Which substances are reabsorbed into the blood




















A three stage process occurs in each nephron: filtration, selective reabsorption and finally excretion. The glomerulus filters the blood and removes water, glucose, salts and waste urea from it. The blood is under high pressure at the start of the nephron, which aids the filtration of the blood. These waste substances all pass from the capillaries in the glomerulus into the Bowman's capsule. This purifies the blood. These waste substances then move from the Bowman's capsule towards the loop of Henle.

Proteins are too large to pass through here and so remain in the blood. This process is called filtration. The kidneys must now reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine.

Solutes and water recovered from these loops are returned to the circulation by way of the vasa recta. The majority of the descending loop is comprised of simple squamous epithelial cells; to simplify the function of the loop, this discussion focuses on these cells. The thin segment of the nephron loop has membranes with permanent aquaporin channel proteins that allow unrestricted movement of water from the tubule into the surrounding interstitium.

About 15 percent of the water found in the original filtrate is reabsorbed here. Most of the solutes that were filtered in the glomerulus have now been recovered along with a majority of water, about 82 percent. As the filtrate enters the ascending loop, major adjustments will be made to the concentration of solutes to create what you perceive as urine. The ascending loop is made of very short thin and longer thick portions. Once again, to simplify the function, this section only considers the thick portion.

The thick segment is lined with simple cuboidal epithelium without a brush border that is completely impermeable to water due to the absence of aquaporin proteins. These are found between cells of the ascending loop, where they allow certain solutes to move according to their concentration gradient.

Therefore, in comparison to the lumen of the loop, the interstitial space is now a negatively charged environment. The presence of aquaporin channels in the thin segment of the descending loop allows large quantities of water to leave the loop and enter the hyperosmolar interstitium, and ultimately, the circulation by the vasa recta. As the loop turns to become the thick segment of the ascending loop, there is an absence of aquaporin channels, so water cannot leave the loop.

The structure of the loop of Henle and associated vasa recta create a countercurrent multiplier system. The countercurrent term comes from the fact that the descending and ascending loops are next to each other and their fluid flows in opposite directions countercurrent.

In addition, collecting ducts have urea pumps that actively pump urea into the interstitial spaces. Ammonia NH 3 is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH 2 , is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes. Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts.

At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct. The amino acid glutamine can be deaminated by the kidney. Ammonia and bicarbonate are exchanged in a one-to-one ratio. T his exchange is yet another means by which the body can buffer and excrete acid.

At the transition from the distal convoluted tubule to the collecting duct, about 20 percent of the original water is still present and about 10 percent of the sodium. If no other mechanism for water reabsorption existed, about 20—25 liters of urine would be produced. Now consider what is happening in the adjacent capillaries, the vasa recta. They are recovering both solutes and water at a rate that preserves the countercurrent multiplier system.

In general, blood flows slowly in capillaries to allow time for exchange of nutrients and wastes. In the vasa recta particularly, this rate of flow is important for two additional reasons. The flow must be slow to allow blood cells to lose and regain water without either crenating or bursting. Approximately 80 percent of filtered water has been recovered by the time the dilute filtrate enters the distal convoluted tubule.

The distal convoluted tubule will recover another 10—15 percent before the filtrate enters the collecting ducts.

Peritubular capillaries receive the solutes and water, returning them to the circulation. Receptors for parathyroid hormone are found in distal convoluted tubule cells and when bound to parathyroid hormone, induce the insertion of calcium channels on their luminal surface.

Finally, calcitriol 1,25 dihydroxyvitamin D, the active form of vitamin D is very important for calcium recovery. These binding proteins are also important for the movement of calcium inside the cell and aid in exocytosis of calcium across the basolateral membrane. Solutes move across the membranes of the collecting ducts, which contain two distinct cell types, principal cells and intercalated cells.

A principal cell possesses channels for the recovery or loss of sodium and potassium. An intercalated cell secretes or absorbs acid or bicarbonate.

As in other portions of the nephron, there is an array of micromachines pumps and channels on display in the membranes of these cells. Regulation of urine volume and osmolarity are major functions of 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. 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 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.

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. Even after filtration has occured, the tubules continue to secrete additional substances into the tubular fluid. This enhances the kidney's ability to eliminate certain wastes and toxins. It is also essential to regulation of plasma potassium concentrations and pH.



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