Functions of the Distal Convoluted Tubules (DCTs) - PDF

# Functions of the Distal Convoluted Tubules (DCTs) - The DCTs receive hypotonic fluid from the ascending limbs of the loops of Henle. - Functionally, they are divided into two parts: - **The initial part:** - Has the same characteristics as the thick segment of the ascending limbs of l...

# Functions of the Distal Convoluted Tubules (DCTs) - The DCTs receive hypotonic fluid from the ascending limbs of the loops of Henle. - Functionally, they are divided into two parts: - **The initial part:** - Has the same characteristics as the thick segment of the ascending limbs of loop of Henle. - Is almost impermeable to both water and solutes (urea, $Na^+$ and Cl). - $Na^+$ is reabsorbed by primary active transport, and Cl follows it passively. - The tubular fluid becomes more hypotonic (about 100 mOsm/liter). - This part is called the diluting segment of the nephron. - The initial parts of DCTs secrete $H^+$ mainly by secondary active transport. - **The late part:** - Performs the following functions: - **Reabsorption:** - $Na^+$ reabsorption: - Occurs by primary active transport. - Is controlled mainly by aldosterone hormone. - Is followed by passive reabsorption of Cl and water. - About only 5% of the filtered water is reabsorbed in the DCTs as they are poorly permeable to water. - There is no urea reabsorption because the DCTs are normally poorly permeable to urea. - The fluid delivered from the DCTs into the collecting ducts is hypotonic. - $Ca^{++}$ reabsorption: - Also occurs by primary active transport. - Increased by the parathyroid hormone. - **Secretion:** - $H^+$ secretion: - Occurs mainly by secondary active transport by $Na^+-H^+$ antiport carrier. - However, certain cells called intercalated, dark or brown cells start to appear in this segment (and become more abundant in the collecting ducts). - These cells secrete $H^+$ independent of $Na^+$(by primary active transport) against high concentration gradient by specific uniport carrier protein and $H^+-ATPase$. - **Secretion of buffers for excess $H^+$ in the DCTs:** - The kidney often excretes urine at pH as low as 4.5 in acidosis or as high as 8 in alkalosis. - In acidosis, the urine is buffered by the following buffer systems to prevent marked decrease of pH below 4.5: - **Bicarbonate buffer:** - $HCO_3^-$ ions and $H^+$ are normally titrate each other in the tubules mainly in the PCTs. - The remaining excess $H^+$ in the tubular fluid is buffered by the phosphate and ammonia buffers. - **Phosphate buffer:** - This buffer is much more powerful in the tubular fluid than in blood (due to its high concentration in the urine). - **Ammonia buffer** - $K^+$ secretion: - Occurs actively as follows: - $K^+$ is transported inside the tubular cells by the $Na^+-K^+$ pump at their basolateral borders. - Then, it is secreted by counter-transport mechanism at their luminal borders of the principle cells (which start to appear in this segment), into the tubular fluid in exchange for $Na^+$ reabsorption (utilizing an antiport carrier). - $K^+$ secretion in the DCTs and cortical collecting ducts is increased by: - Increase the extracellular $K^+$ level. - Increase aldosterone level. - $K^+$ and $H^+$ compete for secretion in the DCTs and cortical collecting ducts. - An increase of any of these ions in the tubular cells favors its secretion. # Acidification of Urine ($H^+$ Secretion) by the Renal Tubules - Urine acidification occurs by secretion of $H^+$ into the tubular lumen. - This mostly occurs in the PCTs, DCTs and collecting ducts. - To lesser extent in the ascending limbs of loop of Henle. - It occurs as following: - $H^+$ is formed inside the renal tubular cells as a result of dissociation of $H_2CO_3$ (The later is formed by combination of $CO_2$ and $H_2O$ under the influence of C.A enzyme). - Then $H^+$ is secreted by either: - **Secondary active transport:** - In exchange for $Na^+$ reabsorption (i.e., by counter-transport utilizing an antiport carrier). - It occurs in the PCTs, loop of Henle, DCTs, and collecting ducts. - **Primary active transport:** - This occurs against high $H^+$ concentration gradient by the intercalated cells. - These cells are abundant in the late parts of the DCTs and the collecting ducts. - It occurs only in the late DCTs and collecting tubules. # Functions of the Collecting Ducts (CTs) - The collecting ducts receive hypotonic fluid from the DCTs. - Functionally, they are divided into two parts: - Cortical part. - Medullary part. - They perform the following functions: - **Reabsorption:** - $Na^+$ reabsorption: - This occurs by primary active transport all over the CDs. - It increases by aldosterone only in the cortical CDs. - It is followed by passive diffusion of Cl and water. - It is coupled with $K^+$ secretion. - Urea reabsorption: - This occurs by