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    name the substances other than water, that are reabsorbed during urine formation. what are the two parameters that decide the amount of water that is reabsorbed in the kidney?

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    Name any two substances which are selectively reabsorbed from the tubules of a nephron.

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    Name any two substances which are selectively reabsorbed from the tubules of a nephron.

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    Updated on : 2022-09-05

    The nephron is the functional unit of the kidney. Some of the substances selectively reabsorbed from the tubules of a nephron are

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    Tubular Reabsorption

    Tubular Reabsorption

    LEARNING OBJECTIVES

    By the end of this section, you will be able to:

    List specific transport mechanisms occurring in different parts of the nephron, including active transport, osmosis, facilitated diffusion, and passive electrochemical gradients

    List the different membrane proteins of the nephron, including channels, transporters, and ATPase pumps

    Compare and contrast passive and active tubular reabsorption

    Explain why the differential permeability or impermeability of specific sections of the nephron tubules is necessary for urine formation

    Describe how and where water, organic compounds, and ions are reabsorbed in the nephron

    Explain the role of the loop of Henle, the vasa recta, and the countercurrent multiplication mechanisms in the concentration of urine

    List the locations in the nephron where tubular secretion occurs

    With up to 180 liters per day passing through the nephrons of the kidney, it is quite obvious that most of that fluid and its contents must be reabsorbed. That recovery occurs in the PCT, loop of Henle, DCT, and the collecting ducts. Various portions of the nephron differ in their capacity to reabsorb water and specific solutes. While much of the reabsorption and secretion occur passively based on concentration gradients, the amount of water that is reabsorbed or lost is tightly regulated. This control is exerted directly by ADH and aldosterone, and indirectly by renin. Most water is recovered in the PCT, loop of Henle, and DCT. About 10 percent (about 18 L) reaches the collecting ducts. The collecting ducts, under the influence of ADH, can recover almost all of the water passing through them, in cases of dehydration, or almost none of the water, in cases of over-hydration.

    Figure 1. Locations of Secretion and Reabsorption in the Nephron

    Table 1. Substances Secreted or Reabsorbed in the Nephron and Their Locations

    Substance PCT Loop of Henle DCT Collecting ducts

    Glucose Almost 100 percent reabsorbed; secondary active transport with Na+

    Oligopeptides, proteins, amino acids Almost 100 percent reabsorbed; symport with Na+

    Vitamins Reabsorbed Lactate Reabsorbed Creatinine Secreted

    Urea 50 percent reabsorbed by diffusion; also secreted Secretion, diffusion in descending limb Reabsorption in medullary collecting ducts; diffusion

    Sodium 65 percent actively reabsorbed 25 percent reabsorbed in thick ascending limb; active transport 5 percent reabsorbed; active 5 percent reabsorbed, stimulated by aldosterone; active

    Chloride Reabsorbed, symport with Na+, diffusion Reabsorbed in thin and thick ascending limb; diffusion in ascending limb Reabsorbed; diffusion Reabsorbed; symport

    Water 67 percent reabsorbed osmotically with solutes 15 percent reabsorbed in descending limb; osmosis 8 percent reabsorbed if ADH; osmosis Variable amounts reabsorbed, controlled by ADH, osmosis

    Bicarbonate 80–90 percent symport reabsorption with Na+ Reabsorbed, symport with Na+ and antiport with Cl–; in ascending limb Reabsorbed antiport with Cl–

    H+ Secreted; diffusion Secreted; active Secreted; active

    NH4+ Secreted; diffusion Secreted; diffusion Secreted; diffusion

    HCO3– Reabsorbed; diffusion Reabsorbed; diffusion in ascending limb Reabsorbed; diffusion Reabsorbed; antiport with Na+

    Some drugs Secreted Secreted; active Secreted; active

    Potassium 65 percent reabsorbed; diffusion 20 percent reabsorbed in thick ascending limb; symport Secreted; active Secretion controlled by aldosterone; active

    Calcium Reabsorbed; diffusion Reabsorbed in thick ascending limb; diffusion Reabsorbed if parathyroid hormone present; active

    Magnesium Reabsorbed; diffusion Reabsorbed in thick ascending limb; diffusion Reabsorbed

    Phosphate 85 percent reabsorbed, inhibited by parathyroid hormone, diffusion Reabsorbed; diffusion

    Mechanisms of Recovery

    Mechanisms by which substances move across membranes for reabsorption or secretion include active transport, diffusion, facilitated diffusion, secondary active transport, and osmosis. These were discussed in an earlier chapter, and you may wish to review them.

    Active transport utilizes energy, usually the energy found in a phosphate bond of ATP, to move a substance across a membrane from a low to a high concentration. It is very specific and must have an appropriately shaped receptor for the substance to be transported. An example would be the active transport of Na+ out of a cell and K+ into a cell by the Na+/K+ pump. Both ions are moved in opposite directions from a lower to a higher concentration.

