Homeostasis

Homeostasis

1. Regulation of Blood Glucose

   • The pancreas is a small organ lying below the stomach - it has both endocrine and exocrine functions
     • The exocrine function of the pancreas is to manufacture and release digestive enzymes. The cells surround small tubules which join up to make the pancreatic duct which carries the fluid into teh small intestine. The fluid contains amylase, lipase, trypsinogen and NaOH to make it alkaline
     • Islets of Langerhans contain alpha cells which secrete glucagon, and beta cells which secrete insulin. The hormones are secreted straight into the blood. This is the endocrine function of the pancreas
   • A high blood concentration is detected by the beta cells. In response the beta cells secrete insulin into the blood. The target cells are the hepatocytes, muscle cells and some cells in the brain. These possess the specific receptors for insulin. When the blood passes these cells the insulin binds to the receptors which activates the adenyl cyclase inside each cell which converts ATP to cAMP. This activates enzyme-controlled reactions in the cell
     • Insulin has several effects on the cell
       • More glucose channels are placed into the plasma membrane
       • More glucose enters cell
       • Glucose is converted to glycogen for storage
       • Glucose is converted to fats
       • Glucose is used for respiration
   • A low blood glucose concentration is detected by the alpha cells. In the response they secrete glucagon. Its target cells are hepatocytes which possess the specific receptors for glucagon
     • The effects of glucagon
       • Conversion of glycogen to glucose (glycogenolysis)
       • Use of fatty acids for respiration
       • Production of glucose by conversion from amino acids and fats (gluconeogenesis)

2. Regulation of Insulin Levels

   • Control of Insulin Secretion
     • 1. Cell membranes of beta cells contain both calcium and potassium ions channels
     • 2. The potassium ion channels are normally open and the calcium ion channels are normally closed. K+ ions diffuse out of the cell making the inside more negative. At rest the p.d is about -70 mV
     • 3. When glucose concentrations outside of the cell are high, glucose diffuses into the cell
     • 4. The glucose is quickly used to produce ATP. The extra ATP causes the potassium ions to close
     • 5.The potassium can no longer diffuse out and this alters the p.d and becomes less negative
     • 6. The change in the p.d opens the calcium ion channels. Calcium ions enter the cell and cause the secretion of insulin by making the vesicles containing insulin move and fuse with the plasma membrane, releasing insluin by exocytosis
   • Diabetes mellitus is a disease in which blood glucose concentrations can't be controlled effectively. This can lead to hyperglycaemia - where the blood glucose concentration is too high. It can also lead to the concentration dropping too low (hypoglycaemia)
     • Type 1 diabetes (insulin-dependent diabetes) - usually starts in childhood. Result of an autoimmune response in which the body's own immune system attacks the beta cells and destroys them. The body can't manufacture sufficient insulin and store excess glucose as glycogen
     • Type 2 diabetes (non-insulin-dependent diabetes) - can still produce insulin but as people age, their responsiveness to insulin declines. This is probably because the receptors on the surface of the liver and muscle cells decline and teh cells lose their ability to respond to the insulin in the blood. Thre insulin levels secreted by beta cells also decline
       • Factors that can bring on an earlier onset of diabetes: obesity, diet high in sugars, being of Asian or Afro-Caribbean origin and family history

3. Control of Heart Rate

   • The rate at which the heart beats is affected by a number of factors
     • The heart muscle is myogenic
     • The SAN is the heart's own pacemaker. It initiates an action potential which travels as a wave of excitation across the atria walls, through the AVN and down the Purkyne fibres to the ventricles, causing them to contract
     • The heart is supplied by nerves from the medulla oblongata of the brain. They connect to the SAN. They can affect the frequency of contractions. Action potentials sent down the accelerator nerve increase the heart rate. Action potentials sent down the vagus nerve reduce the heart rate
     • The heart muscle responds to the presence of adrenaline in the blood
   • Movement of the limbs is detected by stretch receptors in the muscles. These send impulses to the cardiovascular centre informing it that extra oxygen may be needed - tends to increase heart rate
   • When exercising the muscles produce more carbon dioxide. Some of this reacts with water in the blood plasma and reduces its pH. The change in pH is detected by chemoreceptors in the carotid arteries, aorta and brain. The chemoreceptors send impulses to the cardiovascular centre which increases heart rate
     • When we stop exercising the carbon dioxide concentration in the blood falls, which reduces the activity of the accelerator pathway, therefore the heart rate declines.
       • Adrenaline is secreted in response to stress, shock, anticipation or excitement. The presence of adrenaline in the blood increases heart rate
         • Blood pressure is monitored by stretch receptors in the walls of the carotid sinus. This is a small swelling in the carotid artery. If blood pressure rises too high, the stretch receptors send signals to the cardiovascular centre. this responds by reducing heart rate

4. The Endocrine System

   • Hormones are molecules that are released by endocrine glands directly into the blood. They act as messengers, carrying a signal from the endocrine gland to a specific target organ or tissue
     • An endocrine gland is a gland that secretes hormones directly into the blood. Endocrine glands have no ducts. An exocrine gland is a gland that secretes molecules into a duct that carries the molecules to where they are used
   • There are two types of hormone: protein and peptide hormones (for example adrenaline, insulin and glucagon), and steroid hormones (for example sex hormones)
   • Adrenaline is an amino acid derivative - it's unable to enter the target cell. Adrenaline in the blood binds to its specific receptor on the plasma membrane. The adrenaline molecule is the first messenger. When it binds to the receptor it activates the enzyme adenyl cyclase. The adenyl cyclase converts ATP to cyclic AMP. The cAMP is the second messenger. The cAMP can then cause an effect inside the cell by activating enzyme action
   • The adrenal medulla
     • The medulla is found in the centre of the gland. The cells in the medulla release adrenaline. The effects of adrenaline include: relaxes smooth muscle in the bronchioles, increases stroke volume and heart rate, stimulates conversion of glycogen to glucose and raises blood pressure
   • The adrenal cortex
     • The adrenal cortex uses cholesterol to produce certain steroid hormones. The mineralocorticoids (e.g. aldosterone) help to control the concentrations of sodium and potassium in the blood. The glucocorticoids (e.g. cortisol) help to control the metabolism of carbohydrates and proteins in the liver

5. Maintaining Body Temperature

   • An ectotherm is an organism that relies on external sources of heat to regulate its body temperature
     • Advantages
       • Ectotherms use less of their food in respiration
       • Able to survive for long periods without eating
       • Lots of energy from food can be used for growth
     • Disadvantages
       • They are less active in cooler temperatures - greater risk of predation
       • May not be capable of activity during winter as they never warm up sufficiently - must have sufficient stores of energy to survive over winter without eating
   • An endotherm is an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature
     • Advantages
       • Fairly constant body temperature whatever the temperature is externally
       • Activity possible when external temperatures are cool
       • Ability to inhabit colder parts of the planet
     • Disadvantages
       • A significant part of the energy intake is used to maintain body temperature
       • More food is required
       • Less of the energy from food is used for growth