Blood vessels, autonomic regulation

Although skeletal muscle comprises the bulk of muscle tissue in the body, smooth muscle is far more important in terms of homeostasis. Most smooth muscle is found in the walls of tubes and hollow organs. Contraction and relaxation of the smooth muscle in these tissues regulates the movement of substances within them. For example, contraction of the smooth muscle in the wall of a blood vessel narrows the diameter of the vessel and leads to a decrease in the flow of blood through it. Contraction of the smooth muscle in the wall of the stomach exerts pressure on its contents and pushes these substances forward into the small intestine. Smooth muscle functions at a subconscious level and is involuntary. It is innervated by the autonomic nervous system, which regulates its activity. 

Vasomotor center. Autonomic nervous activity to the cardiovascular system is regulated by the vasomotor center (see Figure 15.4). Located in the lower pons and the medulla of the brainstem, the vasomotor center is an integrating center for blood pressure regulation. It receives several sources of input, processes this information, and then adjusts sympathetic and parasympathetic discharge to the heart and blood vessels accordingly. 

Physiologically, in both normal and hypertensive individuals, blood pressure is maintained by moment-to-moment regulation of cardiac output and peripheral vascular resistance, exerted at three anatomic sites (Figure 11-1) arterioles, postcapillary venules (capacitance vessels), and heart. A fourth anatomic control site, the kidney, contributes to maintenance of blood pressure by regulating the volume of intravascular fluid. Baroreflexes, mediated by autonomic nerves, act in combination with humoral mechanisms, including the renin-angiotensin-aldosterone system, to coordinate function at these four control sites and to maintain normal blood pressure. Finally, local release of vasoactive substances from vascular endothelium may also be involved in the regulation of vascular resistance. For example, endothelin-1 (see Chapter 17) constricts and nitric oxide (see Chapter 19) dilates blood vessels. 

Note The autonomic nervous system (ANS) regulates the functions of internal viscera such as the heart, blood vessels, digestive organs, and reproductive organs. 

A low level of tonic activity of the sympathetic nerves to vascular smooth muscle adrenergic receptors exists so that withdrawal of sympathetic vasomotor tone results in vasodilatation and reduced pressure. Conversely, enhancement of sympathetic vasomotor tone augments the level of vasoconstriction leading to elevated pressure. While the parasympathetic branch of the autonomic nervous system innervates some blood vessels, it does not generally play a role in regulating peripheral resistance. 

Blood is supplied to the retina by the central retinal artery and choroidal blood vessels (Oyster, 1999). The central retinal artery arises from the ophthalmic artery, w hich in turn branches off the internal carotid artery. Upon entering the retina, the central retinal artery branches into deep capillary beds in the INL and superficial capillary beds in the GCL. Endothelial cells of retinal capillaries are joined by tight junctions, contributing to the blood/retinal barrier. There is litde or no autonomic regulation of the retinal circulation blood flow through these capillaries is instead primarily controlled by autoregulation (Wangsa-Wirawan and Linsenmeier, 2003). Retinal capillaries drain into the central retinal vein. 

---