Osmolalities of intracellular and extracellular fluid is the same, but the composition of fluid is different.

Water is freely permeable between intracellular extracellular compartments.

2/3 intracellular fluid  (28 liters) and 1/3 extracellular fluid (14 liters)

Extracellular fluid is interstitial and vascular space

Extracellular fluid is predominantly salt solution.

• Nephron
  • 1 million nephrons
  • Glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting ducts.
  • Juxtaglomerular apparatus (JGA) : macula densa (Terminal portion of loop of henle) afferent and efferent vessels and mesangial cells.
  • JGA is the site of Renin synthesis and secretion
• Glomerulus: 
  • Afferent and efferent arterioles with interposed capillaries enclosed by Bowman's capsule.
  • Glomerular filtration: 180 liters in 24 hours
  • Plasma with solutes except cells and protein
  • Calculation of glomerular filtration rate
• Proximal tubule
  • 2/3 of filtrate reabsorbed
  • glucose, amino acids and other organic solutes are completely absorbed
  • Significant amount of phosphate re absorbed
  • Calcium, water absorbed in parallel with sodium
  • Active secretion of H+ secretion and reclamation of most of filtered bicarbonate
  • Secretion of organic acids like uric acid and drugs like penicillin.
• Loop of Henle
  • descending limb, ascending limb, macula densa
  • 15% of volume absorption
  • 25% of sodium chloride absorption
  • Differential absorption of water and sodium chloride, enables the loop to concentrate or dilute urine.
  • Calcium absorbed in thick medullary ascending limb, not regulated by PTH
  • PTH controls Calcium absorption from cortical thick ascending limb
  • Passive absorption of water and sodium in ascending limb
  • active absorption of sodium from thick ascending limb, to maintain interstitial hypertonicity
  • Cortex 300 mOsm and medulla 1200 mOsm/kgm H2O.
• Distal nephron
  • Distal convoluted tubule in cortex water impermeable
    • Active sodium absorption
    • Potassium and H+ secretion
    • Calcium absorption under the influence of PTH
  • Collecting ducts
    • Primary site of ADH action
    • Water absorption influenced by ADH and medullary hypertonicity
    • Distal nephron reabsorbs sodium, secrete H" and potassium under the influence of aldosterone
    • Generation of new bicarbonate

Renal homeostatic functions

• Waste/toxin excretion
• Extracellular fluid volume regulation
• Body fluid osmolar regulation
• Blood pressure regulation
• Aid-base regulation
• Mineral regulation
• Metabolic and hormonal regulation.

Waste/toxin excretion

• Glomerular filtration
  • Endogenous end products
    • Urea
    • Creatinine
  • Exogenous toxins and drugs
• Tubular excretion
  • urate
  • Citrate
  • Lactate
  • Creatinine
  • drugs

Extracellular fluid volume 

Operationally kidney regulates NaCl absorption as the primary adjustment to ECF volume

• Volume loss
  • baroreceptors in the atria, aortic arch sense volume change, and send signals to CNS to influence sympathetic tone, which in turn controls glomerular flow and proximal tubular absorption.
  • Vasopressin and angiotensin II release: net effect to increase salt absorption in the proximal tubule and loop of Henle.
  • JGA secretes renin which generates angiotensin ii: regulates renal and glomerular dynamics and stimulates secretion of aldosterone. Aldosterone acting on the distal tubule facilitates absorption of NaCl.
• Volume expansion
  • In addition to the above
  • Atrial natriuretic peptide (ANP)
  • Increase in filtered load of NaCl
  • Reduce Renin thus aldosterone
  • hinder sodium absorption in distal tubules

Osmolar regulation

Osmolar balance is regulated by ECF water

• Dehydration
  • Osmoreceptors in CNS 
  • stimulate release of ADH by posterior pituitary
  • Increased water absorption from collecting ducts
  • Stimulate sensation of thirst
• Excess of body water
  • Suppress ADH and thirst

Blood pressure regulation

• Hypotension
  • Vasoactive compounds
  • vascular baroreceptors or efferent renal arteriole aids in releasing renin. Renin concerts angiotensin i to angiotensin ii, which is a potent vasoconstrictor.
  • ECF volume control
  • Angiotensin ii stimulates release of aldosterone which increases sodium absorption and extracellular volume.
• Hypertension
  • Less certain
  • vasodepressors : prostaglandins and kinins

Acid-base regulation

• Reclamation of bicarbonate in proximal tubule. Influenced by
  • ECF volume
  • Pco2
  • Potassium
• Generation of bicarbonate in distal tubule
• Active secretion of hydrogen ions by both tubules
  • Excreted in phosphate or ammonia buffer or as a free proton.

Mineral regulation

• Potassium
  • filtered freely
  • reabsorbed throughout the proximal nephron
  • secreted in distal nehron, is influenced by
    • aldosterone promotes
    • rate of fluid flow
    • Sodium
• Calcium
  • 60% of filtered calcium is absorbed in proximal tubule with sodium
  • PTH governs calcium absorption from thick limb of loop of henle and distal convoluted tubule
  • Renal response to hypercalcemia is less satisfactory
• Magnesium
  • Absorbed in proximal tubule and in the loop of henle.
  • Hypermagnesemia is the result of reduced GFR

Renal metabolic and Hormonal regulation

• Contributes to degradation of
  • pituitary hormones
  • glucogon
  • Insulin
• Adds a second hydroxyl ion to cholecalciferol (Vit D) and regulates calcium absorption from gut.
• Erythropoetin
  • hypoxia