passive diffusion only in the inner parts of the medullary CDs (because these parts are partially permeable to urea), especially in the presence of ADH. - Urea is not reabsorbed in both cortical CDs and the outer parts of medullary CDs (because these parts are impermeable to urea). - Water reabsorption in the CDs: - The CDs are relatively impermeable to water in absence of ADH. - However, in the presence of ADH the CDs become highly permeable to water (due to activation of water channels in that segment which are called aquaporins [AQP2 and AQP3]). - Water reabsorption in the CDs and to some extent also in the DCTs is called facultative water reabsorption (because it depends on the blood level of ADH). - Water reabsorption in the cortical CDs occurs as following: - The cortical CDs receive hypotonic fluid from the DCTs. - At the normal rate of ADH secretion - about 10% of the filtered water is passively reabsorbed into the iso-osmotic cortical interstitium in excess of $Na^+$ reabsorption. - So, the tubular fluid becomes isotonic at the end of the cortical CDs. - Water reabsorption in the medullary CDs occurs as following: - This isotonic fluid enters the medullary CDs. - Then, an additional 4.7% of $H_2O$ is reabsorbed by the hyperosmotic medullary interstitium. - **Secretion** - $K^+$ secretion: - This occurs only in the cortical CDs in exchange for $Na^+$ reabsorption. - It is increased by the aldosterone hormone. - $H^+$ secretion: - It is secreted all over the CDs: - In the cortical CDs - it occurs by both primary and secondary active transport. - In the medullary CDs - it occurs mainly by primary active transport. - Secretion of buffers: As in DCTs. # N.B.1: Summary of Water Reabsorption Through the Different Parts of the Renal Tubules - Normally, about 99.7% of the filtered water is reabsorbed in the renal tubules as follows: - 65% in the PCTs. - 15% in the descending limb loop of Henle. - 5% in the DCTs. - 10% in the cortical CDs. - 4.7% in the medullary CDs. - Only 0.3% of the glomerular filtrations excreted producing about 1.5 liters urine daily with osmolarity about 400 mOsm/liter. # Urine Concentration and Dilution - **(1) Mechanism of urine concentration:** - This occurs in: - Hypovolemia (e.g., in dehydration and hemorrhage). - Blood hypertonicity (e.g., due to excessive salt intake). - This mechanism depends only on the facultative water reabsorption in the CDs which is determined by two main factors: - **Blood level of ADH:** - When it increases in (hypovolemia or hypertonicity) - increase the permeability for water reabsorption in the CDs and to the little extent the late part of DCTs. - Increase ADH secretion - larger part of the medullary CDs become water-permeable so, more water is reabsorbed resulting in excretion of concentrated urine. - **The hyperosmolarity of medullary interstitium:** - It is increased by countercurrent mechanism which causes passive diffusion of water from the CDs into the medulla. - The net effect is excretion of small volume of concentrated urine = (0.5 liter daily with an osmolarity about 1400 mOsm/liter). - **(2) Mechanism of urine dilution:** - This occurs in: - Hypervolemia. - Blood hypotonicity. - The following occurs: - **Inhibition of secretion ADH:** - This decreases the water permeability of the CDs - decrease the reabsorption of water. - **Decrease the osmolarity of medullary interstitium:** - The net effect is the excretion of large volume of diluted urine with an osmolarity less than 80 mOsm/liter. - e.g. in diabetes insipidus - complete absence of ADH - so, the urine volume 23.3 liter/day with osmolarity is about 30 mOsm/liter. # Diuresis - **Definitions** - Diuresis means increasing the rate of urine output. - **Types (methods of diuresis:** - **Water Diuresis:** - This is produced by drinking of a large amount of water which increases the urine volume after about 15 minutes. - The maximal diuresis occurs within 40-45 min. - While, the ingested amount is completely excreted after 2 hours. - **Osmotic diuresis:** - This is produced by administration of osmotically active substances that are not readily absorbed in the PCTs. - e.g. mannitol & sucrose. - **Difference between water and osmotic diuresis:** | | Water diuresis | Osmotic diuresis | |-----------------|------------------------------------------|---------------------------------------------------| | Water reabsorption | Facultative water reabsorption only ↓ | Obligatory & facultative water reabsorption are ↓ | | ADH secretion | Normal | Normal | | $Na^+$ excretion | Normal | ↑ | | Tonicity of urine | Hypotonic | Isotonic | - **Pressure diuresis:** - The changes in the ABP within the autoregulation level have a little effect on GFR with parallel changes in urine volume. - Marked increase in ABP - increase urine output. - Drop in the mean ABP below 50mmHg - stop urine formation.