Chapter 15 Homeostasis

What is homeostasis?

• The maintenance of a stable equilibrium in the conditions inside the body with small fluctuations over a small range of conditions.

What is a receptor?

• A component that detects changes to the internal and external environment of an organism.

• For example in homeostasis, changes in the pH of the blood, core body temperature and the concentration of the urea need to be monitored.

What is an effector?

• The muscles or glands that react to bring about a change in response to the stimulus.

What is a negative feedback system and how does it work?

• A small change in one direction detected by sensory receptors which is reversed by the effectors to restore conditions to their initial level.

• Negative feedback systems work to reverse the initial stimulus.

What is a positive feedback system and how does it work?

• A change in the internal environment detected by the sensory receptors, effectors are stimulated to reinforce the change and increase the response.

What is thermoregulation?

• The maintenance of a relatively constant core temperature.

What is an ectotherm?

• Animals that use their surroundings to warm their bodies, their core body temperature is heavily dependent on their environment.

What is an endotherm?

• Animals that rely on their metabolic processes to warm up, they usually maintain a very stable core body temperature regardless of the temperature of the environment.

What are the main behavioural responses of ectotherms?

1. Basking in the sun- orientate their bodies so that the maximum surface area is exposed to absorb the radiation from the sun.

2. Conduction through pressing their bodies to the ground- conduction through the warm ground.

3. Generate metabolic heat- a result of exothermic metabolic reactions.

4. Taking shelter- decrease core temperature to prevent enzymes denaturing, pressing their bodies against cool ground.

What physiological responses do ectotherms have?

1. Dark colours- absorb more radiation than light colours.

2. Alter their heart rate- to increase or decrease the metabolic rate.

What are the main ways in which endotherms detect temperature changes?

1. The peripheral temperature receptors in the skin detect changes in the surface temperature.

2. Temperature receptors in the hypothalamus detect the temperature of the blood deep in the body.

What are the main behavioural responses endotherms use to cool down?

1. Vasodilation

2. Increased sweating

3. Reducing the insulating effect of hair or feathers

What is vasodilation in endotherms?

• The arterioles near the surface of the skin dilate when temperature rises, the vessels constrict and this forces blood through the capillary networks close to the skin. The skin flushes and cools as a result of increased radiation.

What is increased sweating in endotherms?

• As core temperature increases, rates of sweating also increase, sweat spreads out across the surface of the skin.

• As sweat evaporates from the surface of the skin, heat is lost, cooling the blood below the surface.

How is the insulating effect of hair and feathers reduced in endotherms?

• As body temperature increases the erector pili muscles in the skin relax, as a result the hair/ feathers lie flat on the skin, this avoids trapping an insulating layer of hair.

What are the main behavioural responses endotherms use to warm up?

1. Vasoconstriction

2. Decreased sweating

3. Raising body hair or feathers

4. Shivering

What is vasoconstriction in endotherms?

• The arterioles near the surface of the skin constrict, vessels dilate so very little blood flows through to the capillary networks close to the surface of the skin.

• Very little radiation takes place, warm blood kept below the surface.

What is decreased sweating in endotherms?

• As core temperature falls, rates of sweating and sweat production stop entirely.

• This greatly reduces cooling by evaporation of water from the surface of the skin.

How is the insulating effect of hair and feathers increased in endotherms?

• As body temperature falls, the erector pili muscles in the skin contract, pulling the hair or feathers erect.

• This traps an insulating layer of air and so reduces cooling through the skin.

What is shivering in endotherms?

• As core body temperature falls the body may begin to shiver, this is rapid involuntary contracting and relaxing of the large voluntary muscles in the body.

What are the two negative feedback control centres of the hypothalamus?

1. The heat loss centre

2. The heat gain centre

What is the heat loss centre?

• Activated when the temperature of the blood flowing through the hypothalamus increases.

• It sends an impulse through autonomic motor neurones to effectors in the skin and muscles, triggering responses that lower act to lower the core temperature.

What is the heat gain centre?

• This is activated when the temperature of the blood flowing through the hypothalamus decreases.

• It sends impulses through the autonomic nervous system to effectors in the skin and muscles, triggering responses that act to raise the core temperature.

What is excretion?

• The removal of the waste products of metabolism from the body.

What are the main metabolic wastes and where are they excreted from?

1. Carbon dioxide- waste product of cellular respiration, excreted from the lungs.

2. Nitrogenous waste products (urea)- formed from the breakdown of excessive amino acids in the liver, excreted by the kidneys in the urine.

How is oxygenated blood supplied to the liver?

• The hepatic artery.

How is oxygenated blood removed from the liver?

• Removed and returned to the heart by the hepatic vein.

What vein carries blood loaded with the products of digestion to the liver?

• The hepatic portal vein, carries it straight from the intestines to the liver.

