Is continuously maintained by the loop of henle a

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is continuously maintained by the loop of Henle, a hairpin-like loop between the proximal and distal tubules - the opposing flows in the loop, the downward flow in the descending limb and the upward flow in the ascending limb is termed a countercurrent flow – to understand how hyperosmolality is maintained in the medulla it is best to look 1 st at what happens in the ascending limb – Sodium and chloride are actively and passively reabsorbed into the medulla interstitial fluid along the entire length of the ascending limb – b/c the ascending limb is relatively impermeable to water, little water follows and the medulla interstitial fluid becomes hyperosmotic compared w/ the fluid in the ascending limb – the fluid in the ascending limb becomes hypotonic or dilute as sodium and chloride ions are reabsorbed w/o the loss of water, so the ascending limb is often called the diluting
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RENAL FUNCTION OBJECTIVES segment – the descending limb in contrast to the ascending limb, is highly permeable to water and does not reabsorb sodium and chloride – the high osmolality of the surrounding interstitial medulla fluid is the physical force that accelerates the reabsorption of water from the filtrate in the descending limb – interstitial hyperosmolality is maintained b/c the ascending limb continues to pump sodium and chloride ions into it – this interaction of water leaving the descending loop and sodium and chloride leaving the ascending loop to maintain a high osmolality w/in the kidney medulla produces hypoosmolal urine as it leaves the loop – this process is called the countercurrent multiplier system DISTAL TUBULE --- (Anatomy) located in the cortex (Physiology) it is much shorter than the proximal tube w/ 2 or 3 coils that connect to a collecting duct – the filtrate entering it is close to its final composition – about 95% of the sodium and chloride ions and 90% of water have already been reabsorbed from the original glomerular filtrate – the function of the distal tubule is to effect small adjustments to achieve electrolyte and acid base homeostasis – these adjustments occur under the hormonal control of antidiuretic hormone (ADH) and aldosterone ADH - is a peptide hormone secreted by the posterior pituitary , mainly in response to increased blood osmolality – it is also released when blood volume decreases by 5-10% - large decrease of blood volume will stimulate ADH secretion even when plasma osmolality is decreased – ADH stimulates water reabsorption – the walls of the distal collecting tubules are normally impermeable to water (like the ascending loop of Henle) but they become permeable to water in ADH – water diffuses passively from the lumen of the tubules resulting in more concentrated urine and decreased plasma osmolality Aldosterone – produced by adrenal cortex under the influence of the renin- angiotensin mechanism – its secretion is triggered by decreased blood flow of blood pressure in the afferent renal arteriole
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