They are located just below the rib cage, one on each side of your spine. Healthy kidneys filter about a half cup of blood every minute, removing wastes and extra water to make urine. The urine flows from the kidneys to the bladder through two thin tubes of muscle called ureters, one on each side of your bladder. Your bladder stores urine.
Your kidneys, ureters, and bladder are part of your urinary tract. Your kidneys remove wastes and extra fluid from your body. Your kidneys also remove acid that is produced by the cells of your body and maintain a healthy balance of water, salts, and minerals—such as sodium , calcium , phosphorus , and potassium —in your blood. Watch a video about what the kidneys do. Each of your kidneys is made up of about a million filtering units called nephrons.
Each nephron includes a filter, called the glomerulus , and a tubule. The nephrons work through a two-step process: the glomerulus filters your blood, and the tubule returns needed substances to your blood and removes wastes. As blood flows into each nephron, it enters a cluster of tiny blood vessels—the glomerulus.
The thin walls of the glomerulus allow smaller molecules, wastes, and fluid—mostly water—to pass into the tubule. Larger molecules, such as proteins and blood cells, stay in the blood vessel. A blood vessel runs alongside the tubule. The detrusor muscle receives parasympathetic input, while the internal sphincter receives sympathetic input and the external sphincter receives motor input.
While the bladder is filling, there is little parasympathetic input to the detrusor muscle but there are strong sympathetic and motor inputs to the sphincters.
While filling occurs, the detrusor muscle is relaxed, and sphincters are closed. As the bladder fills, stretch receptors stimulate the parasympathetic fibers, resulting in contraction of the detrusor muscle. Sympathetic and motor inputs to sphincters are inhibited and sphincters open to produce urination. There is voluntary control over the external sphincter. They undergo considerable reabsorption in the proximal tubule but the major hormonal controls on reabsorption are exerted in the collecting ducts.
They are not secreted into the tubules. Primary Active Sodium Reabsorption. Coupling of Water Reabsorption. The removal of sodium lowers the osmolarity of the lumen and raises that of the interstitial fluid. This causes a net diffusion of water from the lumen into the interstitial fluid through the epithelium. Water permeability of the proximal tubule is high but only that of collecting ducts is under the control of vasopressin ADH. ADH stimulates the insertion of aquaporin channels, increasing water permeability.
Low ADH leads to water diuresis or diabetes insipidus. Increased urine flow due to increased solute excretion is called osmotic diuresis. Fluid from proximal tubule has the same osmolarity as plasma since it absorbs sodium and water equally. In the ascending limb, sodium, but not water, is actively reabsorbed from the lumen, making the interstitial fluid of the medulla hyperosmotic.
Due to this hyperosmocity, there is passive diffusion of water from the lumen into the interstitial fluid in the descending limb.
Fluid in the distal tubule becomes progressively dilute as sodium is transported out and then in the cortical and medullary ducts, water diffuses out of the tubule into the hyperosmotic interstitial fluid and urine is concentrated.
Since sodium is the major extracellular solute, changes in total body sodium result in changes in the volume of extracellular fluid, changing plasma volume and therefore blood pressure, which is detected by the baroreceptors. Lower total body sodium can decrease GFR by vasoconstriction, resulting in lower pressure in renal arteries. The control of reabsorption is more important for long-term regulation.
Aldosterone stimulates sodium reabsorption by cortical collecting ducts and large intestine, sweat, and salivary glands. The secretion of aldosterone is controlled by angiotensin II, which is produced from angiotensinogen in a reaction, the rate-limiting step of which is controlled by renin from JG cells.
These cells act as internal baroreceptors as well as receive sympathetic inputs from external baroreceptors. Angiotensin II is also a vasoconstrictor itself. Potassium is filtered in the renal corpuscle and most of it absorbed in the tubules. Any changes in potassium excretion, however, are mainly due to changes in potassium secretion by cortical collecting ducts.
Aldosterone-secreting cells are sensitive to potassium concentration of their extracellular fluid and an increased potassium concentration stimulates aldosterone production, thereby increasing potassium secretion and its excretion from the body. In addition to the gastrointestinal tract and kidneys, which determine net intake and excretion of calcium, calcium can be redistributed between extracellular fluid and bone.
Hydrogen ions can be redistributed in the body by binding it reversibly with a buffer such as bicarbonates, phosphates, proteins and Hb.
HCO 3 — is filtered at the renal corpuscle and undergoes reabsorption in the tubule. Reabsorption of water and specific solutes occurs to varying degrees over the entire length of the renal tubule. Bulk reabsorption, which is not under hormonal control, occurs largely in the proximal tubule. In addition, many important solutes glucose, amino acids, bicarbonate are actively transported out of the proximal tubule such that their concentrations are normally extremely low in the remaining fluid.
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.
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