Module 5: Communication and homeostasis

Question 1

Define homeostasis. Why is it important in organisms?

Answer 1

Maintaining constant internal conditions despite external changes. Vital for enzyme action and cell function.
Factors controlled by homeostasis include blood glucose concentration, body temp, blood pH.

Question 2

What is negative feedback? (homeostasis)

Answer 2

Action taken by the body to return something to a normal level after deviation e.g. controlling body temp.

Question 3

What is positive feedback?

Answer 3

Amplifies a response to a stimulus, causing it to deviate further from the normal range e.g. blood clotting.

Question 4

What is an ectotherm?

Answer 4

Relies on heat in surroundings to warm their bodies. Core body temp depends on environment. Ectotherms cannot control their body temp using metabolism.

Lizards living in colder climates tend to be dark coloured as dark absorb more radiation than light colours (which reflect).

Ectotherms need less food as they use less energy regulating their body temp, so they can survive in some habitats where food is scarce.

Question 5

Give 2 examples of an ectotherm.

Answer 5

Lizard, locust.

Question 6

Give an example of an ectotherm warming up or cooling down through:

a) Radiation
b) Conduction
c) Inflating/deflating
d) Evaporation

Answer 6

a) Sunbathe to gain radiation from sunlight
b) Lizard presses against the hot earth to gain heat
c) Maximises/reduces SA exposed to Sun
d) Panting during the day/mouth open cools the animal down by evaporation of water from surfaces.

Question 7

What is an endotherm? (humans)

Answer 7

Rely on metabolic processes to warm up instead of relying on environment. Endotherms survive in a wide range of environments.
The metabolic rate of endotherms as 5x more than ectotherms, so they need to consume more food.

Question 8

The peripheral temperature receptors on skin detect …
Temperature receptors in the hypothalamus detect …

Answer 8

Changes in skin surface temperature.
Detect temperature of the blood.

Question 9

Explain what happens to the body when you are hot.

Answer 9

1. Increased sweat - water evaporates from skin to cool it down. The water takes away the heat from the body.
2. Hairs lie flat - ERECTOR PILI MUSCLES relax. Avoids trapping an insulating layer of air. Heat can be lost more easily.
3. Vasodilation - ARTERIOLES near the surface of the skin dilate. More heat is lost/radiated from the skin.

Question 10

Explain what happens to the body when you are cold.

Answer 10

1. Shiver - increase respiration in muscles so more thermal energy is released.
2. More adrenaline and thyroxine are produced so more metabolism.
3. Hairs rise - traps an insulating layer of air to prevent heat loss. ERECTOR PILI MUSCLES contract.
4. Vasoconstriction - ARTERIOLES near surface of skin contract, little radiation takes place.

Question 11

Thermoreceptors are found in the …

Answer 11

Skin AND hypothalamus.

Question 12

What is the function of thermoreceptors in the skin?

Answer 12

Send impulses along sensory neurones to the hypothalamus, which sends impulses along motor neurones to effectors. The effectors respond by returning the body back to normal.

Question 13

Why are you more hungry when you’re cold?

Answer 13

Higher metabolic rate, so more glucose for respiration of contracting muscles as you shiver.

Question 14

What is the difference between excretion and defecation?

Answer 14

Excretion - removal of the waste products of metabolism from the body, including CO2, urea, and bile.
Defecation - removal of undigested food, dead cells, and bacteria from the body.

Question 15

What are the 3 main metabolic waste products in mammals?

Answer 15

=> CO2 from respiration excreted from the lungs.
=> Bile pigments formed from the breakdown of haemoglobin from old RBC’s in the liver. They are excreted in the bile from the liver into small intestine via gall bladder and bile duct. They colour faeces.
=> Urea formed from the breakdown of excess amino acids by the liver. Urea is excreted by the kidneys in urine.

Question 16

What is the function of the gall bladder?

Answer 16

The gallbladder stores bile, a thick liquid that’s produced by the liver to help us digest fat. When we eat, the gallbladder’s thin, muscular lining squeezes bile into the small intestine through the main bile duct. The more fat we eat, the more bile the gallbladder injects into the digestive tract.

Question 17

What actually is bile?