    Simple diffusion moves a substance from a higher to a lower concentration down its concentration gradient. It requires no energy and only needs to be soluble.

    Facilitated diffusion is similar to diffusion in that it moves a substance down its concentration gradient. The difference is that it requires specific membrane receptors or channel proteins for movement. The movement of glucose and, in certain situations, Na+ ions, is an example of facilitated diffusion. In some cases of facilitated diffusion, two different substances share the same channel protein port; these mechanisms are described by the terms symport and antiport.

    स्रोत : courses.lumenlearning.com

    Tubular reabsorption article (article)

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    What is tubular reabsorption?

    The fluid that filters through the glomerulus and Bowman’s capsule (glomerular filtrate) is very similar to blood plasma without the proteins, and at this point not at all like urine. If this filtrate flowed straight to your bladder and then out your body, you would lose more than 10-times the entire volume of your extracellular body fluids (plasma and interstitial fluid) every day. Fortunately, tubular reabsorption mechanisms in the nephrons of your kidneys return the water and solutes that you need back into your extracellular fluid and circulatory system. In addition to reabsorbing the substances that you need, your nephrons are able to secrete unwanted substances from your bloodstream into the filtrate. Together these processes complete the transformation of the glomerular filtrate into urine.

    Tubular reabsorption is the process that moves solutes and water out of the filtrate and back into your bloodstream. This process is known as reabsorption, because this is the second time they have been absorbed; the first time being when they were absorbed into the bloodstream from the digestive tract after a meal.

    How does reabsorption in the nephrons work?

    The nephrons in your kidneys are specifically designed to maintain body fluid homeostasis. This means keeping extracellular body fluid volumes stable, as well as maintaining the right levels of the salts and minerals that are essential for the normal function of your tissues and organs; regardless of how much you eat, or how active you are. Nephrons are divided into five segments, with different segments responsible for reabsorbing different substances.

    Overview of the nephron showing which substances get reabsorbed along the various structures of the nephron (like the proximal convoluted tubule).

    Reabsorption is a two-step process:

    The first step is the passive or active movement of water and dissolved substances from the fluid inside the tubule through the tubule wall into the space outside.

    The second step is for water and these substances to move through the capillary walls back into your bloodstream, again, either by passive or active transport.

    Nephrons are comprised of different segments that perform specific functions. The walls of the nephron are made of a single layer of cube-like cells, called cuboidal epithelial cells, and their ultrastructure changes depending on the function of the segment they are in. For example, the surface of the cells facing the lumen of the proximal convoluted tubule are covered in microvilli (tiny finger-like structures). This type of surface is called a brush border. The brush border and the extensive length of the proximal tubule dramatically increase the surface area available for reabsorption of substances into the blood enabling around 80% of the glomerular filtrate to be reabsorbed in this segment. Another notable feature of these cells is that they are densely packed with mitochondria (the cell’s energy generators). The mitochondria ensure a good supply of energy is available to fuel the active transport systems needed for efficient reabsorption.

    Diagram showing what a proximal tubule epithelial cell looks like.

    Passive transport is when substances use specific transporters to move down their concentration gradient (from areas of high concentration to areas of low concentration) or in the case of charged ions, down their electrochemical gradient.

    Active transport is when substances are moved up (or against) their concentration or electrochemical gradients (from low to high). In this case, the substances are transported back into the bloodstream via energy-dependent, or active transport proteins.

    Reabsorption of sodium, nutrients, water, and other ions

    Sodium is the major positively charged electrolyte in extracellular body fluid. The amount of sodium in the fluid influences its volume, which in turn determines blood volume and blood pressure. Most of the solute reabsorbed in the proximal tubule is in the form of sodium bicarbonate and sodium chloride, and about 70% of the sodium reabsorption occurs here. Sodium reabsorption is tightly coupled to passive water reabsorption, meaning when sodium moves, water follows. The movement of water balances the osmotic pressure within or across the tubule walls, which maintains extracellular body fluid volume.

    Reabsorption in the early proximal convoluted tubule: The most essential substances in the filtrate are reabsorbed in the first half of the proximal convoluted tubule (early proximal tubule). These include glucose, amino acids, phosphate, lactate and citrate, which “piggy-back” on sodium co-transporters (membrane proteins that link the movement of two or more specific solutes together) that move sodium down its electrochemical gradient into tubule epithelial cells. For this to continue, the sodium gradient must be maintained, which means sodium cannot be allowed to build up inside the epithelial cells of the proximal tubule wall. This is achieved using:

    Sodium/potassium ATPase, a sodium pump (active transporter) located on the opposite side of the epithelial cell that takes care of this by moving three sodium ions out of the cell for reabsorption into the bloodstream, and pumping two potassium ions back into the cell (see diagram below).

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