BIO 360 - Exam 4 - Chapter 20 Notes Flashcards
Insensible Water loss: water loss across the skin and in exhaled air of which we are not normally aware.
Pathological water loss disrupts homeostasis in two ways. Volume depletion of the extracellular compartment decreases blood pressure.
The kidneys cannot replenish lost water: All they can do is conserve it.
How can the kidney reabsorb water without first reabsorbing solute? At one time, scientists speculated that water was actively transported on carriers, just as
and other ions are. However, once scientists developed micropuncture techniques for sampling fluid inside kidney tubules, they discovered that water is reabsorbed by osmosis through water pores (aquaporins).
The mechanism for absorbing water without solute turned out to be simple: make the collecting duct cells and interstitial fluid surrounding them more concentrated than the fluid flowing into the tubule. Then, if the tubule cells have water pores, water can be absorbed from the lumen without first reabsorbing solute.
The loop of Henle is the primary site where the kidney creates hyposmotic fluid.
How do the collecting duct cells alter their permeability to water? The process involves adding or removing water pores in the apical membrane under the direction of the posterior pituitary hormone vasopressin (p.207). Because vasopressin causes the body to retain water, its alternate name is antidiuretic hormone (ADH). When vasopressin acts on target cells, the collecting duct epithelium becomes permeable to water, allowing water to move out of the lumen (Fig. 20.5a). The water moves by osmosis because osmolarity of tubule cells and the medullary interstitial fluid is higher than osmolarity of fluid in the tubule. In the absence of vasopressin, the collecting duct is impermeable to water. the more vasopressin is present, the more water is reabsorbed.
Most membranes in the body are freely permeable to water. What makes the cells of the distal nephron different? The answer lies with the water pores found in these cells. Water pores are aquaporins, a family of membrane channels with at least 10 different isoforms that occur in mammalian tissues. The kidney has multiple isoforms of aquaporins, including aquaporin-2 (AQP2), the water channel regulated by vasopressin.
What stimuli control vasopressin secretion? There are three: plasma osmolarity, blood volume, and blood pressure (Fig. 20.6). The most potent stimulus for vasopressin release is an increase in plasma osmolarity.
The primary osmoreceptors for vasopressin release are in the hypothalamus.
Decreases in blood pressure and blood volume are less powerful stimuli for vasopressin release. The primary receptors for decreased volume are stretch-sensitive receptors in the atria. Blood pressure is monitored by the same carotid and aortic baroreceptors that initiate cardiovascular responses[p. 492]. When blood pressure or blood volume is low, these receptors signal the hypothalamus to secrete vasopressin and conserve fluid.
A countercurrent exchange system in which exchange is enhanced by active transport of solutes is called a countercurrent multiplier.
The regulation of blood levels takes place through one of the most complicated endocrine pathways of the body. The reabsorption of in the distal tubules and collecting ducts of the kidney is regulated by the steroid hormone aldosterone: the more aldosterone, the more reabsorption. Because one target of aldosterone is increased activity of the aldosterone also causes secretion (Fig. 20.9).
Aldosterone is a steroid hormone synthesized in the adrenal cortex, the outer portion of the adrenal gland that sits atop each kidney[p. 200]. Like other steroid hormones, aldosterone is secreted into the blood and transported on a protein carrier to its target.
The primary site of aldosterone action is the last third of the distal tubule and the portion of the collecting duct that runs through the kidney cortex (the cortical collecting duct). The primary target of aldosterone is principal cells (P cells) (Fig. 20.9b), the main cell type found in the distal nephron epithelium.
Vasopressin must be present to make the distal nephron epithelium permeable to water.
What controls physiological aldosterone secretion from the adrenal cortex? There are two primary stimuli: increased extracellular K+ concentration and decreased blood pressure
