Pancreas - the exocrine function
Pancreas has both exocrine and endocrine functions.
There are pancreatic cells which manufacture and secrete pancreatic juice into tiny tubules which join to make up the pancreatic duct. The pancreatic juice is then secreted into the first part of the small intestine.
The pancreatic juice contains the following digestive enzymes and fluid:
- amylase - a carbohydrase
- trypsinogen - an inactive protease
- sodium hydrogencarbonate which makes it alkaline and helps to neutralise the contents of the digestive system that have just left the acidic environment of the stomach.
The Pancreas - Endocrine function
The pancreas contains cells called the Islets of Langerhans which are well supplied with blood capillaries. There are two types of these cells, both of which have different functions;
- a-cells, which manufacture and secrete the hormone glucagon
- b-cells, which manufacture and secrete the hormone insulin
Islets of Langerhans monitor the concentration of blood glucose levels in the blood and if they detect a concentration outside of the acceptable range, either b-cells or a-cells are activated to regulate the concentration. These hormones are directly secreted into the blood.
The Pancreas - Rise in Blood Glucose Concentration
1) Under normal conditions, potassium ions flow out of the cell 2) When there is a glucose concentration gradient, glucose will diffuse into the cell via facilitated diffusion 3) The glucose is then respired and ATP is produced which binds to the potassium channels and therefore closes them 4) The potassium ions now remain in the cell because of the closed channels due to ATP 5) The builid up of K+ increases thw potential difference of the membrane and depolarises it 6) This depolarisations causes voltage gated calcium channels to open so there is an influx of calcium ions 7) The calcium ions enter the cell and bind to vesicles which contain insulin, causing them to fuse with the membrane and release the insulin outside of the membrane via exocytosis into the bloodstream The insulin is transported all over the body and when it passes its target tissues and cells (usually hepatocytes), the hormone binds to receptor sites activating the adenyl cyclase enzyme to produce cyclic AMP. The cAMP activates a number of enzyme intracellular reactions such as:
- more glucose channels inserted into cell surface membrane, so more glucose can enter
- glucose inside cell is polymerised into gluycogen by glycogenesis
- more glucose is converted into fats and more is respired
End result - decreased blood glucose concentration in the bloodstream!! :)
The Pancreas - Drop in Blood Glucose Concentration
For example after doing exercise it is common that your blood glucose concentration would fall as the glucose would have been used up in respiration to provide energy.
This drop in blood glucose is detected by a-cells in the Islet of Langerhans, and therefore in response they secrete the hormone glucagon, which uses hepatocytes as target cells.
Glucagon binds to the receptors on target cells, which activates adenyl cyclase which then produces cAMP. The cAMP triggers the reactions, for example:
- the polymer glycogen is broken down into glucose monomers (glycogenolysis)
- more fatty acids are used in respiration
- amino acids and lipids are converted into glucose by gluconeogenesis
The overall effects of glycogenolysis and gluconeogenesis is to release more glucose in the blood, so that the concentration is increased.