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• • The digestive system typically breaks down macromolecules (proteins, carbohydrates, lipids (fats) to be absorbed and used for energy production, growth, maintenance and repair.
• • The vertebrate digestive system includes:
  • alimentary canal (gastrointestinal tract) and the hollow organs of oesophagus, stomach and small intestine
  • accessory organs (solid organs): salivary glands, liver, gall bladder, pancreas.
• tissues of the digestive system include:

  • muscosa (epithelial tissue)
  • sub-mucosa (connective tissue)
  • muscularis (muscle tissue)
  • serosa (connective tissue).
• • Epithelial tissue has a number of types; its functions include protection, absorption and secretion.
• • Connective tissues perform various functions: they bind and support, they protect and insulate, they store reserve energy as fat and they transport materials.
• • Muscle tissues in the digestive system are largely smooth involuntary muscle cells — the exceptions are the muscle tissues of the throat and upper region of the oesophagus and that at the end of the anal canal.
• • Coordinated contraction of muscle tissues controls peristalsis along the length of the system from oesophagus to anus.
• • Regulation of movement between segments of the gut are controlled by sphincters.
• • The pyloric sphincter separates the large acidic stomach from the smaller alkaline duodenum.
• • Digestion of food relies on many enzymes throughout the digestive system.
• • Mouth: teeth provide mechanical breakdown; enzymes in saliva begin chemical breakdown.
• • Oesophagus: transports food to the stomach via peristalsis
• • Stomach: temporary holding chamber for food; three layers of muscle actively churn the contents, acidic conditions and active enzymes continue digestion of fats and start digestion of proteins.
• • Liver (accessory organ): produces bile which aids in digestion
• • Pancreas (accessory organ): produces enzymes
• • Small intestine: the final stage of digestion and absorption of nutrients; mucosa distinguished by villi, crypts and microvilli.
• • Large intestine: includes the caecum, colon, rectum and anus. Functions include reabsorption of water, formation and storage of faeces, elimination of faeces and maintenance of gut bacteria.
• • Homeostasis is the maintenance within a narrow range of conditions in the internal environment.
• • Many variables, including blood glucose and core body temperature, are kept within a narrow range around their set points by homeostatic control loops.
• stimulus–response with negative feedback is a key mechanism for the homeostatic regulation of physiological variables.
• The first step involved in a homeostatic stimulus–response loop is the detection of a stimulus (such as a change in variable) by a receptor
• • A receptor sends a message to a control centre (such as the hypothalamus in the brain) that evaluates the signal, determines the response needed, and sends this message to an effector.
• • An effector carries out the response that counteracts the stimulus, returning the variable to within the set range.
• • Negative feedback loops are key mechanisms of homeostasis that allow for the return of a variable to its required range.
• • Positive feedback is not a mechanism of homeostasis. It causes an amplification of a stimulus further from a set point.
• • Thermoreceptors detect changes in body temperature and send messages to the hypothalamus.
• • The nervous system is an essential component of thermoregulation. It conveys signals from sensors to the control centre in the hypothalamus and from the hypothalamus to various effectors to produce relevant responses.
• • The thyroid gland produces the hormones triiodothyronine (T3) and thyroxine (T4), which act to regulate basal metabolism, which affects body temperature.
• When the core body temperature rises above normal, homeostatic mechanisms operate to increase heat loss and inhibit heat gain mechanisms
• • Other responses that decrease body temperature include sweating, vasodilation of cutaneous blood vessels and behavioural responses.
• • When the core body temperature drops below normal, homeostatic mechanisms operate to increase heat production and decrease heat loss (heat conservation).
• • Other responses that increase body temperature include shivering, vasoconstriction of cutaneous blood vessels and behavioural responses.
• • Homeostatic mechanisms can be overwhelmed in extremely hot and extremely cold conditions.
• • Blood glucose levels are normally highly regulated.
• • Insulin is produced in beta cells of the pancreatic islets. Its secretion is triggered by a rise in blood glucose above the normal level. It allows blood glucose levels to decrease.
• • Glucagon is produced in alpha cells of the pancreatic islets. Its secretion is triggered by a fall in blood glucose below the normal level. It allows blood glucose levels to increase.
• • Insulin stimulates the uptake of blood glucose by cells of skeletal muscle and adipose tissue.
• Insulin activates key enzymes needed to convert glucose to glycogen in liver and muscle cells
• Insulin inhibits the breakdown of fats, reducing the supply of fatty acids. This results in an increased use of blood glucose as the energy source of many body cells
• • Glucagon stimulates the breakdown of liver glycogen to glucose, which is released into the blood.