulation supplies arterial blood to the tissues; it begins at the aortic valve and ends with the openings of the venae cavae into the right atrium. Its component vessels and their function may be described as follows:

Elastic arteries—the aorta and its major branches—convert the spasmodic left ventricular output into a more continuous distal flow.

Muscular arteries—the internal carotid, coronary, brachial, femoral, renal and mesenteric arteries—distribute blood to the tissues.

Arterioles—by definition, arteries less than 2 mm in diameter—have muscular walls and a rich sympathetic nerve supply that permits adjustment of luminal size. Arterioles regulate the pressure decrease from aortic to capillary levels (Figure 20-1). Adjustment of resistance within the arterioles is a major factor determining systemic blood pressure and distribution of flow.

###### Figure 20–1.

Blood pressures (measured in mm Hg) in the pulmonary and systemic circulations.

The microcirculation consists of capillaries, precapillary sphincters, and postcapillary venules. It is the site of exchange with tissue fluids.

Veins are low pressure capacitance vessels that return blood to the heart. Forward flow in the veins is facilitated by endothelial valves.

### The Pulmonary Circulation

The main function of the pulmonary circulation is to effect respiratory gas exchange in the pulmonary capillary bed; it begins at the pulmonary valve and ends in the left atrial openings.

The pulmonary circulation is at low pressure (25/10 mm Hg). Because this is lower than the plasma osmotic pressure, there is normally no fluid movement out of the alveolar capillaries, permitting the alveoli to remain dry for effective gas exchange.

### The Portal Circulations

Portal circulations within the systemic circulation interpose a second capillary bed, which enables a specific function by the involved tissues. The hepatic portal circulation delivers intestinal and splenic blood to the liver so that organ has first access to substances absorbed from the intestine. A minor portal circulation in the pituitary stalk transports releasing hormones from the hypothalamus directly to the anterior pituitary gland.

### The Lymphatic Circulation

Lymphatic vessels originate in the interstitial compartment of tissues and end in the opening of the thoracic duct into the jugular vein. Their main function is to transport large molecules and excess fluid from the interstitium back into the blood. Lymphatic vessels have thin walls with endothelial valves spaced at intervals that promote central flow; the lymphatic system operates under very low pressure.

Interspersed in the lymphatic system are the lymph nodes, which represent organs of the immune system (Chapter 4: The Immune Response).

### Vascular Endothelium

The endothelium is a simple, flat layer of ...

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