Describe and explain the main function of the proximal convoluted tubule in filtration/absorption.

The proximal convoluted tubule is used for selective re-absorption of glucose, water, peptides and other nutrients from the tubule fluid back into the blood. The cells lining this tubule are ciliated, to ensure maximum surface area for reabsorption. One membrane side of the cell faces the tubule, the other faces the bloodstream. Inside the cell, sodium is pumped out into the bloodstream using active sodium/potassium pumps. This resulting lack of sodium creates a diffusion gradient within the cell. Sodium within the tubule fluid will diffuse along this gradient into the cell, acting as a cotransporter for other important molecules such as glucose or water. These molecules are brought into the cell along with sodium. Once inside, sodium is pumped out, whilst glucose and water diffuse out into the blood along their gradients. Selective re-absorption ensures important molecules such as water and glucose for respiration are retained rather than excreted, to be stored or used immediately.