Pulmonary and Systemic Circuits

3 Types of Capillaries

Structurally there are three types of capillaries. The three are continuous, fenestrated, and sinusoidal capillaries.

The most common, but also the most permeable are the continuous capillaries. They are abundant in the skin and muscles. They are continuous in the sense that their endothelial cells provide an uninterrupted lining, adjunct cells being joined laterally by tight junctions. The tight junctions leave gaps of unjoined membrane that are just large enough to limit passage.

The fenestrated capillaries are similar to the continuous except some of the endothelial cells are riddled with oval pores, or fenestrations. Fenestrated capillaries are found wherever active capillary absorption or filtrate formation occurs. Fenestrated capillaries in the small intestine receive nutrients from digested food, and those in endocrine organs allow hormones rapid entry into the blood.

The third type of capillaries are the sinusoid capillaries, which are highly modified, leaky capillaries found only in the liver, bone marrow, spleen, and adrenal medulla. Their structural adaptions allow large molecules and even blood cells to pass between the blood and surrounding tissues. In the liver, the endothelium's are discontinuous and its lining removes and destroys contained bacteria. Blood flows sluggishly through the fatuous sinusoid channels, which allows time for it to be modified in various ways.

Cardiac Intrinsic Conduction System

Impulses pass across the heart in a process called Cardiac Intrinsic Conduction System, which includes five steps. The impulses start in the sinotrial node, atrioventricular node, atrioventricular bundle, the bundle braches, and end with the Purkinje Fibers.

The SA node typically generates impulses at about 75 times per minute. From the SA node, the depolarization wave spreads thru the gap junctions throughout the atria to the atrioventricular node. At the AV node, the impulse is delay about 0.1 second. Once through the AV node, the signaling impulses passed to the atrioventricular bundle and from there to the right and left bundle branches. The bundle only last briefly before splitting into the separate left and right branches. From the branches the impulses conduct through the introventricular septum, to the Purkinje Fibers, where they depolarize the contractile cells of both ventricles.