15 - homeostasis Flashcards
What is a positive feedback system?
A change in the internal environment of the body is detected by sensory receptors, and effectors are stimulated to reinforce that change and increase that response. e.g. when a blood vessel is damaged, platelets stick to the damaged region and they release factors that initiate clotting and attract more platelets.
What physical processes do changes in an organisms temperature depend on?
- Exothermic chemical reactions
- Latent heat of evaporation; objects cool down as water evaporates from a surface
- Radiation; the transmission of electromagnetic waves to and from the air, water or ground.
- Convection- heating as a result of the collision of molecules. Air is not a good conductor of heat by the ground and water are.
What are ectotherms?
- They use their surroundings to warm their bodies. Their core body temp is heavily dependent on their environments.
- Ectotherms include all invertebrate animals, along with fish, amphibians and reptiles.
- Many ectotherms living in water don’t need to thermoregulate. The high heat capacity of water means that the temp of their environment doesn’t change much.
- Ectotherms living on land have a bigger problem with temp regulation, as a result, they have evolved a range of strategies that enable them to cool down or warm up.
What are endotherms?
Rely on metabolic processes to warm up and they usually maintain a very stable core temp regardless of the environmental temp. Have adaptations which enable them to maintain their body temp and to take advantage of warmth from the environment. As a result, ectotherms survive in a wide range of environments. The metabolic rate of ectotherms is around 5X higher than that of ectotherms, so they need to consume more food to meet their metabolic needs than ectotherms of a similar size.
What behavioural responses so ectotherms use to overcome limitations imposed by the temp of their surroundings?
To warm up;
- bask in the sun
- orientate their bodies so that max SA is exposed to the sun
- extend their bodies to increase the SA exposed to the sun.
- pressing their bodies against the warm ground.
What physiological responses do ectotherms have to warming?
- Dark colours absorb more radiation thank light colours; Lizards living in colder climates tend to be darker coloured than those that live in hotter climates.
- Some ectotherms also alter their HR to increase or decrease the metabolic rate and sometimes to affect the warming or cooling across the body surfaces.
- They’re vulnerable to fluctuations in the environment compared to endotherms. However, by using a variety of behavioural and physiological responses many can maintain relatively stable core temp.
How do endotherms detect changes in temperature?
- The peripheral temp receptors are in the skin and detect changes in the surface temperature.
- Temp receptors in the hypothalamus detect temperature of the blood deep in the body.
- The temp of the skin is much more likely to be affected by external conditions than the temperature of the hypothalamus.
- The combination of the 2 gives the body great sensitivity and allows it to respond not only to actual changes in the temperature of the blood but to pre-empt possible problems that might result from changes in the external environment.
How do endotherms cool down?
Vasodilation
increased sweating
reducing the insulating effect of hairs or feathers.
How does vasodilation help to cool endotherms down?
- The arterioles near the surface of the skin dilate when the temperature rises. The vessels that provide a direct connection between the arterioles and the venules constrict.
- This forces blood through the capillary networks close to the surface of the skin.
How does increased sweating help to cool down endotherms?
- Sweat spreads out across the surface of the skin.
- In some mammals, including humans and horses, there are sweat glands all over the body.
- As sweat evaporates from the surface of the skin, heat is lost, cooling down the blood below the surface.
- In some animals, the sweat glands are restricted to the less hairy areas of the body. These animals often open their mouths and pant when they get hot, again losing heat as the water evaporates.
How does reducing the insulating effect of hairs or feathers help to cool down endotherms?
- As the body temp increases, the erector pili muscles in the skin relax- as a result, the hair or the feathers of the animal lie flat to the skin. This avoids trapping an insulating layer of air. It has little effect in humans.
How do endotherms warm up?
Vasoconstriction
Decreased sweating
Rising the body hair or feathers
Shivering
How does vasoconstriction help an endotherm to increase their body temp?
The arterioles near the surface of the skin constrict. The arteriovenous shunt vessels dilate so very little blood flows through the capillary networks close to the surface of the skin. The skin looks pale and very little radiation takes place. The warm blood is kept well below the surface.
