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The key points covered of this video include:

1. Reabsorption in the Kidney Nephron
2. Reabsorption of Sodium
3. Reabsorption of Glucose and Amino Acids
4. Absorption of Water
5. Adaptations for Reabsorption

Reabsorption in the Kidney Nephron

Ultrafiltration produces filtrate some components of this filtrate are excreted and others are kept. The main solute that is excreted is urea while glucose, amino acids, water and some mineral ions are reabsorbed. Most of the reabsorbing occurs in the proximal convoluted tubule which reabsorbs around 85% of the fluid. The reabsorption occurs from the glomerular filtrate in the tubule into the capillaries. This process is called selective reabsorption. Selective reabsorption is the reabsorption of only some molecules from the glomerular filtrate into the blood.

Reabsorption of Sodium

The epithelial cells of the proximal convoluted tubule use active and secondary active transport to selectively reabsorb molecules. Active Transport is when substances are transported across a membrane against their concentration gradient using proteins and ATP. The epithelial cells of the proximal convoluted tubule have an apical side (the tubule side) and a basal side (the capillary side). This is done using ATP and so is described as active transport. This is done using ATP and so is described as active transport.

Reabsorption of Glucose and Amino Acids

On the apical side of the epithelial cell there are cotransporters. These only transport sodium in when they can also transport glucose or an amino acid in. Because sodium inside the cell is low the cotransporters are able to transport glucose and amino acids up their concentration gradient. This is called secondary transport because it doesn't directly use ATP but is only possible because of active transport of sodium. After sodium and glucose/amino acids enter the cell they are free to diffuse down their concentration gradient into the blood.

Reabsorption of Water

Since the filtrate contains lots of water from the blood it is useful to reabsorb some of it to reduce water loss. The solutes sodium, amino acids and glucose are pumped into tubular epithelial cells. This means the water potential in the tubular fluid is higher than the water potential in the epithelial cell and blood. As a result water follows the passage of solutes down its water potential gradient into the cell and then the blood. Most of the water reabsorption is done in the proximal convoluted tubule the rest is done in the collecting duct.

Adaptations for Reabsorption

The role of the epithelium of the proximal convoluted tubule is reabsorption it has specialisations which help it achieve this. The epithelial cells have protrusions on their apical side called microvilli which increase their surface area. This allows there to be many protein pumps and transporters on the membrane to maximise transport. Their basal side is also slightly infolded to maximise transport from the cell into the blood. The proximal convoluted tubule is also tangled to increase the distance along which reabsorption can occur. Finally, ATP directly and indirectly drives reabsorption so the cells have many mitochondria.

Summary

Some components of glomerular filtrate are reabsorbed whilst others (like urea) are excreted
Sodium ions are reabsorbed by actively transporting them into the blood
The low sodium ion concentration allows for the cotransport of amino acids and glucose into the cell
Water follows the passage of solutes down its water potential gradient into the blood
Cells of the proximal convoluted tubule are adapted with microvilli, infoldings, and many mitochondria for reabsorption