What are liver cells called and what is their basic structure?

• Liver cells are called hepatocytes.

• They have a large nucleus, prominent Golgi apparatus and lots of mitochondria as they are metabolically active cells.

What area do the blood from the hepatic artery and hepatic portal vein mix?

• Areas called the sinusoid.

Why do the blood supplies of the two blood vessels mix in the sinusoid?

• The mixing increases the oxygen content of the blood from the hepatic portal vein.

• It also supplies the hepatocytes with enough oxygen for their needs.

What are Kupffer cells and what is their function?

• The Kupffer cells are found in the sinusoid and are the resident macrophages of the liver, they ingest foreign particles to protect against disease.

What is the bile canliculus?

• The hepatocytes secrete bile from the breakdown of blood, this goes to spaces called the canaliculi, and from these the bile drains into bile ductules which take it to the gall bladder.

What are the three main roles of the liver?

1. Storage of glycogen

2. Detoxification

3. The formation of urea from ammonia reacting with carbon dioxide as part of the ornithine cycle

How is the liver involved in the storage of glycogen?

• 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.

How is the liver involved in detoxification?

• Many metabolic pathways produce potentially poisonous substances, the liver is the site that detoxifies toxins and makes them harmless.

• For example, hepatocytes contain catalase which breaks down hydrogen peroxide into oxygen and water.

How is the liver involved in the formation of urea?

• Ammonia is a very soluble toxic compound that can be damaging if it builds up in the blood, to avoid the toxic effects it is converted into urea during the ornithine cycle.

• Ammonia is combined with carbon dioxide to form urea which is less toxic.

What are the two homeostatic roles of the kidneys?

1. Excretion

2. Osmoregulation

How is blood supplied to the kidneys?

• By the renal arteries with arterial pressure that branch off the abdominal aorta.

How is blood removed from the kidneys?

• Removed by the renal vein that drains into the inferior vena cava.

What are kidneys made up of?

• Made up of millions of small structures called nephrons that act as filtering units that produce urine.

What are the three main areas of the kidney?

1. Cortex- dark outer layer where blood filtering takes place, very dense capillary network.

2. Medulla- contains the tubules of the nephrons that from the pyramids of the kidney and collecting ducts.

3. Pelvis- central chamber where the urine collects.

What is osmoregulation?

• The balancing and control of the water potential of the blood.

What are the five main structures of the nephron?

1. Bowman’s capsule

2. Proximal convoluted tubule

3. Loop of Henle

4. Distal convoluted tubule

5. Collecting duct

What is the structure and function of the Bowman’s capsule?

• Cup-shaped structure that contains the glomerulus (tangle of capillaries).

• More blood goes into the glomerus than leaves it due to ultrafiltration processes that take place.

What is the structure and function of the Proximal convoluted tubule?

• 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.

What is the structure and function of the Loop of Henle?

• Long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the medulla.

• The descending loop runs from the cortex to the medulla to a hairpin bend at the bottom.

• The ascending limb travels back up through the medulla to the cortex.

What is the structure and function of the Distal convoluted tubule?

• A second twisted tubule where the fine-tuning of the water balance takes place.

• Permeability of the walls to water varies in response to levels of antidiuretic hormone (ADH) in the blood.

What is the structure and function of the Collecting duct?

• Urine passes down the collecting duct through the medulla to the pelvis, more fine-tuning of water balance takes place.

What solutes are in the blood when it leaves the kidney?

• Greatly reduced levels of urea but levels of glucose, and amino acids are almost the same as when the blood entered the kidney.

What is ultrafiltration?

• The process by which blood plasma is filtered through the walls of the Bowman’s capsule under pressure.

Describe the process of ultrafiltration of the blood.

1. The glomerulus is supplied with blood from an afferent arteriole from the renal artery.

2. The blood leaves through a narrower efferent arteriole which as a result creates pressure in the capillaries of the glomerulus.

3. This forces the blood out through the capillary wall, acting like a sieve.

4. The blood then passes through the basement membrane made of collagen fibres and other proteins, that act as a second sieve.

5. Most of the plasma contents can pass through, but blood cells and many proteins cannot due to their size.

6. The wall of the Bowman’s capsule also has podocytes which act as an additional filter.

What are the main substances that pass out of the capillaries and form the glomerular filtrate?

1. Amino acids

2. Water

3. Glucose

4. Urea

5. Inorganic ions

What are the podocytes and what is their function?

• Cells that act as an additional filter located on the wall of the Bowman’s capsule.

• They have extensions that wrap around the capillaries, forming slits to make sure any cells, platelets or large plasma proteins do not make it into the tubule.

Why is selective reabsorption needed after ultrafiltration?

• Ultrafiltration removes urea from the blood but also lots of water, glucose, salt and other substances present in the plasma.

