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. 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.
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. Both mechanisms may utilize concentration gradients maintained by ATP pumps.
The glucose molecule then diffuses across the basal membrane by facilitated diffusion into the interstitial space and from there into peritubular capillaries. In the case of urea, about 50 percent is passively reabsorbed by the PCT. More is recovered by in the collecting ducts as needed. ADH induces the insertion of urea transporters and aquaporin channel proteins.
The renal corpuscle filters the blood to create a filtrate that still contains many important molecules that the body needs to reclaim. The PCT reclaims more of these than any other portion of the nephron. The cells of the PCT have two surfaces: apical faces the lumen of the tubule and is in contact with the filtrate.
The basal surface of the PCT cell faces the interstitial space near the peritubular capillary. Sodium is actively pumped by the PCT cells into the interstitial space and diffuses down its concentration gradient into the peritubular capillary. As it does so, water follows passively by osmosis. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Glucose leaves the cell to enter the interstitial space by facilitated diffusion.
The numbers and particular types of pumps and channels vary between the apical and basilar surfaces Table Some molecules do not require cellular transport proteins but instead move between adjacent cell membranes paracellular across the tubule and back into the blood. Almost percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded transport maximum, or Tm like that seen with diabetes mellitus.
The significant recovery of solutes from the PCT lumen to the interstitial space creates an osmotic gradient that promotes water recovery. Much of the relevant information is included in the reading. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded transport maximum, or T m.
Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine.
Recovery of bicarbonate HCO 3 — is vital to the maintenance of acid—base balance, since it is a very powerful and fast-acting buffer. Reabsorption is a finely tuned process that is altered to maintain homeostasis of blood volume, blood pressure, plasma osmolarity, and blood pH. Reabsorbed fluids, ions, and molecules are returned to the bloodstream through the peri-tubular capillaries, and are not excreted as urine.
Tubular secretion : Diagram showing the basic physiologic mechanisms of the kidney and the three steps involved in urine formation. Namely filtration, reabsorption, secretion, and excretion. Reabsorption in the nephron may be either a passive or active process, and the specific permeability of the each part of the nephron varies considerably in terms of the amount and type of substance reabsorbed.
The mechanisms of reabsorption into the peri-tubular capillaries include:. These processes involve the substance passing though the luminal barrier and the basolateral membrane, two plasma membranes of the kidney epithelial cells, and into the peri-tubular capillaries on the other side.
Some substances can also pass through tiny spaces in between the renal epithelial cells, called tight junctions. 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. 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.
Both mechanisms may utilize concentration gradients maintained by ATP pumps. The glucose molecule then diffuses across the basal membrane by facilitated diffusion into the interstitial space and from there into peritubular capillaries. In the case of urea, about 50 percent is passively reabsorbed by the PCT. More is recovered by in the collecting ducts as needed. ADH induces the insertion of urea transporters and aquaporin channel proteins. The renal corpuscle filters the blood to create a filtrate that differs from blood mainly in the absence of cells and large proteins.
From this point to the ends of the collecting ducts, the filtrate or forming urine is undergoing modification through secretion and reabsorption before true urine is produced. The first point at which the forming urine is modified is in the PCT. Here, some substances are reabsorbed, whereas others are secreted. Water and substances that are reabsorbed are returned to the circulation by the peritubular and vasa recta capillaries.
It is important to understand the difference between the glomerulus and the peritubular and vasa recta capillaries. The glomerulus has a relatively high pressure inside its capillaries and can sustain this by dilating the afferent arteriole while constricting the efferent arteriole. This assures adequate filtration pressure even as the systemic blood pressure varies. Movement of water into the peritubular capillaries and vasa recta will be influenced primarily by osmolarity and concentration gradients.
Sodium is actively pumped out of the PCT into the interstitial spaces between cells and diffuses down its concentration gradient into the peritubular capillary.
As it does so, water will follow passively to maintain an isotonic fluid environment inside the capillary. More substances move across the membranes of the PCT than any other portion of the nephron. Antiport, active transport, diffusion, and facilitated diffusion are additional mechanisms by which substances are moved from one side of a membrane to the other. Recall that cells have two surfaces: apical and basal.
The apical surface is the one facing the lumen or open space of a cavity or tube, in this case, the inside of the PCT. The basal surface of the cell faces the connective tissue base to which the cell attaches basement membrane or the cell membrane closer to the basement membrane if there is a stratified layer of cells. In the PCT, there is a single layer of simple cuboidal endothelial cells against the basement membrane.
The numbers and particular types of pumps and channels vary between the apical and basilar surfaces. Most of the substances transported by a symport mechanism on the apical membrane are transported by facilitated diffusion on the basal membrane. Almost percent of glucose, amino acids, and other organic substances such as vitamins are normally recovered here. Some glucose may appear in the urine if circulating glucose levels are high enough that all the glucose transporters in the PCT are saturated, so that their capacity to move glucose is exceeded transport maximum, or T m.
Though an exceptionally high sugar intake might cause sugar to appear briefly in the urine, the appearance of glycosuria usually points to type I or II diabetes mellitus. The transport of glucose from the lumen of the PCT to the interstitial space is similar to the way it is absorbed by the small intestine. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it.
Glucose leaves the cell to enter the interstitial space by facilitated diffusion. Recovery of bicarbonate HCO 3 — is vital to the maintenance of acid—base balance, since it is a very powerful and fast-acting buffer.
An important enzyme is used to catalyze this mechanism: carbonic anhydrase CA. This same enzyme and reaction is used in red blood cells in the transportation of CO 2 , in the stomach to produce hydrochloric acid, and in the pancreas to produce HCO 3 — to buffer acidic chyme from the stomach. In the kidney, most of the CA is located within the cell, but a small amount is bound to the brush border of the membrane on the apical surface of the cell. This is enzymatically catalyzed into CO 2 and water, which diffuse across the apical membrane into the cell.
Water can move osmotically across the lipid bilayer membrane due to the presence of aquaporin water channels. Inside the cell, the reverse reaction occurs to produce bicarbonate ions HCO 3 —. Note how the hydrogen ion is recycled so that bicarbonate can be recovered. The significant recovery of solutes from the PCT lumen to the interstitial space creates an osmotic gradient that promotes water recovery.
As noted before, water moves through channels created by the aquaporin proteins. These proteins are found in all cells in varying amounts and help regulate water movement across membranes and through cells by creating a passageway across the hydrophobic lipid bilayer membrane.
Changing the number of aquaporin proteins in membranes of the collecting ducts also helps to regulate the osmolarity of the blood. The movement of many positively charged ions also creates an electrochemical gradient. This charge promotes the movement of negative ions toward the interstitial spaces and the movement of positive ions toward the lumen.
The loop of Henle consists of two sections: thick and thin descending and thin and thick ascending sections. The loops of cortical nephrons do not extend into the renal medulla very far, if at all. Juxtamedullary nephrons have loops that extend variable distances, some very deep into the medulla.
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