The Endocrine System
- The endocrine system is a communication system that uses hormones as signalling molecules
- Hormones only affect target cells which have a specific receptor for a given hormone
- Endocrine glands are ductless glands and include the pancreas, pituitary gland, testes and ovaries.
- The adrenal glands are endocrine glands that sit just above the kidney
- The adrenal medulla is responsible for the secretion of adrenaline
- The adrenal cortex is made up of the zona glomerulosa, the zona fasciculata and the zona reticularis
- The zona glomerulosa secretes mineralocorticoids involved in controlling the levels of Na+ and k+ in the blood. E.g. aldosterone increases Na+ absorption and decreases k+ absorption via the distal tubules and collecting ducts in the kidney
- The zona fasciculata secretes glucocorticoids hormones which control the metabolism of different respiratory substrates in the body e.g. cortisol stimulates glucose production from glycogen
- The zona reticularis can secrete cortisol and precursor androgens which are converted to sex hormones
- Adrenaline is a peptide hormone and activates G-proteins, which usually activates cAMP as a second messenger.
Control of Blood Glucose Concentration
- The pancreas acts as an exocrine gland by synthesising and secreting pancreatic juices (lipase, amylase, trypsinogen, NaOH) within acini. It also acts as an endocrine gland by synthesising and secreting hormones into blood vessels.
- Cells within islets of Langerhans monitor blood glucose concentration
- Insulin is a hormone released from β-cells in the pancreas when blood glucose concentration rises in order to lower the concentration back to its optimum via negative feedback.
- Mechanism of insulin release:
- At rest in β-cells, K^+ channels are open, resulting in a negative resting potential.
- When glucose concentration rises, glucose diffuses into the cell and is phosphorylated. It is converted to ATP via respiration causing k^+ channels to close and Ca^2+ open. The Ca^2+ causes vesicle of insulin to be exocytosed
- When insulin binds to an insulin receptor, tyrosine kinase is activated which phosphorylates enzymes. The cascade of enzyme controlled intracellular reactions results in vesicles of glucose transporters fuse with the plasma membrane to allow more glucose to enter the cell. The cell also uses more glucose in respiration and activated enzymes covert glucose into glycogen (glycogenesis).
- Glucagon is a hormone released from α-cells in the pancreas in response to low glucose concentration in order to increase the concentration back to its optimum. Binding of glucagon to its receptor activates G proteins and adenyl cyclase, resulting in increased level of cAMP. It return glucose concentration back to its optimum by:
- Activating enzymes which break down glycogen into glucose (glycogenesis).
- Producing glucose from other molecules
- Activating enzymes that convert glycerol (from lipids) and amino acids into glucose (gluconeogenesis)
Diabetes
- Diabetes is a condition where the concentration of glucose in the blood cannot be controlled effectively. It can lead to hyperglycaemia after meals and hypoglycaemia after exercising.
- Type 1 diabetes is caused by an autoimmune attack on the β-cells of the pancreas, so the body cannot produce insulin. It can be treated by insulin injections.
- Type 2 diabetes is caused because the body does not produce enough insulin & the insulin receptors become less responsive. It can be treated by lifestyle changes (losing weight & exercising), drugs to stimulate insulin production and reduce glucose absorption and insulin injections in severe cases.