• Many of these substances are needed in the body, like glucose for cellular respiration.

• The main function of the nephron after the Bowman’s capsule is to return most of the filtered substances back to the blood.

Where does most reabsorption occur?

• In the proximal convoluted tubule.

Describe the process of selective reabsorption.

1. In the proximal convoluted tubule all of the glucose, amino acids, vitamins and hormones are moved from the filtrate back into the blood by active transport.

2. Some sodium chloride and water is reabsorbed. The sodium ions are moved by active transport while the chloride ions and water follow passively down concentration gradients.

3. Once removed from the nephron, the substances move into the capillary network which surrounds the tubules down steep concentration gradients. These are maintained by the constant flow of blood in the capillaries.

4. At this stage, most of the glomerular filtrate has been reabsorbed back into the blood.

How does the loop of Henle act as a countercurrent multiplier?

• It uses energy to produce concentration gradients that result in the movement of substances from once area to another.

What do the changes that take place in the descending limb depend on?

• Depends on the high concentrations of sodium and chloride ions in the tissue fluid of the medulla that are the result of events in the ascending limb of the loop.

Describe the countercurrent mechanism of the descending limb of the loop of Henle.

1. Descending limb leads from the proximal convoluted tubule, this is the region where water moves out of the filtrate down a concentration gradient. The upper part is impermeable to water but the lower part is permeable and runs down into the medulla.

2. The concentration of sodium and chloride ions in the tissue fluid of the medulla gets higher moving through the cortex to the pyramids, as a result of activity in the ascending limb.

3. The filtrate entering the descending limb is isotonic with the blood, as it travels down water passes out of the loop into the tissue fluid by osmosis down a concentration gradient. It then moves down a concentration gradient into the blood of the surrounding capillaries.

4. The descending limb is not permeable to sodium and chloride ions, no active transport takes place, the fluid that reaches the bend is very concentrated and hypertonic to the blood in the capillaries.

Describe the countercurrent mechanism of the ascending limb of the loop of Henle.

1. The first section of the ascending limb is very permeable to sodium and chloride ions and they move out of the concentrated solution down a concentration gradient by diffusion.

2. In the second section, sodium and chloride ions are actively pumped out into the medulla tissue fluid against a concentration gradient. It is also impermeable to water, so water cannot follow the ions.

3. The fluid left in the ascending limb is increasingly dilute while the tissue fluid of the medulla develops a very high concentration of ions essential to producing urine.

4. By the time it reaches the top, it is hypotonic to blood again and it enters the distal convoluted tubule and the collecting duct.

Describe how water is balanced in the distal convoluted tubule in response to ADH.

• The permeability of the walls and tubules varies depending on the levels of ADH.

• If the body lacks salt, sodium ions will be actively pumped out of the distal convoluted tubule with chloride ions following down the electrochemical gradient.

• Water can also leave the distal convoluted tubule, concentrating the urine, if the walls are permeable in response to ADH.

• It also plays a role in balancing the pH of the blood.

How is the distal convoluted tubule adapted to balancing water?

• The cells lining the walls have many mitochondria so they are adapted to carry out active transport.

Describe the collecting ducts role in determining the concentration and volume of the urine.

• Water moves out of the collecting duct by diffusion down a concentration gradient as it passes through the renal medulla, as a result the urine becomes more concentrated.

• The level of sodium ions surrounding the fluid increases through the medulla, this means water can be removed from the collecting duct all the way along its length. This produces very hypertonic urine when the body needs to conserve water.

• The permeability of the collecting duct to water is controlled by the level of ADH, which determines how much or how little water is reabsorbed.

What are osmoreceptors?

• Osmoreceptors are sensory receptors that respond to the changes in the water potential of the blood.

What are osmoreceptors sensitive to?

• The concentration of inorganic ions in the blood and are linked to the release of ADH.

Describe how the walls of the collecting duct respond to ADH when water is in short supply.

1. When water is in short supply, the concentration of inorganic ions in the blood rises and the water potential of the blood and tissue fluid becomes more negative. This is detected by the osmoreceptors in the hypothalamus.

2. They send nerve impulses to the posterior pituitary which in turn releases stored ADH into the blood.

3. The ADH is picked up by receptors in the cells of the collecting duct and increases the permeability of the tubules to water.

4. Water leaves the filtrate in the tubules and passes into the blood in the surrounding capillary network.

Describe how the walls of the collecting duct respond to ADH when water is in excess.

1. When large amount of liquid are taken in, the blood becomes more dilute and its water potential becomes less negative. This change is detected by the osmoreceptors of the hypothalamus.

2. Nerve impulses to the posterior pituitary are reduced or stopped and so the release of ADH is inhibited.

3. Very little reabsorption can occur because the walls of the collecting duct remain impermeable to water.

Describe the mechanism of ADH action in the control of water potential.