Answer 17

Fluid that is made and released by the liver and stored in the gallbladder. Bile breaks down fats from the foods you consume into fatty acids.

Question 18

What is bilirubin?

Answer 18

Yellow pigment that gives bile its colour. It is a waste product formed from the breakdown of old or damaged RBC’s. The liver processes bilirubin and helps excrete it from the body in bile.

Question 19

Describe the structure of the liver.

Answer 19

Oxygenated blood is supplied to liver by hepatic artery.
Hepatic portal vein carries nutrient-rich blood and toxins after digestion, from intestines and gall bladder, to the liver. They’re processed and filtered in the liver.
Hepatic vein removes deoxygenated blood carrying products of detoxification e.g. urea from liver to vena cava. Then into systemic circulation and to the heart.

Question 20

What happens to the blood that reaches the liver?

Answer 20

Blood from hepatic artery and hepatic portal vein is mixed in spaces called sinusoids surrounded by hepatocytes. This mixing increases O2 content of the blood, supplying hepatocytes with enough oxygen.

Question 21

What do the sinusoids contain?

Answer 21

Kupffer cells that act as macrophages to ingest foreign particles and help to protect against disease. Also break down old RBC’s.

Question 22

What do the hepatocytes do? (think about function of liver)

Answer 22

Secrete bile from the breakdown of haemoglobin into spaces called canaliculi. From these the bile drains into bile ducts which take it to the gall bladder.

Question 23

How is the liver involved in controlling blood glucose levels?

Answer 23

When blood glucose rises, insulin level rises and stimulate hepatocytes to convert glucose to storage carbohydrate glycogen. Vice versa but with glucagon.

Question 24

Why does the liver need to break down excess amino acids? (Excretion)

Answer 24

Nitrogenous substances are damaging to the body so if they aren’t used up, they must be excreted.

Question 25

Explain how the liver is involved in the breakdown of excess amino acids coming from the digestion of protein.

Answer 25

Hepatocytes carry out transamination - converting 1 amino acid into another. This is vital as the body doesn't always contain the right balance of amino acids. The first step of amino acid excretion is the deamination - the removal of an amine group from a molecule, leading to formation of ammonia. Amino acid + oxygen = keto acid + ammonia This is then converted to urea by the addition of CO2 in the ornithine cycle. This is because ammonia is very soluble and toxic. All of this occurs in the hepatocytes. Finally, the urea is released from liver into the blood and subsequently filtered out by the kidneys to produce urine. The rest of the amino acid molecules are used in respiration.

Question 26

Why is the ammonia converted straight away to urea?

Answer 26

Urea is much less toxic than ammonia, allowing urea to be stored before excretion as urine.

Question 27

Describe how the liver is involved in detoxification.

Answer 27

The liver is involved in the removal of harmful substances such as drugs and alcohol: Hepatocytes contain catalase. hepatocytes contain alcohol dehydrogenase that breaks down ethanol to ethanal, which is then converted to ethanoate which is used in respiration or converted to fatty acids.

Question 28

How can continuous alcohol detoxification cause problems in the liver?

Answer 28

=> Metabolism of ethanol makes ATP, so hepatocytes don't metabolise as much as usual and so store fat, which leads to fatty liver. => Stored fat reduces ability of hepatocytes to function, leading to cirrhosis.

Question 29

2 main functions of the kidneys?

Answer 29

Excretion and osmoregulation.

Question 30

Describe the 3 main areas in the kidney.

Answer 30

=> Cortex is the darker outer layer. It contains the glomerulus, Bowman's capsule, PCT and DCT. => Medulla contains loop of Henle and collecting duct. => Renal pelvis is the central chamber where urine collects before passing down the ureter.

Question 31

Renal artery - carries oxygenated blood containing urea and salts to kidneys. Renal vein - carries oxygenated blood (that has had urea and excess salts removed) away form kidneys. Ureter - carries urine from kidneys to bladder. Bladder - stores urine temporarily. Urethra - releases urine outside the body.

Answer 31

Cortex: The outer layer of the kidney, where the glomerulus, Bowman's capsule, proximal convoluted tubule, and distal convoluted tubule of the nephron are situated. Medulla: The inner layer of the kidney, where the loop of Henle and collecting duct of the nephron are located.

