Osmoregulation is the process by which the osmotic pressure in the urine is regulated by the concentration of water and ions. This takes place between the tubules of the nephrons and the surrounding capillaries known as peritubular capillaries.
Reabsorption in the nephron may be either a passive or active process, and the specific permeability of each part of the nephron varies considerably in terms of the amount and type of substance reabsorbed. The mechanisms of reabsorption into the peritubular capillaries include:
1. Passive diffusion—passing through plasma membranes of the kidney epithelial cells by concentration gradients.
2. Active transport—membrane-bound ATPase pumps (such as NA+/K+ ATPase pumps) with carrier proteins that carry substances across the plasma membranes of the kidney epithelial cells by consuming ATP.
3. Cotransport—this process is particularly important for the reabsorption of water. Water can follow other molecules that are actively transported, mainly glucose and sodium ions in the nephron.
As the filtrate passes through the nephron, its osmolarity (ion concentration) changes as ions and water are reabsorbed. The filtrate entering the proximal convoluted tubule is 300 mOsm/L, which is the same osmolarity as normal plasma osmolarity.
In the proximal convoluted tubules, all the glucose in the filtrate is reabsorbed, along with an equal concentration of ions and water (through cotransport), so that the filtrate is still 300 mOsm/L as it leaves the tubule. The filtrate osmolarity drops to 1200 mOsm/L as water leaves through the descending loop of Henle, which is impermeable to ions. In the ascending loop of Henle, which is permeable to ions but not water, osmolarity falls to 100–200 mOsm/L.
Finally, in the distal convoluted tubule and collecting duct, a variable amount of ions and water are reabsorbed depending on the hormonal stimulus. The final osmolarity of urine is therefore dependent on whether or not the final collecting tubules and ducts are permeable to water or not, which is regulated by homeostasis.
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Key Points
• Reabsorption includes passive diffusion, active transport, and cotransport.
• Water is mostly reabsorbed by the cotransport of glucose and sodium.
• Filtrate osmolarity changes drastically throughout the nephron as varying amounts of the components of filtrate are reabsorbed in the different parts of the nephron.
Key Terms
NA+/K+ ATPase: An ATPase pump that consumes ATP to facilitate the active transport of ions in the filtrate of the nephron.
Peritubular capillaries: The capillaries through which components of filtrate are reabsorbed from the lumen of the nephron.
Nephron: structural and functional unit of the kidney
Plasma membranes: the phospholipid bilayer surrounding most cells and tissues
Loop of Henle: a structure in a kidney’s nephron that connects the proximal convoluted tubule to the distal convoluted tubule.
Proximal convoluted tubule: involved in the resorption of sugar, sodium and chloride ions, and water from the glomerular filtrate
Distal convoluted tubule: have an essential role in the absorption of many ions, and in water reabsorption
Cotransport: when water can follow other molecules that are actively transported over a membrane
Osmolarity (Osm/L): is the total concentration of all solutes in the solution.