homeostasis

• Homeostatic control of water potential of blood is called osmoregulation. In kidney, functional unit is the nephron
• Mammalian kidney

  • Fibrous capsule: outer membrane that protects the kidney
  • Cortex: lighter coloured region made up of renal (Bowman's) capsules, convoluted tubules and blood vessels
  • Medulla: darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels
  • Renal pelvis: funnel-shaped cavity collects urine into ureter
  • Ureter: tube that carries urine to the bladder
  • Renal artery: supply kidney with blood from heart via aorta
  • Renal vein: return blood to heart via vena cave
• Microscopic examination of cortex and medulla has millions of tiny tubular structures in each kidney- called nephrons
• Structure of the nephron:

  • Nephron is a functional unt of the kidney
  • Narrow tube, twisted regions, separated by long hairpin
• Nephron


  • Renal (Bowman's) capsule
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting duct
• Renal (Bowman's) capsule

  Closed end at start of nephron, cup-shaped, surrounds mass of blood capillaries called glomerulus
• Inner layer of renal capsule

  Made of specialised cells called podocytes
• Proximal convoluted tubule

  Series of loops surrounded by blood capillaries, walls made of epithelial cells which have microvilli
• Loop of Henle


  Long, hairpin loop extends from cortex into medulla and kidney and back, surrounded by blood capillaries
• Distal convoluted tubule


  Series of loops surrounded by fewer capillaries than proximal tubule
• Collecting duct


  Tube where number of distal convoluted tubules from number of nephrons empty, lined by epithelial cells and become wider as it empties into pelvis of kidney
• Afferent arteriole


  • Tiny vessel from renal artery
  • Supplies nephron with blood
  • Enters renal capsule of nephron to form glomerulus
• Glomerulus


  • Branched knot of capillaries
  • Where fluid is forced out of the blood
  • Glomerular capillaries recombine and form efferent arteriole
• Efferent arteriole

  • Tiny vessel leaving renal capsule
  • Has smaller diameter than afferent arteriole
  • Causes increase in blood pressure within glomerulus
  • Carries blood away from renal capsule and later branches to form blood capillaries
• Blood capillaries


  • Concentrated network of capillaries
  • Surround proximal convoluted tubule, Loop of Henle and distal convoluted tubule
  • Where they reabsorb mineral salt, glucose and water
  • Capillaries merge to form venules, which merge to form renal vein
• structure of the nephron

  A) afferent arteriole

  B) efferent arteriole

  C) distal convoluted tubule

  D) glomerular capillary

  E) renal (Bowman's) capsule

  F) renal artery

  G) renal vein

  H) proximal convoluted tubule

  I) collecting duct

  J) blood capillaries

  K) loop of Henle

  
• Role of nephron in osmoregulation

  Important function of kidney: maintain water potential of plasma and tissue fluid (osmoregulation).

  Nephron carries out role of osmoregulation in series of stages:

  • Formation of glomerular filtrate by ultrafiltration
  • Reabsorption of glucose and water by proximal convoluted tubule
  • Maintenance of gradient of Na ions in medulla by loop of Henle
  • Reabsorption of water distal convoluted tubule and collecting ducts
• Formation of glomerular filtrate by ultrafiltration

  1. Blood enter kidney through renal artery

  2. Renal artery branches to millions of tiny arterioles

  3. Arterioles enter renal (Bowman's) capsule of nephron

  4. Arteriole is called afferent arteriole which divides to capillaries- glomerulus

  5. Glomerular capillary merge form efferent arteriole

  6. Efferent arteriole sub-divides to form capillaries

  7. Capillaries wind around various tubules of nephron

  8. Capillaries combine to form renal vein
• Walls of glomerular capillary


  • Made of endothelial cells which has pores between them
• There is build-up of hydrostatic pressure within glomerulus


  Since diameter of afferent arteriole greater than efferent arteriole
• Formation of glomerular filtrate

  Water, glucose and mineral ions squeezed out of capillary forms glomerular filtrate
• Blood cells

  • Cannot pass across into renal capsule since too large
• Movement of filtrate out of glomerulus

  • Resisted by capillary endothelial cells
  • Resisted by connective tissue and endothelial cells of blood capillary
  • Resisted by epithelial cells of renal capsule
  • Resisted by hydrostatic pressure of blood fluid in renal capsule space
  • Resisted by low water potential of blood in glomerulus
• Inner layer of renal capsule

  • Made of specialised cells (podocytes) have space between them, filtrate can pass beneath them and through gaps between branches
• Endothelium of glomerular capillaries


  • Has spaces between cells, fluid can pass between these cells
• Hydrostatic pressure of blood


  Sufficient to overcome resistance and so filtrate passes from blood into renal capsule
• Filtrate containing urea, not contain cells or plasma proteins which are too large to pass across connective tissue
• Many substances of filtrate passing out of blood each minute useful to body are reabsorbed
• Proximal convoluted tubule


  Where most of filtrate reabsorbed back into blood
• Ultrafiltration

  • Operates on basis of size of molecules- small ones removed e.g. urea and wastes, most useful ones are reabsorbed
• Proximal convoluted tubule

  • Adapted to reabsorb substances in blood by having epithelial cells that have:
  • Microvilli provide large SA reabsorb substances from filtrate
  • Infoldings at bases give large SA to transfer reabsorbed substances into blood capillaries
  • High density of mitochondria to provide ATP for active transport
• Reabsorption in proximal convoluted tubule


  1. Na ions actively transported out of cells lining proximal convoluted tubule into blood capillaries which carry them away

  2. Na ions diffuse down conc gradient from lumen of proximal convoluted tubule into epithelial lining cells through special carrier proteins by facilitated diffusion

  3. Carrier proteins specific types, which carry another molecule (glucose/AA/ Cl ions etc) along with Na ions. Called co-transport

  4. Molecules co-transported into cells of proximal convoluted tubule diffuse into blood. So, glucose and other valuable molecule reabsorbed as well as water
• Maintenance of gradient of sodium ions by loop of Henle:

  • Loop of Henle hairpin shaped tubule extend to medulla of kidney
  • Reabsorbs water from collecting duct, concentrates urine so has a lower water potential than blood
  • Conc of urine produced directly related to length of loop of Henle
  • Loop of Henle has 2 regions:
  • Descending limb: narrow, thin walls highly permeable to water
  • Ascending limb: wider, thick walls impermeable to membrane
• Distal convoluted tubule:

  • Cells that make up walls of this tube have microvilli, and many mitochondria, allows to reabsorb materal rapidly by active transport
  • Main role: make final adjustments to water and salts that are reabsorbed and to control pH of blood by selecting which ions to reabsorb. By permeability of walls altered under different hormones
• Counter-current multiplier:

  • Loop of Henle, counter-current flow: filtrate of collecting duct with lower water potential meas interstial fluid has an even lower water potential
  • Means that, although water potential gradient between collecting duct and interstitial fluid is small, exists for whole length of collecting duct
  • Steady flow of water in interstitial fluid, so around 80% of water enter interstitial fluid and hence blood
  • If two flows in same direction (parallel) less water enters blood