Question 32

Summary of kidney function.

Answer 32

Blood enters each kidney through the renal artery (aorta) and subsequently passes through the capillaries in the cortex of the kidneys. Blood leaves the kidneys through the renal vein which is attached to vena cava.

Question 33

Ultrafiltration.

Answer 33

Blood arrives at the glomerulus (a bundle of capillaries) in the Bowman's capsule of the nephron, through the afferent arteriole which has branched from the renal artery. Blood leaves the glomerulus through the efferent arteriole to the renal vein. The afferent arteriole has a wider lumen than the efferent arteriole, so a greater volume of blood can pass into the glomerulus via the afferent arteriole. Glomerulus is therefore under hydrostatic pressure, allowing small molecules to pass out of the blood and into the Bowman's capsule. Small molecules like urea/water/glucose are absorbed into the Bowman's capsule, leading to the PCT. RBC's and plasma proteins can't pass into the nephron due to large size. The Bowman's capsule contains cells called podocytes with lots of capillaries. Podocytes have pedicels which are wrapped around the outside of the capillary. The gaps between the pedicels allow small molecules to pass through. There are pores between the endothelial cells of the capillary that allow small molecules to leave the blood.

Question 34

Selective reabsorption.

Answer 34

Many useful molecules in the Bowman's capsule like glucose/amino acids are needed by the body so selective reabsorption happens in the proximal convoluted tubule (PCT). PCT is found in the cortex. 100% glucose and amino acids are reabsorbed back into the blood. Lots of water and salts are also reabsorbed into the blood. Urea remains in the tubule. Glucose and amino acids are reabsorbed by active transport by co-transporter proteins (Na+ needs to be reabsorbed at the same time). The remaining structures are: loop of Henle, distal convoluted tubule and collecting duct. These are involved in regulating water concentration in blood.

Question 35

Describe some adaptations of the kidney to carry out selective reabsorption.

Answer 35

=> Cells lining the PCT have microvilli - large SA for reabsorption of molecules. Contain many mitochondria - to provide the ATP for active transport. => Infoldings on the membrane near the blood capillary provide a large SA for transfer of molecules into blood.

Question 36

What is the loop of Henle and its function?

Answer 36

Has a very high solute concentration deep in the kidney medulla. The main function of the Loop of Henle is to produce a low water potential in the medulla of the kidney. It does this by acting as a counter-current multiplier to produce concentration gradients.

Question 37

How does the counter-current multiplier work?

Answer 37

First, fluid moves down the descending limb which has thin walls and is very permeable to water, so as it moves down, water potential decreases. The filtrate's water potential is lowest at the bottom of the loop. The fluid makes its way up the ascending limb, which is thick walled and IMPERMEABLE to water. It allows the active transport of Na+ and Cl- out of the filtrate and into the interstitial region so the water potential of the filtrate rises again. In between the limbs is the interstitial region. This process requires ATP, which is supplied by the mitochondria in the cells of the ascending limb. Now therefore the medulla has a very low water potential. As descending limb passes down, water moves out of fluid and into the medulla by osmosis down the concentration gradient. Then the water moves into the blood through the capillaries by osmosis and is carried away.

Question 38

Why is the counter-current multiplier system important?

Answer 38

Results in low water potential in the medulla. This would mean that water moves out of the collecting duct by osmosis. So humans produce a small volume of concentrated urine.

Question 39

Distal convoluted tubule.

Answer 39

Permeability of walls to water depends on levels of ADH and aquaporins in the blood. Also controls ion balance. The cells lining the distal convoluted tubule have many mitochondria so are adapted to carry out active transport. If the body lacks salt, Na+ will be actively pumped out of the distal convoluted tubule with Cl- ions down an electrochemical gradient. Water can also leave the distal tubule, concentrating urine. Water is reabsorbed by osmosis back into the blood from these tubules.

Question 40

Collecting duct.

Answer 40

Permeability of walls to water depends on levels of ADH and aquaporins in the blood. The collecting duct passes down the concentrated tissue fluid of the renal medulla. Water moves out of collecting duct by diffusion down a concentration gradient as it passes through the renal medulla. Urine passes down the collecting duct through the medulla to pelvis.

