Title: Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology - WikiVet English
Headings: Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology
A Tribute to Nick Short
Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology
Contents
Introduction to Reabsorption
Proportion of Filtered Substances Reabsorbed in the Proximal Tubule
Epithelial Transport
Ions and Compounds
Sodium
Water
Potassium
Urea
Glucose
T Max and Splay
The Kidneys Role In Glucose Regulation
The Secretion of H+ and the Reabsorption of HCO3-
Reabsorption of HCO3-
Protein
Primary Active Secretion - Organic Acids and Bases
Calcium
Revision

Content: 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
Small proteins and peptide hormones are reabsorbed by endocytosis
Other substances are secreted or reabsorbed in the tubules passively by facilitated, non-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.