How does raising the body hair of feathers help endotherms to warm up?
As temp falls, erector pili muscles in the skin contract, pulling the hair or feathers of the animal erect.
- This traps an insulating layer of air and so reduces cooling though the skin.
How does shivering help endotherms to warm up?
- Rapid involuntary contracting and relaxing of the large voluntary muscles in the body.
- The metabolic heat from the exothermic reactions warm up the body instead of moving it and is an effective way of raising the core temp.
What additional anatomical adaptations do endotherms living in cool climates have to keep them warm?
- Adaptations to minimise their SA:V ratio e.g. small ears
- Thick layer of insulating fat underneath the skin
- Skin underneath is black, so it absorbs warming radiation.
How does the heat loss centre work in controlling thermoregulation?
It is activated when the temp of the blood flowing through the hypothalamus increases.
- It sends impulse through autonomic motor neurones to effectors in the skin and muscles, triggering responses that act to lower core temperature.
How does the heat gain centre work in controlling thermoregulation?
It is activated when the temp of the blood flowing through the hypothalamus decreases.
- It sends impulse through autonomic motor neurones to effectors in the skin and muscles, triggering responses that act to increase core temperature.
What are the main metabolic waste products in mammals?
- CO2; from cellular respiration
- Bile pigments; from breakdown of haemoglobin from old RBCs in the liver.
- Nitrogenous waste products (urea); formed from the breakdown of excess amino acids by the liver.
What are the different veins/ arteries that flow to and from the liver?
- Oxygenated blood is supplied to the liver by the hepatic artery
- Blood is returned to the heart via the hepatic vein
- The hepatic portal vein carries blood loaded with the products of digestion straight from the intestines to the liver and this is used as the starting point of many metabolic activities of the liver.
What are the main structures within the liver?
- Liver cells= hepatocytes. They have a large nuclei, prominent golgi and lots of mitochondria, as they are metabolically active cells.
- The blood from the hepatic artery and portal vein is mixed in spaces called sinusoids which are surrounded by hepatocytes. This increases the oxygen content of the blood from the hepatic portal vein, supplying the hepatocytes with enough oxygen for their needs.
- The sinusoids contain Kupffer cells, which act as macrophages in the liver. They ingest foreign particles and help protect against disease.
- The hepatocytes secrete bile from the breakdown of the blood into spaces called canaliculi, and from these, the bile drains into the bile ductules which take it to the gall bladder.
What are the main functions of the liver?
- Carbohydrate metabolism
- Deamination of excess amino acids
- detoxification
How does the liver control carbohydrate metabolism?
- Hepatocytes are involved in the homeostatic control of glucose levels in the blood by their interaction with insulin and glucagon.
- When blood glucose levels rise, insulin levels rise and stimulate hepatocytes to convert glucose to the storage carbohydrate glycogen.
- When blood glucose levels start to fall, the hepatocytes convert the glycogen back to glucose under the influence of glucagon.
How does the liver deaminate excess amino acids?
- Hepatocytes carry out transamination- the conversion of one amino acid into another. This is important because the diet doesn’t always contain the required balance of amino acids but transamination can overcome this problem.
- Deamination is the removal of an amine group from a molecule. The body cannot store proteins or amino acids. Any excess ingested protein would be excreted and therefore wasted.
- Hepatocytes deaminate the amino acids, removing the amino group and converting it first into ammonia (which is very toxic) and then into urea.
- Urea is toxic in very high concs. but not normally in those found in the blood. Urea is excreted by the kidneys.
- The remainder of the amino acid can then be fed into cellular respiration or converted into lipids for storage.
- Ammonia is produced in the deamination of proteins and is converted into urea during the ornithine cycle.
Explain the ornithine cycle.
- Deamination of excess amino acids.
- amino group used in respiration.
- NH3 group is combined with ornithine and CO2, producing water and citrulline.
- More NH3 is then combines with the citrulline to produce water and arginine.
