Anatomy & Physiology

Anatomy & Physiology Study Guide Reabsorption of potassium occurs by active transport. As sodium is reabsorbed from the lumen, potassium is secreted. Reabsorption of potassium occurs in the proximal tubule and secretion of potassium occurs in the distal tubule. Most of the water filtered from the plasma is reabsorbed in the proximal convoluted tubule. The flow of urine and the concentration of solute particles in the urine vary. The concentration of antidiuretic hormone (ADH) controls the concentration of solute in the urine; yet, the amount of ADH that is released depends on the concentration of solutes in the plasma. If the concentration is high, the posterior pituitary releases ADH into the blood. When this hormone reaches the kidney, water reabsorption begins. If the concentration of solutes in the plasma is low, no ADH is released, and water reabsorption decreases. In the proximal convoluted tubule, the reabsorption of glucose and amino acids begins. Sodium, chlorides, and bicarbonate are reabsorbed. Most of the sodium is reabsorbed in the proximal tubule. Water is reabsorbed in the proximal tubule as needed and is determined by the concentration of solutes in the plasma. In the distal tubule, sodium is reabsorbed in exchange for hydrogen and potassium. In the presence of ADH, the distal tubule and collecting tubules become permeable to water, but, if ADH is not present, these tubules act only as conduits. 24.5 Diuresis There are two main types of diuresis: water diuresis and osmotic diuresis. Water diuresis occurs when there are insufficient amounts of ADH in the blood. Osmotic diuresis is caused by an increase in the rate of solute excretion. An increase in solute excretion causes an increase in the amount of water that is excreted in urine. 24.6 Hormonal Involvement Antidiuretic Hormone The effect of ADH on water reabsorption occurs in the tubules and is very rapid. ADH increases water reabsorption when blood volume decreases. Renin and Aldosterone When the sodium concentration in the blood decreases, or when the blood pressure falls (for example, in hemorrhage), juxtaglomerular cells in the nephron release a hormone called renin , which acts on a plasma protein (angiotensinogen) to form angiotensin-2. Angiotensin-2, in turn, acts upon the cortex of the adrenal gland to produce aldosterone . Aldosterone increases Na + reabsorption and, thus, increases water reabsorption. This action increases plasma volume and blood pressure.

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