Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration.
Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion , which is, in essence, the opposite process to tubular reabsorption.
The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis. This entire process is illustrated in Figure Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole.
The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate GFR is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.
To learn more about the vascular system of kidneys, click through this review and the steps of blood flow. Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones.
Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries.
However, every solute has a transport maximum and the excess is not reabsorbed. In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate.
The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, as illustrated in Figure The vasa recta around it acts as the countercurrent exchanger.
Loop diuretics are drugs sometimes used to treat hypertension. A side effect is that they increase urination. Why do you think this is the case? By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. If the body requires additional water, all of it can be reabsorbed at this point. Further reabsorption is controlled by hormones, which will be discussed in a later section.
Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct. Although parts of the renal tubules are named proximal and distal, in a cross-section of the kidney, the tubules are placed close together and in contact with each other and the glomerulus.
This allows for exchange of chemical messengers between the different cell types. For example, the DCT ascending limb of the loop of Henle has masses of cells called macula densa , which are in contact with cells of the afferent arterioles called juxtaglomerular cells. Together, the macula densa and juxtaglomerular cells form the juxtaglomerular complex JGC. The JGC is an endocrine structure that secretes the enzyme renin and the hormone erythropoietin.
When hormones trigger the macula densa cells in the DCT due to variations in blood volume, blood pressure, or electrolyte balance, these cells can immediately communicate the problem to the capillaries in the afferent and efferent arterioles, which can constrict or relax to change the glomerular filtration rate of the kidneys. A nephrologist studies and deals with diseases of the kidneys—both those that cause kidney failure such as diabetes and the conditions that are produced by kidney disease such as hypertension.
Blood pressure, blood volume, and changes in electrolyte balance come under the purview of a nephrologist. Nephrologists usually work with other physicians who refer patients to them or consult with them about specific diagnoses and treatment plans.
Patients are usually referred to a nephrologist for symptoms such as blood or protein in the urine, very high blood pressure, kidney stones, or renal failure.
Nephrology is a subspecialty of internal medicine. To become a nephrologist, medical school is followed by additional training to become certified in internal medicine. An additional two or more years is spent specifically studying kidney disorders and their accompanying effects on the body. The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the osmolarity of body fluids at mOsm.
They are surrounded by three layers and are made up internally of three distinct regions—the cortex, medulla, and pelvis. The nephron of the kidney is involved in the regulation of water and soluble substances in blood. A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity. It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone. The basic physiology of a nephron within a kidney : The labels are: 1. Glomerulus, 2. Efferent arteriole, 3. Proximal tube, 5. Cortical collecting tube, 6. Distal tube, 7. Loop of Henle, 8. Collecting duct, 9.
Peritubular capillaries, Arcuate vein, Arcuate artery, Afferent arteriole, and Juxtaglomerular apparatus. The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. A group of specialized cells known as juxtaglomerular apparatus JGA are located around the afferent arteriole where it enters the renal corpuscle.
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. This increase results in reabsorption of up to 15 percent of the water entering the nephron.
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 forming urine 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 portion is lined with simple cuboidal epithelium without a brush border. 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 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. This exchange is yet another means by which the body can buffer and excrete acid.
The presence of aquaporin channels in the descending loop allows prodigious quantities of water to leave the loop and enter the hyperosmolar interstitium of the pyramid, where it is returned to the circulation by the vasa recta.
As the loop turns to become the ascending loop, there is an absence of aquaporin channels, so water cannot leave the loop. At the transition from the DCT 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 forming urine enters the DCT.
The DCT will recover another 10—15 percent before the forming urine enters the collecting ducts. Peritubular capillaries receive the solutes and water, returning them to the circulation. 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. If the blood becomes hyperosmotic, the collecting ducts recover more water to dilute the blood; if the blood becomes hyposmotic, the collecting ducts recover less of the water, leading to concentration of the blood.
Another way of saying this is: If plasma osmolarity rises, more water is recovered and urine volume decreases; if plasma osmolarity decreases, less water is recovered and urine volume increases. This function is regulated by the posterior pituitary hormone ADH vasopressin. With mild dehydration, plasma osmolarity rises slightly.
This increase is detected by osmoreceptors in the hypothalamus, which stimulates the release of ADH from the posterior pituitary. If plasma osmolarity decreases slightly, the opposite occurs. When stimulated by ADH, aquaporin channels are inserted into the apical membrane of principal cells, which line the collecting ducts. As the ducts descend through the medulla, the osmolarity surrounding them increases due to the countercurrent mechanisms described above. If aquaporin water channels are present, water will be osmotically pulled from the collecting duct into the surrounding interstitial space and into the peritubular capillaries.
Therefore, the final urine will be more concentrated. If less ADH is secreted, fewer aquaporin channels are inserted and less water is recovered, resulting in dilute urine. By altering the number of aquaporin channels, the volume of water recovered or lost is altered.
This, in turn, regulates the blood osmolarity, blood pressure, and osmolarity of the urine.
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