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:

= Water

H⁺ and HCO₃^-

Organic Acids and Bases