By V. Vak. Knox Theological Seminary.
The NADH generated from glycolysis can be used to reduce methemoglobin (Fe 3) to normal hemoglobin (Fe 2) generic v-gel 30gm online, or to con- vert pyruvate to lactate best v-gel 30gm, such that NAD can be regenerated and used for glycolysis. Pathways unique to the erythrocyte are indicated in blue. Some of the NADH produced by glycolysis is used to regenerate hemoglobin from methemoglobin by the NADH-cytochrome b5 methemoglobin reductase system. The oxi- 5 dized cytochrome b5 is then reduced by a flavin-containing enzyme, cytochrome b5 reductase (also called methemoglobin reductase), using NADH as the reducing agent. An inherited deficiency in pyruvate kinase leads to hemolytic anemia (an anemia caused by the destruction of red blood cells; hemo- globin values typically drop to 4 to 10 g/dL in this condition). Because the amount of ATP formed from glycolysis is decreased by 50%, red blood cell ion transporters cannot function effectively. The red blood cells tend to gain Ca2 and lose K and water. The water loss increases the intracellular hemoglobin concentration. With the increase in intracellular hemoglobin concentration, the internal viscosity of the cell is increased to the point that the cell becomes rigid and, therefore, more susceptible to damage by shear forces in the circulation. Once damaged, the red blood cells are removed from circulation, leading to the anemia. However, the effects of the anemia are frequently moder- ated by the twofold to threefold elevation in 2,3-BPG concentration that results from the blockage of the conversion of phosphoenol pyruvate to pyruvate. Because 2,3-BPG binding to hemoglobin decreases the affinity of hemoglobin of oxygen, the red blood cells that remain in circu- lation are highly efficient in releasing their bound oxygen to the tissues. The NADPH is globin, is found in people with an used to maintain glutathione in the reduced state. The glutathione cycle is the red enzymatic deficiency in cytochrome b5 red- blood cell’s chief defense against damage to proteins and lipids by reactive oxygen uctase or in people who have inherited hem- species (see Chapter 24). In hemoglobin M, a single amino The enzyme that catalyzes the first step of the hexose monophosphate shunt is acid substitution in the heme-binding pocket stabilizes the ferric (Fe 3) oxygen. The lifetime of the red blood cell cor- als with congenital methemoglobinemia relates with G6PD activity. Lacking ribosomes, the red blood cell cannot synthesize appear cyanotic but have few clinical prob- new G6PD protein.
ZOOMING IN Which vessels have valves that control blood flow? Rates of exchange vary because generic v-gel 30 gm overnight delivery, based on their many holes discount v-gel 30 gm free shipping, or fenestrations, in the endothelium. These sieve- structure, different types of capillaries vary in permeability. In the digestive tract, fenestrated capillaries permit found in muscle, connective tissue, the lungs, and the central rapid absorption of water and nutrients into the bloodstream. These capillaries are composed of a In the kidneys, they permit rapid filtration of blood plasma, continuous layer of endothelial cells. Adjacent cells are the first step in urine formation. In addition to fenestrations, they have large spaces be- ies are the least permeable, water and small molecules can dif- tween endothelial cells that allow the exchange of water, large fuse easily through their walls. Large molecules, such as solutes, such as plasma proteins, and even blood cells. In certain regions of soids are found in the liver and red bone marrow, for exam- the body, like the CNS, adjacent endothelial cells are joined ple. Albumin, clotting factors, and other proteins formed in tightly together, making the capillaries impermeable to many the liver enter the bloodstream through sinusoids. In red bone substances (see Box 10-1 in Chapter 10, The Blood Brain Bar- marrow, newly formed blood cells travel through sinusoids to rier: Access Denied). The aorta is one contin- vessels also have less elastic tissue between the layers. As uous artery, but it may be divided into sections: a result, the blood within the veins is carried under much ◗ The ascending aorta is near the heart and inside the lower pressure. Because of their thinner walls, the veins pericardial sac. Only slight pressure on a vein by a ◗ The aortic arch curves from the right to the left and tumor or other mass may interfere with return blood also extends posteriorly. Such valves are most numerous in the veins of the extremities. Figure 15-3 is a cross-section of Smooth muscle an artery and a vein as seen through a microscope.