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    most water is reabsorbed from the filtrate by which region of the nephron?

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    Tubular Reabsorption

    Tubular Reabsorption

    LEARNING OBJECTIVES

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

    Describe how the renal tubules reabsorb useful solutes from the glomerular filtrate and return them to the blood.

    Describe how the nephron regulates water excretion.

    Explain the role of aldosterone and of atrial natriuretic factor in sodium and water balance.

    Describe the mechanism that maintains the medullary osmotic gradient.

    The process of producing urine occurs in three stages: filtration, reabsorption, and secretion. The physiologic goal is to modify the composition of the blood plasma and, in doing so, eliminate only waste in the form of urine. In the last section, we discussed filtrate formation.  Now, we will examine how most nutrients are selectively returned into the blood, and how the composition of urine is regulated.

    Reabsorption

    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. Reabsorption occurs in the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and to a lesser degree, 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. Most water is recovered in the proximal convoluted tubule, loop of Henle, and distal convoluted tubule. About 10 percent (about 18 L) reaches the collecting ducts. Antidiuretic hormone and aldosterone are responsible for regulating how much water is retained in urine.  The collecting ducts, under the influence of antidiuretic hormone, 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. Arrows pointing away from the tubule indicate substances that are returning to the blood.  Arrows pointing towards the tubule indicate additional substances being removed from the blood and moved into the filtrate.

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

    Substance Proximal convoluted tubule Loop of Henle Distal convoluted tubule 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 antidiuretic hormone; osmosis Variable amounts reabsorbed, controlled by antidiuretic hormone, 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 simple diffusion, facilitated diffusion, active transport, secondary active transport, and osmosis.

    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 simple diffusion in that it moves a substance down its concentration gradient. The difference is that it requires specific membrane transporters 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. Symport mechanisms move two or more substances in the same direction at the same time, whereas antiport mechanisms move two or more substances in opposite directions across the cell membrane.

    Source : courses.lumenlearning.com

    Water Absorption

    Water Absorption

    Water absorption and porosity influence the mechanical strength and nutrient and metabolic waste transport within the polymer matrix.

    From: 3D Bioprinting for Reconstructive Surgery, 2018

    Related terms:

    Secretion (Process)ExcretionOsmolalitySoluteUrineVasopressin ReleaseDiuresisSodium AbsorptionNephron

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    Tubular Reabsorption and Secretion

    Joseph Feher, in Quantitative Human Physiology (Second Edition), 2017

    Water Follows the Osmotic Pressure Gradient Through Water Channels

    Water reabsorption is by osmosis through water channels in the membrane. These water channels consist of a family of proteins called aquaporin. At least seven different aquaporin isoforms are expressed in the kidney. The proximal tubule has abundant AQP1 on the apical and basolateral membranes and AQP7 on the apical membrane of the late proximal tubule.

    The blood that flows through the peritubular capillaries that surround the proximal tubules originates from the efferent arterioles of cortical nephrons. This blood has passed through the glomerulus and has had a protein-free filtrate abstracted from it. Thus the protein concentration in the efferent arterioles is increased by removal of 20% of the plasma volume while leaving the proteins behind. Therefore, the peritubular capillaries contain plasma with a higher oncotic pressure. As Na+ is reabsorbed with other solutes, the concentration of osmolytes in the spaces between the cells increases, causing a local increase in the osmotic pressure in this space. Water moves in response to the high oncotic pressure of the peritubular capillaries and the slight hyperosmolarity of the lateral intracellular space, so that water flows across the basolateral membrane into the lateral intracellular space and into the interstitial space surrounding the capillaries, and from there into the peritubular capillaries. As water moves from the cell, it concentrates the cell contents so that the osmotic gradient is transferred to the apical membrane. Water moves from the tubular fluid into the cell in response to this gradient. The net effect is water reabsorption from the tubular fluid into the peritubular capillaries, caused by the increased oncotic pressure of the capillary blood and the active reabsorption of Na+ and other solutes (see Figure 7.4.11).

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    Figure 7.4.11. Mechanism of urea, water, and protein reabsorption in the proximal tubule. See text for details.

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    Sex Differences in Renal Physiology and Pathophysiology

    Carolyn M. Ecelbarger, in Sex Differences in Physiology, 2016

    Regulation of Renal Aquaporins

    Water reabsorption along the renal tubule is generally passive (does not directly require ATP/energy) to occur, but rather follows a concentration gradient created by the active (ATP utilizing) reabsorption of NaCl. Therefore, a primary determinant of water reabsorption by the renal tubule is the tubule’s permeability. One might expect a lipid bilayer, which constitutes the outer membrane of epithelial cells lining the renal tubule, to be fairly hydrophobic, and not allow efficient water entry/exit. This is useful, for example, in the TAL, for generating the cortico-medullary osmotic gradient to allow for eventual fine-tuning of water reabsorption. However, in the PT and distal tubule (including CD), numerous intramembrane hydrophilic proteins known as water channels or “aquaporins” increase the porosity of the lipid bilayer allowing for H2O molecules to be reabsorbed from the luminal fluid across the apical membrane, into the cell, and then across the basolateral membrane and back into the circulation. Aquaporins (AQPs), as a class, have six-membrane-spanning regions, with charged amino acids (hydrophilic) forming the actual water channel and intracellular amino and carboxy tails.

    AQP1 is the isoform found expressed in PT and thin limbs. Most reports suggest that this water channel is constitutively expressed at high levels allowing for reabsorption of a large number of water molecules in an unregulated fashion. A recent study by Herak-Kramberger et al. [26] demonstrated that male rats had greater expression of AQP1 in kidney (80% protein, 40% higher mRNA) as compared to females. Gonadectomy appeared to reduce expression in both sexes. It is unclear from this study if the number of channels is higher in males due to the greater length or size of the PT in male animals (as discussed in the early portion of this chapter), or if the number of channels per cell is different.

    AQP2 is the apical water channel expressed in the late DCT, CNT, and CD (cortical through inner medullary). AQP2 appears to be the most highly expressed mRNA transcript in the CCD, as assessed by deep-sequencing in rat [22]. Recent studies also show AQP2 expression in bladder, reproductive tissues, and brain. AQP2 transcription is upregulated by AVP [27]; therefore this provides one mode for increased permeability of the CD during periods of high AVP. A second mechanism involves translocation (trafficking) of existing AQP2 molecules from cytosolic sites into the apical membrane [27]. This mode of regulation, which may involve phosphorylation, can rapidly (in a matter of minutes) alter the water permeability of the membrane and increase water reabsorption [28]. A handful of studies have reported sex or sex hormone differences in the regulation of AQP2. Prepubertal levels of urinary AQP2 (a marker for renal levels) have been shown to be higher than postpubertal levels in a study conducted on healthy children of different ages [29]. Sharma et al. [30] reported a greater baseline protein expression of AQP2 in the kidney of female mice. However, in this study, females also experienced a greater downregulation of AQP2 on a high-fructose diet, as compared to male mice [30]. Kim et al. [31] demonstrated increased AQP2 expression in rat bladder in response to estradiol repletion in ovariectomized rats. Moreover, in support of greater expression in females, and with estradiol, Zou et al. [32] recently identified an estrogen-response element in the AQP2 gene promoter region.

    Source : www.sciencedirect.com

    Tubular reabsorption article (article)

    Renal system

    Tubular reabsorption 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).

    Source : www.khanacademy.org

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