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 glomerus (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.