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The permeability of the distal convoluted tubule and the collecting duct is controlled by ADH as it increases the permeability. An area of the brain called the hypothalamus can detect when the body is dehydrated (i.e. water potential is less than normal blood) and stimulate the pituitary gland to produce more ADH so that more water is reabsorbed into the blood in the collecting.

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The proximal convoluted tubule is the part of the nephron which the glomerular filtrate enters. This is also where molecules such as glucose, amino acids, vitamins, hormones, water and other soluble molecules are reabsorbed back into the blood. Some of them by active transport such as glucose and Na⁺. This allows (about 85% of the) H₂O to follow by osmosis as there is a concentration gradient.

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The loop of Henle has a:

-       Thin descending limb, which has a low permeability to ions and urea. It is highly permeable to water.

-       Thin ascending limb is impermeable to water but is to ions.

-       The ascending limb of loop of Henle reabsorbs these Na, K, Cl ions from the urine by secondary active transport.

-       The Cortical thick ascending limb drains urine into the distal convoluted tublule.

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The aims of the loop of henle is to reduce the volume of water and solutes within the urine but without changing the concentration. It tries to create a hypertonic medulla (this draws water out of from the thin descending limb of the loop of Henle and the collecting duct system to produce more concentrated urine) but to do this it has to create a hypertonic countercurrent -

this is maintained by the Vasa recta. 20% of the filtered sodium is reabsorbed due to this countercurrent that is located in the thick ascending limb.

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The thin descending limb has low permeability to urea and ions, but is highly permeable to water. There is a sharp bend in the renal medulla.
The thin ascending limb of the loop of Henle is permeable to ions, but impermeable to water.
The ascending limb of the loop of Henle reabsorbs sodium, potassium and chloride ions from the urine by secondary active transport.
The cortical thick ascending limb drains urine into the distal convoluted tubule.
The "thick" and "thin" terminology refers to the size of the epithelial cells.

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The terminal portion of the distal tubule empties through collecting tubules into a straight collecting duct in the medullary ray. The collecting duct system is under the control of antidiuretic hormone (ADH). When ADH is present, the collecting duct becomes permeable to water. The high osmotic pressure in the medulla (generated by the counter-current multiplier system/loop of Henle) then draws out water from the renal tubule, back to vasa recta.

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The renal collecting duct, composed of ureteric epithelia, is formed by successive branching of the ureteric bud. The duct extends from the periphery of the kidney to the inner medulla. Nephrons are linked via renal connecting tubules to the collecting duct, where the urine is collected. The urine is then drained from the collecting ducts into the pelvis, and from there to the bladder, via the ureter.

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The loop of Henle (LOH) is a long u shaped portion of the nephron which is part of the kidney. It descends into the medulla from the cortex. The loop of Henle is surrounded by the Vasa Recta, which is the vessel that carriers blood from the glomerulus to the renal vein. The transfer of H2O and NaCL from the LOH happens all along the loop simultaneously. NaCL is pumped out of the ascending limb into the tissue fluid, this is pumped out by active transport. Therefore a high concentration gradient is formed as the H2O is moved by osmosis into the medulla. The concentration of NaCL in the descending limb is greater than in the ascending limb as the NaCL leaves in the descending limb. The loop of Henle employs a counter current multiplier. This means that the fluid in and around the LOH becomes saltier as it goes down the loop. The salt levels are kept high in the medulla because of the counter current exchange mechanism. Thus these salty tissues create a concentration gradient between the tissue fluid and the urine in the collecting duct. The longer the loop the more NaCL moves into tissue fluid. The loop is also longer in organisms which live in the desert. This is so more water can be reabsorbed so the organisms are more water efficient. The vasa recta as well as receiving water and NaCL it will also supply oxygen and nutrients to the LOH which allows the cells and particularly the mitochondria to respire and produce ATP which allows active transport to take place.

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Gorgeous osmoregualtion - take note!

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Cortex - glomeruli

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Osmoregulation image

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The distal tubule consists of two functionally distinct regions. The early distal tubule is impermeable to water and urea and its permeability is not regulated by ADH. The late distal tubule has water channels and its permeability is regulated by ADH. The late distal tubule is also called the connecting duct because it connects the rest of the nephron to the collecting duct. As in most parts of the nephron, the Na+,K+-ATPase powers Na+reabsorption in the early distal tubule.

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Microvilli and active transport in the PCT

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Bowman's capsule Tues notes