- Arginine is then combine with water, which produces urea, and more ornithine.
How does the liver carry out detoxification?
- One example = the breakdown of hydrogen peroxide.
- it is a by-product of many metabolic pathways. Hepatocytes contain catalase, which splits H2O2 into water and oxygen.
- Another example is how the liver can detoxify ethanol.
- Hepatocytes contain the enzyme alcohol dehydrogenase which breaks down the ethanol to ethanal. Ethanal is then converted to ethanoate which may be used to build up fatty acids or used in cellular respiration.
What is the structure of the kidney?
- The cortex is the dark outer layer. This is where the filtering of blood takes place and it has a very dense capillary network carrying blood from the renal artery to the nephrons.
- The medulla is lighter in colour, contains the tubules of the nephrons that form the pyramids of the kidney and the collecting ducts
- The pelvis of the kidney is the central chamber where the urine collects before passing out down the ureter.
What are the different parts of the nephron and what are their functions?
- Bowman’s capsule- cup shaped structure that contains the glomerulus, a tangle of capillaries. More blood goes into the glomerulus than leaves it due to ultrafiltration that takes place.
- PCT- the first, coiled region of the tubule after the bowman’s capsule, found in the cortex of the kidney. This is where many of the substances needed by the body are reabsorbed into the blood.
- Loop of henle- a long loop of tubule that creates a region with a very high solute conc in the tissue fluid deep in the medulla. The descending loop runs down from the cortex through the medulla to a hairpin bend at the bottom of the loop. The ascending limb travels back up through the medulla to the cortex.
- DCT- a second twisted tubule where the fine tuning of the water balance of the body takes place. The permeability of the walls to water varies in response to ADH in the blood. Further regulation of the ion balance and pH of the blood also takes place in this tubule.
- Collecting duct- the urine passes down the collecting duct through the medulla to the pelvis. More fine-tuning of the water balance takes place- the walls of this part of the tubule are also sensitive to ADH.
Explain the process of ultrafiltration.
- The glomerulus is supplies with blood by a relatively wide afferent arteriole from the renal artery.
- Blood leaves through a narrower efferent arteriole, as a result, there is considerable pressure in the capillaries of the glomerulus.
- This forces the blood out through the capillary wall. Then the fluid passes through the basement membrane.
- The basement membrane is made up of a network of collagen fibres and other proteins that make up a second filter.
- Most of the plasma contents pass through the basement membrane, but the blood cells and many proteins are retained in the capillary due to their size.
- Bowman’s capsule has podocytes that act as an additional filter. They have extensions called pedicels that wrap around the capillaries, forming slits that make sure any cells, platelets or large plasma proteins that have managed to get through the epithelial cells and the basement membrane don’t get through the tubule itself.
- Filtrate contains glucose, salt, urea and many other substances.
Explain the role of the PCT in reabsorption.
- In the PCT all the glucose, amino acids, vitamins and hormones are moved from the filtrate back into the blood by active transport. Around 85% of the NaCl and water is reabsorbed as well. The Na+ ions are moved back by active transport whilst the water and Cl- ions flow passively down concentration gradients.
- The substances removed from the nephron diffuse into a capillary network which surrounds the tubules down steep conc. gradients. These are maintained by the constant flow of blood.
- The filtrate reaching the loop of Henle at the end of the PCT is isotonic (same conc. as) the tissue fluid surrounding the tubule and isotonic with the blood.
- Over 80% of the glomerular filtrate has been reabsorbed into the the blood. This remains the same regardless of the conditions in the body.
What are the adaptations of the PCT?
- The cells lining the PCT are covered in microvilli, greatly increasing the SA over which substances can be reabsorbed.
- They also have many mitochondria to provide the ATP needed in active transport systems.
Explain the role of the descending loop of Henle in reabsorption.
- The kidney has a counter current multiplier, using energy to produce conc. gradients that result in the movement of substances such as water from one area to another.
- The descending limb leads from the PCT.
