Control of Blood Glucose Concentration

Insulin lowers blood glucose concentration when it's too high:

• β cells in the islets of langerhans of the pancreas detect an increase in blood glucose concentration
• They then secrete insulin into the blood plasma
• Insulin binds to receptors on the cell surface membranes of liver and muscle cells
• It increases the permeability of muscle-cell membranes to glucose, by increasing the number of carrier proteins and opening glucose transport proteins
• Enymes that convert glucose to glycogen are activated and glycogen is formed by glycogenesis
• Insulin also increases the rate of respiration of glucose, especially in muscle cells

Overall:

• Rate of glucose absorption is increased
• Respiratory rate of cells is increased
• Rate of glycogenesis is increased
• Conversion of glucose to fat is increased

Glucagon increases blood glucose concentration when its too low

• α cells in the Islets of Langerhans of the pancreas detect a decrease in blood glucose concentration
• They then secrete glucagon directly into the blood plasma
• Only liver cells have receptors for glucagon
• Glucagon activates enzymes that convert glycogen to glucose by glycogenolysis
• It also activates the enzymes which convert amino acids and glycerol into glucose by gluconeogenesis
• Glucagon decreases the rate of respiration in cells

Overall:

• Rate of glycogenolysis is increased
• Rate of gluconeogenesis is increased
• Rate of respiration is decreased

Adrenaline is a hormone secreted by the adrenal glands when blood glucose concentration is low. It binds to receptors in the cell membranes of liver cells, inhibiting glycogenesis and activating glycogenolysis, as well as activating glucagon secretion and inhibiting insulin secretion.

Glucagon and adrenaline work to increase blood glucose concentration via a second messenger:

• They bind to their specific receptors on cell membranes of liver cells, which activates an enzyme called adenylate cyclase
• Activated adenylate cyclase converts ATP into a chemical signal called cyclic AMP (cAMP). This is the second messenger
• cAMP then activates an enyme called protein kinase A that activates a chain of reactions which breaks down glycogen into glucose- glycogenolysis

• The main functions of the kidneys are to excrete urea, regulate the water potential of the blood, and to regulate mineral ion content of the blood.
• They produce urine which is around 98% water, 1% urea, and 1% mineral salts
• Nephrons are the basic unit of the kidney

• Blood from the renal artery enters the afferent arteriole in the cortex of the kidney
• It then enters the glomerulus which is a bundle of capillaries inside the Bowman's capsule
• Ultrafiltration takes place here- the blood is under high pressure which forces liquid and small molecules out of the capillary and into the Bowman's capsule, forming the glomerular filtrate
• The filtrate then travels through the proximal convoluted tubule (PCT), loop of Henle, and distal convoluted tubule (DCT) where useful substances like water and glucose are reabsorbed in selective reabsorption. Glucose is mainly reabsorbed along the PCT by active transport and facilitated diffusion.
• The epithilium of the wall of the PCT has microvilli for an increased rate of reabsorption
• The filtrate then travels to the collecting duct in the medulla, where more water is reabsorbed, and the filtrate is then called urine

The loop of Henle is located in the medulla and is made up of a descending and an ascending limb. These control the movement of sodium ions so water can be reabsorbed into the blood.

• The walls of the descending limb are permeable to water so water moves into the medulla by osmosis because the water potential is lower in the medulla than in the descending limb. This water in the medulla is reabsorbed into the blood through the capillary network
• This causes the water potential of the filtrate to become more negative, but sodium ions can't diffuse out as the walls of the descending limb are impermeable to them
  
• At the bottom of the ascending limb, sodium ions diffuse out into the medulla by facilitated diffusion, lowering the water potential of the medulla. The ascending limb is impermeable to water so it stays in the tubule.
• As the concentration of ions in the medulla becomes higher than that of the ascending limb, sodium ions are pumped out into the medulla against their concentration gradient using active transport
• As a result, the medulla has a very high concentration of salts. This is important as it allows more water to move out of the collecting duct by osmosis to be reabsorbed into the blood

When dehydrated:

• The water potential of the blood decreases so water moves out of the osmoreceptors in the hypothalamus by osmosis, causing them to decrease in volume
• This sends an impulse to the posterior pituitary gland, causing it to secrete more ADH into the blood
• ADH causes the walls of the DCT and collecting duct to become more permeable to water, so more water is reabsorbed into the medulla
• This causes a small volume of concentrated urine to be produced

When hydrated:

• The water potential of the blood increases and osmoreceptors in the hypothalamus detect this
• Less ADH is released into the blood by the posterior pituitary gland
• The walls of the DCT and collecting duct become less permeable to water so less water moves into the medulla by osmosis to be reabsorbed
• This causes a large volume of dilute urine to be produced