Osmolalities of intracellular and extracellular fluid is the
same, but the composition of fluid is different.
Water is freely permeable between intracellular extracellular
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.
- 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
- JGA is the site of Renin synthesis and secretion
- 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
- 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
- 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
- Passive absorption of water and sodium in ascending
- 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
- 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
- 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.
- Glomerular filtration
- Endogenous end products
- Exogenous toxins and drugs
- Tubular excretion
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 balance is regulated by ECF water
- Osmoreceptors in CNS
- stimulate release of ADH by posterior pituitary
- Increased water absorption from collecting ducts
- Stimulate sensation of thirst
- Excess of body water
Blood pressure regulation
- 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.
- Less certain
- vasodepressors : prostaglandins and kinins
- Reclamation of bicarbonate in proximal tubule. Influenced
- ECF volume
- Generation of bicarbonate in distal tubule
- Active secretion of hydrogen ions by both tubules
- Excreted in phosphate or ammonia buffer or as a free
- filtered freely
- reabsorbed throughout the proximal nephron
- secreted in distal nehron, is influenced by
- aldosterone promotes
- rate of fluid flow
- 60% of filtered calcium is absorbed in proximal tubule
- PTH governs calcium absorption from thick limb of loop
of henle and distal convoluted tubule
- Renal response to hypercalcemia is less satisfactory
- 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
- Adds a second hydroxyl ion to cholecalciferol (Vit D) and
regulates calcium absorption from gut.