- The upper part is impermeable to water but the lower part of the descending limb is permeable to water and runs down into the medulla. The conc. of sodium and chloride ions in the tissue fluid in the medulla gets higher and higher moving through from the cortex to the pyramids, as a result of the activity of the ascending limb.
- The filtrate entering the descending limb is isotonic with the blood. As it travels down the limb, water passes out into the tissue fluid by osmosis. It then moves down a conc. gradient into the blood of the surrounding capillaries.
- The descending limb is not permeable to sodium and chloride ions, so no active transport takes place.
- The fluid that reaches the hairpin bend is very concentrated and hypertonic to the blood in the capillaries.
What is the role of the ascending loop of Henle in reabsorbtion?
- It is very permeable to sodium and chloride ions and they move out of the conc. solution by diffusion down a conc. gradient.
- The second section of the limb, sodium and chloride ions are pumped out into the medulla tissue against the conc. gradient.
- The produces a high Na+ and Cl- ion conc. in the medulla tissue.
- The ascending limb is impermeable to water, so water cannot follow the chloride and sodium ions down a conc. gradient. This means that the fluid left in the ascending limb becomes increasingly dilute, whilst the tissue fluid in the medulla develops the very high conc. of ions that is essential for the kidney to produce urine that is more conc. than the blood.
What is the role of the DCT in reabsorption?
- Balancing the water needs of the body.
- The permeability of the walls vary with the levels of ADH. The cells lining the DCT also have many mitochondria so they are adapted to carry out active transport.
- If the body lacks salt, Na+ ions will be pumped out of the DCT with Cl- ions flowing down an electrochemical gradient. Water can also leave the DCT, concentrating the urine.
- It also plays a role in balancing the pH of the blood.
What is the role of the collecting duct in reabsorption?
- The collecting duct passes down through the conc. tissue fluid of the renal medulla. This is the main site where the conc. and volume of the urine produced is determined.
- Water moves out the collecting duct by diffusion down a conc. gradient as it passes through the renal medulla.
- As a result, the urine becomes more concentrated.
- The level of sodium ions in the surrounding fluid increases through the medulla from the cortex to the pelvis. This means that water can be removed from the collecting duct all the way along it’s length, producing very hypertonic urine when the body needs to conserve water.
- The permeability is controlled by the level of ADH, which determines how much or little water is reabsorbed.
Explain the mechanism of ADH action.
- A change in the water potential of the blood is detected by osmoreceptors in the hypothalamus.
- ADH is released by the posterior pituitary gland, and carried in the blood to the cells of the collecting duct where it has its effect.
- The hormone can’t cross the membrane of tubule cells- it binds to receptors on the cell membrane and triggers the formation of cAMP as a secondary messenger.
cAMP causes; - Vesicles in the cells lining the collecting duct to fuse with the cell surface membranes on the side of the cell in contact with the tissue fluid in the medulla.
- The membranes of these vesicles contain protein- based water channels and when they are inserted into the cell surface membrane, they make it permeable to water.
- This provides a route for water to move out of the tubule cells into the tissue fluid of the medulla and the blood capillaries by osmosis.
What happens when water is in short supply in the body?
- The conc. of inorganic ions in the blood rises and the water potential of the blood and tissue fluid becomes more negative.
- This is detected by osmoreceptors in the hypothalamus. They send nerve impulses to the posterior pituitary gland which in turn, releases stored ADH into the blood.
- ADH is picked up by receptors in the cells of the collecting duct and increases the permeability of the tubules to water. Water leaves the filtrate in the tubules and passes into the blood in the surrounding capillary network. A small vol. of conc. urine in produced.
What happens when you consume excess water?
- The blood becomes more dilute and its water potential becomes negative.
- The change is detected by the osmoreceptors of the hypothalamus. Nerve impulses to the posterior pituitary are reduced or stopped, and so, the release of ADH by the posterior pituitary gland is inhibited.
- Very little reabsorption takes place of the water as the walls of the collecting duct remain impermeable to water.
- In this way, the conc. of blood is maintained, and large amounts of dilute urine are produced.
