Skeletal-muscle pump

The effect of the skeletal muscle pump is essential during exercise. Although a mass sympathetic discharge and venous vasoconstriction enhance VR, this mechanism alone is insufficient to increase VR and, therefore, CO to meet the metabolic demands of strenuous exercise. The skeletal muscle pump mobilizes the blood stored in these tissues and keeps it flowing toward the heart. As the number of muscles involved in the exercise increases, so does the magnitude of the increase in VR and CO. 

Differences in Pc between capillaries of the head and feet are due to gravitational variation of the blood pressure such that the pressure p = pgh. For this reason, volumes of transcapillary filtration and lymph flows are generally higher in tissues of the lower body as compared to those of the upper body. Moreover, one might expect much more sparse distribution of lymphatic vessels in upper body tissues. The brain has no lymphatics, but most other vascular tissues have lymphatics. In fact, tissues of the lower body of humans and other tall animals have efficient skeletal muscle pumps, prominent lymphatic systems, and noncompliant skin and fascial boundaries to prevent dependent edema [Hargens et al., 1987]. 

In the human body, the soleus is a powerful muscle in the back part of the lower leg, i.e., the calf. It runs from just below the knee to the heel, and is involved in standing and walking. In the upright posture it is responsible for pumping venous blood back into the heart from the periphery, and is often called the skeletal-muscle pump or peripheral heart. The reduction of vein diameter by means of compression bandages helps to improve the function of the muscle pump by enhancing the efficiency of the reverse flow. 

The trigger for all musele eontraetion is an increase in Ca eoneentration in the vicinity of the muscle fibers of skeletal muscle or the myocytes of cardiac and smooth muscle. In all these cases, this increase in Ca is due to the flow of Ca through calcium channels (Figure 17.24). A muscle contraction ends when the Ca concentration is reduced by specific calcium pumps (such as the SR Ca -ATPase, Chapter 10). The sarcoplasmic reticulum, t-tubule, and sarcolemmal membranes all contain Ca channels. As we shall see, the Ca channels of the SR function together with the t-tubules in a remarkable coupled process. 

In striated muscles, SR is well developed to surround the myofibrils and is divided into two parts, the terminal cisternae (TC) and longitudinal tubules (LT). TC forms triad (skeletal muscle) or dyad (heart) structure with transverse tubules. The ryanodine receptor is located only in the TC, whereas the Ca2+ pump/SERCA is densely packed in both TC and LT. 

Site symmetry symbols, I, 128 Six-coordinate compounds stereochemistry, 1, 49-69 Six-membered rings metal complexes, 2, 79 Skeletal muscle sarcoplasmic reticulum calcium pump, 6, 565 Slags... 

Figure 49-8. Diagram of the relationships among the sarcolemma (plasma membrane), a T tubule, and two cisternae of the sarcoplasmic reticulum of skeletal muscle (not to scale). The T tubule extends inward from the sarcolemma. A wave of depolarization, initiated by a nerve impulse, is transmitted from the sarcolemma down the T tubule. It is then conveyed to the Ca release channel (ryanodine receptor), perhaps by interaction between it and the dihydropyridine receptor (slow Ca voltage channel), which are shown in close proximity. Release of Ca from the Ca release channel into the cytosol initiates contraction. Subsequently, Ca is pumped back into the cisternae of the sarcoplasmic reticulum by the Ca ATPase (Ca pump) and stored there, in part bound to calsequestrin.

Lymphatic capillaries join together to form larger lymphatic vessels that have valves within them to ensure the one-way flow of lymph. The lymph is moved along by two mechanisms. Automatic, rhythmic waves of contraction of the smooth muscle in the walls of these vessels are the primary mechanism by which lymph is propelled through the system. Second, the contraction of skeletal muscles causes compression of lymphatic vessels. As in the veins, this pumping action of the surrounding skeletal muscles contributes to movement of the lymph. Ultimately, the lymph is returned to the blood when it empties into the subclavian and jugular veins near the heart. 

Genetic evidence supports the importance of coordinated expression and distributions of a2 or a3 Na+/K+ pump isoforms with the Na+/Ca2+ exchanger (NCX) and Ca2+ pumps to function in excitable and contractile cells deletion of one copy of the a2 Na+/K+ pump gene in mice leads to increased contractile force in cardiac and skeletal muscle while deletion of one copy of the al gene leads to reduction of contractile force [25]. In rat optic nerve astrocytes, deletion of the a2 gene or ouabain treatment of cells expressing a2 leads to increased capacitative calcium entrance responses, which reflect a decreased ability to rapidly remove cytosolic Ca2+ [26]. 

The smooth endoplasmic reticulum calcium pumps (SERCA) found in brain were first identified in sarcoplasmic reticulum. The three isoforms of SERCA are products of separate genes SERCA-1 is expressed in fast-twitch skeletal muscle SERCA-2a in cardiac/slow-twitch muscle SERCA-2b, an alternatively spliced form, is expressed in smooth muscle and non-muscle tissues SERCA-3 is... 

Mutations in the SERCA1 pump of skeletal muscle lead to Brody s disease, in which there is delayed muscle relaxation, particularly after exercise, leading to cramps. It is thought to be due to reduced efficiency of the SERCA1 pumps reducing the rate at which Ca2+ can be removed from the cytoplasm (Odermatt et al 2000). 

In both cases, there is a need for vigorous activity of skeletal musculature. To ensure adequate supply of oxygen and nutrients, blood flow in skeletal muscle is increased cardiac rate and contractility are enhanced, resulting in a larger blood volume being pumped into the circulation. Narrowing of splanchnic blood vessels diverts blood into vascular beds in muscle. 

There are three types of muscles smooth, cardiac, and skeletal. Smooth muscles help move food and fluids through your body. Cardiac muscles pump blood through your heart. Skeletal muscles are the kind that attach to your bones. They allow you to run, dance, lift, turn, and, in general, move through your environment. Physical fitness involves resilience in all muscle types. For this essay, however, our focus will be on the more physically apparent skeletal muscles, which can account for up to 40 percent of your total mass. 

The best studied example of a Group IIA cation transport system is the calcium pump of the sarcoplasmic reticulum of skeletal muscle. Indeed, the calcium pump and the sodium pump represent the most studied of all transport processes. The calcium pump involves a membrane-bound (Ca2+, Mg2+)-ATPase and uptake of Ca2+ is associated with hydrolysis of ATP. While the... 

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