Lecture 26 - Plasma Calcium Homeostasis

• Plasma (Ca2+) Homeostasis


  Importance of plasma Ca2+ for proper functioning of cardiac and skeletal muscle tissue, and nerve synapses
• Glands/organs and hormones involved in plasma [Ca2+] homeostasis

  • Parathyroid Gland
  • Kidneys
  • Parathyroid hormone (PTH)
  • Calcitriol
• Parathyroid Gland


  • 4 endocrine glands on the posterior of the thyroid gland
  • Secrete Parathyroid hormone (PTH)
  • PTH is the most important hormone for [Ca2+] homeostasis
• Kidneys


  • 2 kidneys anterior to lower ribs (posterior view)
  • Stimulated by PTH to secrete Calcitriol
  • Filter blood to make urine - 180 litres of blood a day, we have about 6 litres that are filtered over and over!!
  • Sites for filtration of Ca2+ out of and reabsorption of Ca2+ back into the blood
• Thyroid Gland


  • Wrapped around the trachea and windpipe in a butterfly like shape
  • Secretes calcitonin which plays a minor role in Ca2+ homeostasis
• How the body corrects hypocalcemia - low plasma [Ca2+]

  1. Parathyroid gland detects low Ca2+ concentration and releases Parathyroid Hormone (PTH)

  2. Increased calcium resorption by osteoclasts in the bone

  3. Kidneys reabsorb more calcium to put into the blood plasma rather than peeing out

  4. Digestive tract increases Calcium absorption from food
• How the body corrects hypercalcemia - high plasma [Ca2+]

  1. Parathyroid gland detects high Ca2+ concentration and decreases Parathyroid Hormone (PTH)

  2. Decreased calcium resorption by osteoclasts in the bone

  3. Kidneys perform less reabsorption of calcium to put into the blood plasma rather than peeing out

  4. Digestive tract decreases Calcium absorption from food
• Calcitonin


  • Secreted from the thyroid gland when it senses a change in the plasma calcium levels
  • Effects the osteoclasts and kidney behaviours in a similar way to parathyroid hormone, but is much less common in humans
• Memory hooks


  • Milk has calcium - swallow milk down throat
  • Th... throat (parathyroid gland, thyroid gland)
  • Parathyroid is a larger word than thyroid – the Parathyroid has a larger role than the thyroid in controlling plasma calcium homeostasis
• Plasma Glucose Homeostasis
• Uses of glucose


  • All body cells use glucose as the main substrate for making ATP
  • Glucose is also used to make other substances essential for body functioning (e.g. glycoproteins)
  • Glucose is stored in the form of glycogen (long chains of glucose molecules) or fat
• How glucose enters or exits the blood plasma

  1. Carbohydrates in food enter digestive tract and glucose is absorbed into blood

  2. Glucose is released into the blood by the liver, either new glucose or from glycogen breakdown

  3. Glucose is removed from the blood by body cells to use as fuel

  4. Excess glucose is removed from the blood and stored as glycogen or fat
• Glands/organs and hormones involved in plasma [glucose] homeostasis

  • Pancreas
  • Glucagon
  • Insulin
• Pancreas


  • Sits just behind the stomach, on the left side of the abdomen
  • Endocrine organ that secretes hormones for plasma [glucose] homeostasis
  • Both an exocrine gland (secretes digestive enzymes) and an endocrine gland (secretes insulin and glucagon)
• Pancreatic islet cells


  • Alpha cells secrete glucagon
  • Beta cells secrete insulin
• How the body corrects hypoglycaemia - low plasma glucose

  1. Alpha islet cells detect low glucose levels and release glucagon into the blood stream

  2. In the liver: Gluconeogenesis (building new glucose molecules) and Glycogenolysis (breakdown of glycogen to release glucose)
• Ketone synthesis - Building ketones to release into the blood, this is when fat is broken down as another source. Only occurs in desperate situations, the brains preferred source of fuel is glucose.
• Fasting state


  Glucagon signals to the liver to release glucose into plasma
• Fed state


  Insulin signals to body cells to use glucose, liver and muscle to store excess as glycogen, and fat cells to store excess as fat
• Daily fluctuations in plasma (Glucose)
• Eating complex carbs (starch-rich) causes less of a plasma glucose spike, and thus drop. Eating simple carbs (sucrose-rich) causes a much higher plasma glucose spike, and thus a steeper drop.
• Exercise can help bring down plasma glucose after a meal
• How the body corrects hypoglycaemia (low plasma glucose)

  1. Glucagon secreted by alpha islet cells

  2. Gluconeogenesis in liver to build new glucose molecules

  3. Glycogenolysis in liver to break down glycogen and release glucose

  4. Ketone synthesis to provide alternative fuel source
• Hypoglycaemia


  • Occurs in a fasting state when plasma glucose levels are too low
• Alpha islet cells


  Detect low glucose levels compared to set point and release glucagon
• How the body corrects hyperglycaemia (high plasma glucose)

  1. Insulin secreted by beta islet cells

  2. Insulin causes adipose fat to store excess glucose

  3. Insulin causes liver and skeletal muscle to store excess glucose as glycogen
• Hyperglycaemia


  • Occurs in the fed state when plasma glucose levels are too high
• Beta islet cells


  Detect high glucose levels and secrete insulin
• Feeding state


  High glucose, high insulin secretion
• Fasting state


  Low glucose, high glucagon secretion
• Anabolism


  Growth and building, synthesis of glycogen, protein and fat. Storing of energy
• Catabolism


  Breaking down food for energy, breakdown of glycogen, protein and fat