1. ADH is released from the posterior pituitary gland and carried to the wall of the collecting duct where it has an effect.

2. It binds to receptors on the cell surface membrane and triggers the formation of cyclic AMP (cAMP) as a secondary messenger inside the cell. The cAMP causes a cascade of events:

• Vesicles in the cells lining the collecting duct fuse with the cell surface membranes on the side of the cell in contact with the tissue fluid of the medulla.

• The membranes of these vesicles contain aquaporins (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 by osmosis.

What could happen as a result of a kidney infection or high blood pressure?

1. Protein in the urine- if the basement membrane or podocytes of the Bowman’s capsule are damaged, they no longer act as filters and large plasma proteins can pass into the filtrate and then into the urine.

2. Blood in the urine- the filtering process is not working.

What will happen to glomerular filtration rate (GFR) if the kidneys fail?

• Rate of filtration is not measured directly but instead through a blood test that measures the level of creatinine in the blood. Creatinine is a breakdown product of muscles and used to give an estimated glomerular filtration rate.

• If the levels of creatinine in the blood go up, it is an indication that the kidneys are not working properly.

What will happen to electrolyte balance if the kidneys fail?

• There will be a loss of electrolyte balance if the kidneys fail as it means that the body cannot excrete excess sodium, potassium and chloride ions.

• This causes osmotic imbalances in the tissues and eventual death.

What are the two main ways of treating kidney failure?

1. Renal disalysis

2. Transplant

What is renal dialysis?

• The process where the functions of the kidney are carried out artificially to maintain the salt and water balance of the blood.

Describe the process of Haemodialysis.

1. Blood leaves the patients body from an artery and flows into the dialysis machine, where it flows between partially permeable dialysis membranes. These membranes mimic the basement membranes of the Bowman’s capsule.

2. On the other side of the membranes is the dialysis fluid. During dialysis it is vital that patients lose the excess urea and mineral ions that have built up in the blood but is also equally as important to not lose useful substances like glucose.

3. The loss of these substances is prevented by the careful control of the dialysis fluid. It contains normal plasma levels of glucose to ensure there is no net movement of glucose out of the blood. The fluid also contains normal plasma levels of mineral ions so any excess in the blood move out by diffusion down a concentration gradient, restoring the electrolyte balance.

4. The dialysis fluid contains normal plasma urea meaning there is a steep concentration gradient from the blood to the fluid and as a result much of the urea leaves the blood. The blood and dialysis fluid flow in opposite directions to maintain a countercurrent exchange system and maximise exchange that takes place.

How can we treat kidney failure through transplantation?

• A single healthy kidney is placed in the body from a donor.

• The blood vessels are joined and the ureter of the new kidney is inserted into the bladder.

What is the main problem with kidney transplants?

• The main problem is the risk of rejection.

• The antigens on the donor organ differ from the antigens on the cells of the recipient and the immune system is likely to recognise this.

• This can result in the destruction and rejection of the new kidney.

What are monoclonal antibodies?

• Antibodies from a single clone of cells that are produced to target particular cells or chemicals in the body.

What are the main stages of a pregnancy test?

1. Wick is soaked in the first urine passed in the morning- this will have the highest levels of hCG.

2. Test contains mobile monoclonal antibodies that have very small coloured beads attached to them, these beads will only bind to hCG. If the person is pregnant the hCG in her urine binds to the mobile monoclonal antibodies and forms a hCG/ antibody complex.

3. The urine carries on along the test strip until it reaches a window.

4. Here there are immobilised monoclonal antibodies arranged in a line or a pattern such as a positive sign that only bind to the hCG/ antibody complex. If the woman is pregnant a coloured line or pattern will appear in the window.

5. The urine continues up the strip to a second window.

6. Here there is usually a line of immobilised monoclonal antibodies that only bind to mobilised antibodies. The coloured line forms regardless of whether the woman is pregnant or not- it simply indicates the test is working.

What are anabolic steroids?

• Drugs that mimic the action of testosterone and stimulate the growth of muscles. They are excreted in the urine.

How can we test for anabolic steroids?

• We can test the urine using gas chromatography and mass spectrometry, the urine sample is vaporised with a known solvent and passed along a tube.

• The lining of the tube absorbs the gases and is analysed to give a chromatogram that can be read to show the presence of the drugs.

How can we use urine for drug testing?

• Urine is tested because the breakdown products of drugs are filtered through the kidneys and stored in the bladder, so it is possible to find traces of the drug in the urine some time after it has been used.

• A urine sample is first tested by an immunoassay using monoclonal antibodies to immobilise the breakdown products, if that shows positive the second sample will be passed through a gas chromatography to confirm the presence of the drug.