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Tubular Function Notes

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Tubular Function The tubular system functions to transform a large glomerular filtrate into a small volume of urine with variable composition. This is achieved via reabsorption and secretion. The tubular wall consists of a single layer of epithelial cells in close proximity to the peritubular capillaries. They are joined by tight junctions. The tight junctions separate the apical membrane facing the lumen from the basolateral membrane at the base and lateral sides of the epithelial cells.

1. Tubular reabsorption Solutes and water are transported out of the tubular lumen and into the peritubular capillaries. Some substances, such as glucose and amino acids, are completely reabsorbed, whereas others are partially reabsorbed to regulate homeostasis. Whilst glomerular filtration is indiscriminate, tubular reabsorption is highly selective. Cells in different parts of the tubular system have different functions, and so different structures. In the proximal tubules, large amounts of water and solutes are reabsorbed. The cells in this region have numerous microvilli on the apical membrane which creates a large surface area for extensive transport. The epithelial cells here are also highly permeable to water and small ions. In the distal tubule and collecting ducts, there are fewer and smaller microvilli. The permeability of epithelial cells here is relatively low. The result is that substances are not easily reabsorbed, but secreted substances are well retained in the luminal fluid. There are several mechanisms of reabsorption. These include primary and secondary active reabsorption and passive reabsorption. a) Primary active reabsorption In primary active reabsorption, energy from ATP is directly coupled to the transport. Sodium reabsorption is via primary active transport. The active step is in the basolateral membrane which contains Na+/K+ pumps, which pump potassium into and sodium out of the cells. Energy for this is obtained through hydrolysis of ATP. The result is that sodium concentration in the interstitial fluid increases, and so sodium passively moves into the peritubular capillaries. The potassium transported into the cell easily leaks back into the interstitial fluid. The epithelial cells is therefore negatively charged relative to its surroundings. This helps promotes the diffusion of sodium into from the lumen into the epithelial cell, along with the concentration difference across the apical membrane.

b) Secondary active reabsorption Secondary active transport does not require energy directly from ATP. When sodium is transported in the kidney by carriers down its electrochemical gradient, energy is released. Secondary active transport utilises some of this energy for the simultaneous transport of another substance against its electrochemical gradient by the same carrier. In the proximal tubules, diffusion of sodium from the lumen into the epithelial cells occurs by both simple diffusion through ion channels and carrier-mediated transport. The carriers often carry both sodium and another molecules - co-transport. One example is a carrier that interacts with both sodium and glucose. Glucose is actively transported from the lumen into the epithelial cells by a symport exchanger without any extra enery expenditure. Some substances are co-transported in the opposite direction of their partner molecule. This is an antiport exchanger. For example, hydrogen ions are transported out of the epithelial cells whilst sodium is transported in. Glucose, amino acids and other organic substances are all reabsorbed by secondary active reabsorption. After they have been transported into the epithelial cell, they enter the peritubular capillaries by facilitated diffusion across the basolateral membrane.

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