Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology

()Map REABSORPTION AND SECRETION ALONG THE NEPHRON (Map)

Introduction to Reabsorption

• The proximal tubule is a major site for reabsorption and some secretion.
• Gradients are small across the epithelium so tight regulation is not possible
• This occurs in the distal tubule
• 65-80% of the filtrate is reabsorbed
• Most reabsorption is coupled to sodium ion movement

A sodium pottasium ATPase. (Courtesy of Phi-Gastrein)

• Small proteins and peptide hormones are reabsorbed by endocytosis
• Other substances are secreted or reabsorbed in the tubules passively by facilitated, none facilitated diffusion or ion channels down chemical or electrical gradients.
• Other substances are secreted or reabsorbed by active transport against such gradients
  • Movement is via ATPases.

• The reabsorbed material enters peritubular capillaries
• This is mainly driven by the Sodium/Pottasium ATPase in the basolateral membrane.
  • This protein removes sodium from the cell maintaining the gradient between the lumen and the epithelium.
• Sodium reabsorption is driven by this protein
• Water and chloride then follow the transported sodium
• This is the most important transport mechanism in the proximal tubule

Proportion of Filtered Substances Reabsorbed in the Proximal Tubule

Substance	% of Filtrate Reabsorbed
Sodium and Water	~66%
Organic solutes e.g. glucose and amino acids	~100%
Potassium	~65%
Urea	~50%
Phosphate	~80%

Epithelial Transport

Sodium is the most important ion in relation to reabsorption from the proximal tubule. Water and chloride follow sodium passively and many other ions, compounds and molecules are absorbed through co-transporters with sodium. However it is vital that intracellular levels of sodium remain low to favour this reabsorption so it falls to the sodium/potassium ATPase and sodium pump to remove sodium from the cell. Also to a lesser extent active transport of protons (H⁺). As well as directly sodium linked transport secondary active transport also plays a part however this does tend to be powered by sodium movement. Passive transport also has a role

Transport capacity is well above what is needed for normal plasma concentrations to ensure that adequate absorption occurs and that there is little/no wastage. As you are getting transport of sodium, chloride and water simultaneously the concentration does not increase along the proximal tubule. However the volume of filtrate does decrease

Ions and Compounds

The following ions and compounds are reabsorbed or secreted partly or completely in the proximal tubule:

Sodium

The majority (70%) of sodium is reabsorbed in the proximal tubule. It is reabsorbed into the cytosol of the epithelial cells either alone by Diffusion through ion channels followed by water and chlorine or together with another product using a co-transporter.

To maintain the concentration gradients and allow the diffusion to continue it is essential that sodium is not allowed to build up within the cell. This is the job of the sodium/potassium ATPase Pump and is an example of primary active transport. This pump removes sodium from the cell and puts potassium in. This creates a high concentration of potassium within the cell but this is corrected because their are also potassium ion channels in the basolateral membrane which allow potassium to diffuse back into the interstitium. Because both sodium and potassium are leaving the cell the net effect is that the tubular cells are negatively charged. This creates an electro gradient which further increases sodium uptake from the cells. The combined electrochemical gradient is very large allowing for great amounts of sodium to be reabsorbed.

Sodium is then able to move from the interstial fluid into the blood thanks to a combination of the blood having a low hydrostatic pressure and a high protein osmotic pressure. These conditions exist thanks to the selective filtration of water, ions and glucose but the selective obstruction of proteins and promote the reabsorption of water and the associated dissolved ions within it back into the blood.

Potassium

• Potassium is absorbed mainly by the paracellular route with water via osmosis
• Na⁺ / K⁺ ATPases in the basolateral membrane move potassium into epithelial cells from the interstitial spaces in order to remove sodium
• Potassium is then cleared from the cells using a co-transporter with chlorine
• No potassium excretion occurs here due to the lack of potassium ion channels in the apical membrane

Urea

50% of filtered urea is reabsorbed in the proximal tubule. However the concentration of urea actually increases thanks to the reabsorption of 70% of the filtered water in the same portion of the nephron. Urea is not able to be reabsorbed from this point until it reaches the lower portion of the collecting duct therefore its concentration further increases with the reabsorption of water.

Calcium

Half the plasma calcium is bound to proteins so it is only the ionised form which is available for filtration. Reabsorption of calcium occurs in the proximal tubule however the regulation of how much is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts.

H⁺ and HCO₃^-

Organic Acids and Bases