Further bulk reabsorption of sodium occurs in the loop of Henle. Regulated reabsorption, in which hormones control the rate of transport of sodium and water depending on systemic conditions, takes place in the distal tubule and collecting duct.
Even after filtration has occured, the tubules continue to secrete additional substances into the tubular fluid. This enhances the kidney's ability to eliminate certain wastes and toxins. It is also essential to regulation of plasma potassium concentrations and pH. Most of the resulting ammonia is converted into urea by liver hepatocytes.
Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts. At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct.
The amino acid glutamine can be deaminated by the kidney. Ammonia and bicarbonate are exchanged in a one-to-one ratio.
T his exchange is yet another means by which the body can buffer and excrete acid. At the transition from the distal convoluted tubule to the collecting duct, about 20 percent of the original water is still present and about 10 percent of the sodium. If no other mechanism for water reabsorption existed, about 20—25 liters of urine would be produced. Now consider what is happening in the adjacent capillaries, the vasa recta. They are recovering both solutes and water at a rate that preserves the countercurrent multiplier system.
In general, blood flows slowly in capillaries to allow time for exchange of nutrients and wastes. In the vasa recta particularly, this rate of flow is important for two additional reasons. The flow must be slow to allow blood cells to lose and regain water without either crenating or bursting. Approximately 80 percent of filtered water has been recovered by the time the dilute filtrate enters the distal convoluted tubule.
The distal convoluted tubule will recover another 10—15 percent before the filtrate enters the collecting ducts. Peritubular capillaries receive the solutes and water, returning them to the circulation. Receptors for parathyroid hormone are found in distal convoluted tubule cells and when bound to parathyroid hormone, induce the insertion of calcium channels on their luminal surface.
Finally, calcitriol 1,25 dihydroxyvitamin D, the active form of vitamin D is very important for calcium recovery. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane.
Solutes move across the membranes of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells. A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cell secretes or absorbs acid or bicarbonate.
As in other portions of the nephron, there is an array of micromachines pumps and channels on display in the membranes of these cells. Regulation of urine volume and osmolarity are major functions of the collecting ducts. If the blood becomes hyperosmotic, the collecting ducts recover more water to dilute the blood; if the blood becomes hyposmotic, the collecting ducts recover less of the water, leading to concentration of the blood.
Another way of saying this is: If plasma osmolarity rises, more water is recovered and urine volume decreases ; if plasma osmolarity decreases, less water is recovered and urine volume increases. This function is regulated by the posterior pituitary hormone antidiuretic hormone vasopressin. With mild dehydration, plasma osmolarity rises slightly. This increase is detected by osmoreceptors in the hypothalamus, which stimulates the release of antidiuretic hormone from the posterior pituitary.
If plasma osmolarity decreases slightly, the opposite occurs. When stimulated by antidiuretic hormone, aquaporin channels are inserted into the apical membrane of principal cells, which line the collecting ducts. As the ducts descend through the medulla, the osmolarity surrounding them increases due to the countercurrent mechanisms described above. If aquaporin water channels are present, water will be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries.
Therefore, the final urine will be more concentrated. If less antidiuretic hormone is secreted, fewer aquaporin channels are inserted and less water is recovered, resulting in dilute urine.
By altering the number of aquaporin channels, the volume of water recovered or lost is altered. This, in turn, regulates the blood osmolarity, blood pressure, and osmolarity of the urine. Aldosterone is secreted by the adrenal cortex in response to angiotensin II stimulation. As an extremely potent vasoconstrictor, angiotensin II functions immediately to increase blood pressure. By also stimulating aldosterone production, it provides a longer-lasting mechanism to support blood pressure by maintaining vascular volume water recovery.
In addition to receptors for antidiuretic hormone, principal cells have receptors for the steroid hormone aldosterone. Intercalated cells play significant roles in regulating blood pH. This function lowers the acidity of the plasma while increasing the acidity of the urine. The kidney regulates water recovery and blood pressure by producing the enzyme renin. It is renin that starts a series of reactions, leading to the production of the vasoconstrictor angiotensin II and the salt-retaining steroid aldosterone.
Water recovery is also powerfully and directly influenced by the hormone antidiuretic hormone. Even so, it only influences the last 10 percent of water available for recovery after filtration at the glomerulus, because 90 percent of water is recovered before reaching the collecting ducts.
Mechanisms of solute recovery include active transport, simple diffusion, and facilitated diffusion. Most filtered substances are reabsorbed. Urea, NH 3 , creatinine, and some drugs are filtered or secreted as wastes. Movement of water from the glomerulus is primarily due to pressure, whereas that of peritubular capillaries and vasa recta is due to osmolarity and concentration gradients. The proximal convoluted tubule is the most metabolically active part of the nephron and uses a wide array of protein micromachines to maintain homeostasis—symporters, antiporters, and ATPase active transporters—in conjunction with diffusion, both simple and facilitated.
Almost percent of glucose, amino acids, and vitamins are recovered in the proximal convoluted tubule. Bicarbonate HCO 3 — is recovered using the same enzyme, carbonic anhydrase CA , found in erythrocytes. The recovery of solutes creates an osmotic gradient to promote the recovery of water. The collecting ducts actively pump urea into the medulla, further contributing to the high osmotic environment.
The vasa recta recover the solute and water in the medulla, returning them to the circulation. Urea is produced in the liver when excess amino acids are broken down.
Urea is the main waste product removed in the urine, as it is not reabsorbed in the kidney. Each kidney contains over one million microscopic filtering units called nephrons. Each nephron is made of a tubule and is responsible for 'cleaning' the blood by removing urea , excess water and mineral ions. The structure of the nephron is shown in the diagram below. A three stage process occurs in each nephron: filtration, selective reabsorption and finally excretion.
The glomerulus filters the blood and removes water, glucose, salts and waste urea from it. The blood is under high pressure at the start of the nephron, which aids the filtration of the blood. These waste substances all pass from the capillaries in the glomerulus into the Bowman's capsule. This purifies the blood. These waste substances then move from the Bowman's capsule towards the loop of Henle.
Proteins are too large to pass through here and so remain in the blood.
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