Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Anatomy & Physiology

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

Distal Tubule

Exchange in the Principal Cells of the Distal Tubule

Histology section showing the juxtaglomerular apparatus (© RVC 2008)

• Important site of regulation of ions and water
• Less emphasis on bulk transport compared with proximal tubule
• More emphasis on fine management
• It is able to do this as it has high resistance epithelia. Allowing it to maintain substantial gradients across it
• Very important for the homeostasis of:
  • Sodium
  • Potassium
  • Acid / Base

• There are two cell types present each with different functions. They are similar to the cells of the collecting ducts
  • Principal cells
    • Absorb sodium
    • Excrete potassium and hydrogen
    • Site of action of Aldosterone
  • Intercalated cells
    • ATP driven proton secretion

Juxtaglomerular Apparatus

• The terminal portion of the straight distal tubule contacts the afferent and efferent vessels supplying its own glomerulus
• These vessels are said to embrace the distal tubule
• Here a special apparatus called the Juxtaglomerular Apparatus has 3 different structures:
  • The tubular epithelial cells of the distal tubule which are in contact with the arterioles supplying the glomerulus of that nephron are called the macula densa. They play a vital role in the regulation of the GFR.
  • The Juxtaglomerular Cells are smooth muscle cells which adjoin the macula densa in the capillary wall.
  • The Extraglomerular Mesangium has an unclear function

Juxtaglomerular Cells

Function

• Smooth muscle cells
• Synthesise renin
• From the zymogen prerenin
• Described as intra-renal baroreceptors
  • Respond to stretch
• The secretion of renin from these cells depends on the stretch
  • If stretch decreases - blood pressure has decreased - renin secretion increases
• Also respond to the sympathetic nervous system
  • Increases renin output

Renin

• A proteolytic enzyme synthesised in the kidney
• Renin affects blood pressure indirectly
• Part of the first stage of the renin-angiotensin-aldosterone-system
  • Its effects are through its activation of this system

Developmental

Develops from metanephric tubule

Collecting Duct

Exchange in the Principal Cells of the Collecting Duct

This part of the nephron has two cell types

Histology section of the collecting duct showing the close proximity of the loop of henle (© RVC 2008)

Principal Cells

• ADH acts on these cells inserting aquaporins into the cell membranes
• It is released from the posterior pituitary gland

Intercalated cells

• The intercalated cells can be subdivided further to:
  • Alpha intercalated cells secrete H⁺
  • Beta intercalated cells secrete HCO₃^-

Developmental

• Develops from branched ureteric bud

The Concentrating Mechanism, Aquaporins and ADH

Water is drawn from the lumen of the tubule by the increasing hypertonicity of the surrounding tissue as the duct makes its way deeper into the medulla. However this reabsorption is only possible thanks to ADH inserting aquaporins into the apical membrane. These channels are always present on the basolateral membrane of the epithelial cells but not on the apical membrane. The reabsorption would not be possible if the urine did not go back up the thick ascending limb of the loop of henle, where its concentration was decreased by the reabsorption of salt, but instead went straight into the collecting ducts. Although this would mean very concentrated urine it would result in massive salt losses. Thus the collecting duct allows for very concentrated urine with minimal salt loss. It also allows the concentration of the urine to vary from dilute to concentrated under the control of the hypothalamus and ADH concentrations.

Ions and Compounds

The following ions and compounds are reabsorbed or secreted partly or completely in the distal tubules and collecting ducts:

Sodium

The remaining 10% is reabsorbed in the distal tubule. Although the transport capacity here is minimal the high resistance epithelia mean that very little back flow into the lumen occurs allowing the remaining sodium to be easily reabsorbed. Some of this reabsorption takes place using a sodium chloride co-transporter. The reabsorption of sodium from here and the collecting ducts is under hormonal control allowing "fine tuning" to occur.

See:

• Aldosterone
• Atrial Natriuretic Peptide

Potassium

Potassium Transport in the Principal Cells of the Collecting Duct

Distal Tubule

• Here the potassium and chloride symporter actually excretes the two ions into the lumen of the tubule
• Na⁺ / K⁺ ATPases in the basolateral membrane move potassium into epithelial cells from the intercellular spaces in order to remove sodium. The potassium then diffuses into the lumen via channels
• The ability to secrete potassium here is vital in animals with high potassium diets such as herbivores where excreting only the amount filtered would not be suffice

Principal Cells of Collecting Duct

• Potassium transported into lumen
• Is both absorbed and secreted via paracellular route
• Na⁺ / K⁺ ATPases in the basolateral membrane move potassium into epithelial cells from the intercellular spaces in order to remove sodium and keep potassium concentrations high.
  • Aldosterone works here

Intercalated Cells of Collecting Duct

• Potassium is absorbed here in exchange for hydrogen

H⁺ and HCO₃^-

• H⁺ ions are secreted in the cortical and medullary collecting tubules
• This is an active secretion using a H⁺ ATPase
• Also a H⁺/K⁺ ATPase antiporter
• The main function of this is maintaining potassium
• HCO₃^- is reabsorbed here

Urea

In the lower portion of the collecting duct urea freely diffuses across the epithelia. The permeability is also increased with ADH. This adds to the hypertonicity of the medulla and increases water reabsorption.

Urea and Water Reabsorption

Water follows urea as it is reabsorbed from the collecting duct. This actually increases the concentration of urea in the tubular fluid despite some of it being reabsorbed. This means that urea concentration only drops moderately when the urine concentration falls due to increased reabsorption of water.

Calcium

The regulation of how much calcium is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts. Parathyroid hormone stimulates reabsorption. It is normal for a 1-2% excretion of calcium to occur