176 C I R C U L A T O R Y S Y S T E M

•The sinoatrial node (SA node), the pacemaker of the heart, is located at the junction of the superior vena cava and the right atrium. The SA node generates impulses that result in the contraction of the atrial muscles; blood from the atria then enters the ventricles.

•Impulses generated at the SA node are then conducted to the atrioventricular node (AV node), which is located on the medial wall of the right ventricle near the tricuspid valve, as well as to the atrial myocardium.

•Arising from the AV node is the bundle of His, which bifurcates in the septum membranaceum to serve both ventricles.

•As these fibers reach the subendocardium, they ramify and are known as Purkinje fibers, which deliver the impulse to the cardiac muscle cells of the ventricles that contract to pump the blood from the right ventricle into the pulmonary trunk and from the left ventricle into the aorta.

The arrangement of the cardiac myocytes as well as the atrioventricular bundle permits the contraction of the atria first, followed, after a time lag, by contraction of the ventricles. In this fashion, blood from the atria can enter the ventricles, and once the ventricles are filled, they contract and propel the blood into the systemic and pulmonary circuits.

The inherent rhythm of the SA node is modulated by the autonomic nervous system, in that parasympathetic fibers derived from the vagus nerve decrease the rate of the heartbeat, whereas fibers derived from sympathetic ganglia increase it.

ARTERIES

Arteries, by definition, conduct blood away from the heart; they are classified into three categories: elastic (also known as conducting or large), muscular (also known as distributing or medium), and arterioles (see Graphic 8-1 and Table 8-1).

•Elastic arteries, such as the aorta, receive blood directly from the heart and consequently are the largest of the arteries.

Since they arise directly from the heart, they are subject to cyclic changes of blood pressure, high as the ventricles pump blood into their lumina and low between the emptying of these chambers.

To compensate for these intermittent pressure alterations, an abundance of elastic fibers are located in the walls of these vessels.

These elastic fibers not only provide structural stability and permit distention of the elastic arteries but they also assist in the maintenance of blood pressure in between heartbeats.

•Muscular arteries comprise most of the named arteries of the body and supply blood to various organs. Their tunica media is composed mostly of many layers of smooth muscle cells. Both elastic and muscular arteries are supplied by vasa vasorum (see Graphic 8-1) and nerve fibers.

•Arterioles regulate blood pressure and the distribution of blood to capillary beds via vasoconstriction and vasodilatation of vessel walls.

Metarterioles are the terminal ends of the arterioles, and they are characterized by the presence of incomplete rings of smooth muscle cells (precapillary sphincters) that encircle the origins of the capillaries.

Metarterioles form the arterial (proximal) end of a central channel, and they are responsible for delivering blood into the capillary bed. The venous (distal) end of the central channel, known as a thoroughfare channel, is responsible for draining blood from the capillary bed and delivering it into venules.

Contraction of precapillary sphincters of the metarteriole shunts the blood into the thoroughfare channel and from there into the venule; this way, the blood bypasses the capillary bed (see Graphic 8-2).

Arteriovenous anastomoses are direct connections between arteries and venules, and they also function in having blood bypass the capillary bed. These shunts function in thermoregulation and blood pressure control.

•Capillaries are very small vessels that consist of a single layer of endothelial cells surrounded by a basal lamina and occasional pericytes (see Graphic 8-2), but these vessels possess no smooth muscle cells; therefore, they do not exhibit vasomotor activities. Capillaries exhibit selective permeability, and they, along with venules, are responsible for the exchange of gases, metabolites, and other substances between the bloodstream and the tissues of the body. Capillaries are composed of highly attenuated endothelial cells that form narrow vascular channels 8 to 10 mm in diameter and are usually less than 1 mm long. There are three types of capillaries: continuous, fenestrated, and sinusoidal (Table 8-2).

Continuous capillaries lack fenestrae, display only occasional pinocytotic vesicles, and possess a continuous basal lamina. They are present in regions such as peripheral nerve fibers, skeletal muscle, lungs, and thymus.

C I R C U L A T O R Y S Y S T E M 177

Fenestrated capillaries are penetrated by relatively large diaphragm-covered pores. These cells also possess pinocytotic vesicles and are enveloped by a continuous basal lamina. Fenestrated capillaries are located in endocrine glands, pancreas, and lamina propria of the intestines, and they also constitute the glomeruli of the kidneys, although their fenestrae are not covered by a diaphragm.

Sinusoidal capillaries (also known as sinusoids, discontinuous capillaries) are much larger than their fenestrated or continuous counterparts. They are enveloped by a discontinuous basal lamina, and their endothelial cells do not possess pinocytotic vesicles. The intercellular junctions of their endothelial cells display gaps, thus permitting leakage of material into and out of these vessels. Frequently, macrophages are associated with sinusoidal capillaries. Sinusoidal capillaries are located in the liver, spleen, lymph nodes, bone marrow, and the suprarenal cortex.

Capillary Permeability

Capillary permeability is dependent not only on the endothelial cells comprising the capillary but also on the (physico)-chemical characteristics, such as size, charge, and shape, of the traversing substance.

•Some molecules, such as H2O, diffuse through, whereas others are actively transported by carrier proteins across the endothelial cell plasma membrane.

•Other molecules move through fenestrae or through gaps in the intercellular junctions.

Certain pharmacological agents, such as bradykinin and histamine, have the ability to alter capillary permeability.

Leukocytes leave the bloodstream by passing through intercellular junctions of the endothelial cells (diapedesis) to enter the extracellular spaces of tissues and organs.