Blood Glucose and Water Regulation

Action of Insulin

  • when blood glucose conc. is too high - detected by beta islet of Langerhans

  • islets release insulin, which binds to receptors on liver cells

  • stimulates vesicles containing glucose transport proteins

  • more glucose transport proteins expressed, more glucose moves into the cell by facilitated diffusion

  • enzymes converting glucose to glycogen activated - glycogenesis (glucose also converted to fat)

  • lowers glucose concentration in cell, more glucose diffuses into cell by facilitated diffusion

  • respiratory rate of cells increases, using up more glucose


Action of Glucagon

  • when blood glucose conc. is too low - detected by alpha islets of Langerhans

  • pancreas releases glucagon into blood that reaches liver cells and binds to receptors on CSM

  • activates enzymes for glycogenolysis - hydrolyses glycogen into glucose

  • also activates enzymes converting fatty acids, glycerol and amino acids glucose - gluconeogenesis


Second Messenger Model

→ model activated by adrenaline and glucagon

  • adrenaline binds to transmembrane protein causing it to change shape

  • activates enzyme adenyl cyclase

  • activated adenyl cyclase converts ATP to cAMP

  • cAMP acts as second messenger, binding to and activating protein kinase

  • protein kinase enzyme catalyses conversion of glycogen to glucose, which moves out of the liver cells by facilitated diffusion



Glomerular Filtrate at PCT

  • blood enters through afferent arteriole and reaches glomerular capillaries and efferent arteriole, which have a narrower diameter that creates a high hydrostatic pressure

  • water, glucose, urea and other small molecules are forced out as a result through the basement membrane and pores in capillary endothelieum

  • large proteins remain in the blood capillary, too large to be forced through

resisted by:

→ capillary endothelial cells

→ low w.p. of blood in glomerulus

→ epithelial cells in renal capsule

→ connective tissue

Reabsorption of water and glucose at PCT

  • Na+ ions are actively transported out of the cells lining PCT into blood capillaries that carry them away, using energy released by ATP

  • creates a low conc. gradient of Na+ ions in the cell, which move into the cell by facilitated diffusion through co-transport protein, from lumen of PCT to epithelial cells

  • a molecule of glucose/AA/ion is co-transported with Na+ ions

  • molecule diffuses into blood capillary

adaptations:

  • many mitochondria produce ATP for active transprot

  • many channel proteins for faster rate of facilitated diffusion

  • many carrier proteins for faster rate of co-transport and active transport

  • microvilli provides large SA for absorption (by f.d. or a.t.)

  • many ribosomes to produce channel/carrier proteins

Loop of Henle

  • walls of descending limb are permeable to water - water moves out of limb by osmosis down w.p. gradient into interstitial region

    • this water enter blood capillaries by osmosis and is carried away

  • Na+ ions diffuse into descending limb by facilitated diffusion, lowering w.p. in descending limb

  • w.p. is lowest at tip of hairpin

  • walls of ascending limb are impermeable to water - no water moves out

  • Na+ ions are actively transported out of ascending limb into interstitial region using energy released by ATP

  • lowers w.p. in interstitial region

  • water leaves to distal convoluted tubule and then collecting duct

  • water in loop of henle moves on to distal convoluted tubule


distal convoluted tubule: makes final adjustments to salts and water reabsorbed