Kidney

• kidney structure

  A) nephron

  B) collecting duct

  C) pyramid

  D) medulla

  E) cortex

  F) renal pelvis

  G) renal artery

  H) renal vein

  I) ureter

  
• nephron diagram

  A) medulla

  B) cortex

  C) collecting duct

  D) distal convoluted tubule

  E) glomerulus

  F) efferent arteriole

  G) afferent arteriole

  H) artery

  I) vein

  J) Bowman's capsule

  K) proximal convoluted tubule

  L) descending limb

  M) ascending limb

  N) loop of Henle

  
• The nephron is a tube with 4 regions: The Bowman’s capsule extends into a coiled tube called the proximal convoluted (first coiled) tubule. This leads into a long hairpin like tube called the loop of Henlè.
• The loop of Henlè runs deep into the medulla of the kidney, bends sharply and returns back into the cortex where it forms another twisted region, the distal convoluted tubule.
• Many distal convoluted tubules join to a collecting duct, which transfers the fluid (urine) toward the pelvis of the kidney and the contents are emptied into the ureter which takes urine to the bladder.
• Ultrafiltration

  •Plasma in the glomerulus is filtered into Bowman’s capsule under high hydrostatic pressure.

  •Only  small substances below a certain size are filtered out of the capillaries and so the filtrate contains useful molecules as well as toxic ones.
• Ultrafiltration is driven by high hydrostatic pressure: as the blood entering the glomerulus is under high pressure.

  This happens because:

  1.The renal arteries are wide, short and relatively close to the heart

  2.The afferent arteriole of each glomerulus is wider than its efferent arteriole.

  3.The coiling of the capillaries in the glomerulus further restricts blood flow, increasing pressure.
• The Microstructure of the Bowman's capsule consists of three layers separating plasma from filtrate (glomerular filtrate):

  • 1.Endothelium of capillaries in the glomerulus – this is a single layer of squamous endothelial cells with pores.
  • 2.The effective filter is the basement membrane of the glomerular capillaries.  The membrane is an extracellular matrix formed from different substances eg. proteins. 
  • 3.Inner wall of BC consists of specialised cells called podocytes, with foot-like processes which surround the capillaries but which have spacious gaps between them called filtration slits.

  
• For filtration to occur, the water potential within the glomerular capillaries (blood plasma) must exceed the water potential within the Bowman’s capsule (glomerular filtrate). Why does this occur?


  The hydrostatic pressure (pressure potential) of the blood is much greater than the hydrostatic pressure (back pressure) created by the filtrate in the nephron. and it is the net filtration pressure that causes fluid to move from the glomerular capillaries into the Bowman’s capsule.
• Some substances are reabsorbed back into the blood as the filtrate flows along the PCT. Certain molecules are actively reabsorbed – selective (urea is toxic and not actively reabsorbed but some passes back by diffusion to the blood). All glucose, salt, amino acids and some sodium and chloride ions pass are selectively reabsorbed into the blood by facilitated diffusion (when a gradient exists) and active transport (to ensure all the glucose is reabsorbed). Small proteins are reabsorbed by pinocytosis.
• The uptake of these substances means the blood in the capillaries surrounding the nephron has a high solute concentration (a lower water potential). So a large amount of water (70-80%) passes out of the filtrate in the PCT and back into the blood by osmosis. As water is absorbed, the urea concentration increases. At the end of the PCT, the filtrate is isotonic (of equal concentrations) with the blood plasma.
• The epithelial cells of the PCT exhibit a high level of metabolic activity and carry out energy-demanding processes. They have a number of adaptations:

  • Cuboidal epithelium
  • Surface microvilli
  • Basal invaginations (infolding of membrane)

  All of these adaptations increase the surface area available for reabsorptive processes. Additionally, the cells contain numerous mitochondria, which supply the extra ATP needed for active transport. The cell surface membrane also contains protein carrier molecules for selective reabsorption.
• Whats filtered from blood in the Bowman's capsule?

  • Water
  • Glucose
  • Amino acids
  • Salt
  • Urea
  • Vitamins/minerals
  • Small proteins

  Whats not filtered?

  • Blood cells
  • Large proteins