6.7. Hormones and the Kidney:

• Renin increases the production of angiotensin II which is released when there is a fall in intravascular volume e.g. hemorrhage and dehydration. This leads to: Constriction of the efferent arteriole to maintain GFR, by increasing the filtration pressure in the glomerulus.
  
• Release of aldosterone from the adrenal cortex
  
• Increased release of ADH from
  
• the posterior pituitary
  
• Thirst
  
• Inotropic myocardial stimulation and systemic arterial constriction
  

The opposite occurs when fluid overload occurs.

• Aldosterone promotes sodium ion and water reabsorption in the distal tubule and collecting duct where Na+ is exchanged for potassium (K+) and hydrogen ions by a specific cellular pump. Aldosterone is also released when there is a decrease in serum sodium ion concentration
  
• Atrial Natruretic Peptide(ANP) is released when atrial pressure is increased e.g. in heart failure or fluid overload. It promotes loss of sodium and chloride ions and water chiefly by increasing GFR.
  
• Antidiuretic Hormone (ADH) increases the water permeability of the distal tubule and collecting duct, thus increasing the concentration of urine. In contrast, when secretion of ADH is inhibited, it allows dilute urine to be formed. This occurs mainly when plasma sodium concentration falls such as following drinking large quantities of water. This fall is detected by the osmoreceptors. The hormones interact when blood loss or dehydration occurs to maintain intravascular volume.
  
• Other Substances Produced by the kidney
  
• 1,25 dihydroxy vitamin D (the most active form vitamin D) which promotes calcium absorptionfrom the gut
  
• Erythropoietin which stimulates red cell production.