Water enters the tubular cell from the tubular lumen through the inserted water channels. 23
Water exits the cell through a different, always-open water channel permanently positioned at the basolateral border, and then enters the blood, in this way being reabsorbed. FACT: Aldosterone increases Na + reabsorption into the peritubular capillaries from the distal and collecting tubules of the nephron (p. 521). How is this accomplished? o Na + leak channels (ENaCs) into the luminal membranes of the distal & collecting tubules of the nephron o Na + /K + pumps into the basolateral membranes of the tubular epithelial cells of the nephron. Both leads to greater passive movement of Na + into the distal & collecting tubular cells from the lumen and increased active pumping of Na + out of the cells into the plasma (i.e. an increase in Na + reabsorption with Cl - following passively. Atrial natriuretic peptide (ANP) A powerful vasodilator, and a polypeptide hormone secreted by heart muscle cells involved in the control of body water, Na & K ions, water and adipose tissue. ANP is released by atrial myocytes in response to high blood pressure. ANP acts to reduce the water, sodium and adipose loads on the circulatory system, thereby reducing blood pressure Dilates the afferent & constricts the efferent glomerular arterioles, and relaxes the mesangial cells which increases pressure in the glomerular capillaries increasing the GFR, resulting in greater excretion of sodium and water. Increases blood flow through the vasa recta- leads to less reabsorption of tubular fluid and increased excretion. Decreases sodium reabsorption into the blood in the distal convoluted tubules and collecting ducts. Inhibits renin secretion, thereby inhibiting the renin-angiotensin-aldosterone system. Reduces aldosterone secretion by the adrenal cortex. Brain natriuretic peptide Brain natriuretic peptide (BNP is a 32 amino acid polypeptide secreted by the ventricles of the heart in response to excessive stretching of heart muscle cells (cardiomyocytes).The release of BNP is modulated by calcium ions. BNP is named as such because it was originally identified in extracts of porcine brain, although in humans it is produced mainly in the cardiac ventricles. The physiologic actions of BNP are similar to ANP and include decrease in systemic vascular resistance and central venous pressure as well as an increase in natriuresis (excretion of sodium in the urine ). Thus, the net effect of BNP and ANP is a decrease in blood volume which lowers systemic blood pressure. When protein in the urine does not mean kidney disease Following strenuous exercise (“athletic pseudonephritis”): proteinuria occurs in both contact & noncontact sports Not a result of physical trauma to kidneys Causes: o Increased glomerular permeability with no change in tubular reabsorption (mild to moderate exercise).
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