Question 41

ADH is produced by the ... ADH is secreted into the ...

Answer 41

Hypothalamus. Osmoreceptors detect water potential of the blood. Posterior pituitary gland where it is stored.

Question 42

What actually does ADH do?

Answer 42

Increase the permeability of the collecting duct and distal convoluted tubule to water. ADH causes kidneys to reabsorb more water.

Question 43

How does ADH have its effect?

Answer 43

ADH binds to receptors on the cell membrane and triggers the formation of cAMP as a secondary messenger (relays signals received at cell surface receotirs ti molecules inside the cell).

Question 44

The cAMP causes a cascade of events. What are these?

Answer 44

Vesicles in the collecting duct fuse with the cell surface membrane in contact with medulla. These vesicles contain aquaporins that insert into cell surface membrane and make it permeable to water. The more ADH that is released, the more water channels are inserted into the membranes of tubule cells/collecting duct becomes more permeable. This it makes it easier for water to leave the tubules by diffusion, resulting a small amount of concentrated urine. Water is returned to the capillaries.

Question 45

Explain what happens when you are dehydrated.

Answer 45

Osmoreceptors in the hypothalamus detect the low water potential of the blood and send nerve impulses to the posterior pituitary gland to release the stored ADH into the blood. ADH binds to receptors on the cells of the collecting duct and increases the permeability of the tubules to water. Water leaves the filtrate and enters blood. Small volume of concentrated urine is produced.

Question 46

Explain what happens in an excess of water.

Answer 46

Osmoreceptors detect high water potential in blood so nerve impulses to posterior pituitary gland are reduced/stopped. and Less ADH released mean the aquaporin molecules return back to their vesicles, making the wall of the collecting duct less permeable to water. Large amounts of dilute urine is produced.

Question 47

Maybe card?

Answer 47

ADH also causes the wall of the collecting duct to be more permeable to urea. Urea now moves from fluid in collecting duct into medulla, lowering the water potential of medulla, increasing reabsorption of water.

Question 48

Medical diagnosis.

Answer 48

Urine samples can be used in diagnostic tests such as pregnancy testing with the use of monoclonal antibodies which test for the presence of human chorionic gonadotropin which is found in the urine of pregnant women. Urine samples can also be used to test for the presence of anabolic steroids. Anabolic steroids are used to build muscle mass which are banned due to their dangerous side effects as well as to ensure that the competition between athletes is fair. The presence of steroids is detected via gas chromatography by measuring the time taken for the urine sample to pass the column compared to the time taken for a steroid to pass through.

Question 49

Kidney failure.

Answer 49

Kidney failure can be triggered by various kidney infections which cause inflammation. Certain viral infection can weaken the connections between cells of glomerulus and Bowman's capsule. High blood pressure can also damage Bowman's capsule. The resulting damage causes the kidneys to perform processes such as filtration and reabsorption less efficiently. High blood pressure can also cause damage to the kidney by damaging the capillaries of glomeruli thus meaning that larger molecules can find their way into urine. The consequences of kidney failure include the build-up of toxic waste products such as urea which causes symptoms such as vomiting. In cases where excess water cannot be removed from the blood by the kidneys, fluid accumulation occurs which leads to swelling. Apart from this, kidney failure can disrupt the balance of ions, resulting in making the bones more brittle or causing water to be retained.

Question 50

How can kidney failure be treated?

Answer 50

can be treated with a renal dialysis which filters the blood with the help of a machine containing dialysis fluid which serves as a means of removing the waste products as well as excess water and ions. Dialysis is only a temporary solution while the patient awaits a transplant. Dialysis needs to be performed several times a week and causes the patient to feel unwell between sessions as the toxic waste builds up. There are 2 types of dialysis: * Haemodialysis – removes blood from the body and pumps it through a machine where the blood is run in countercurrent flow alongside dialysis fluid. These fluids are separated by an artificial membrane so must rely on diffusion gradients for molecules to move from one fluid to the other. A blood thinning agent must be added to avoid the blood clotting outside of the body. * Peritoneal dialysis – dialysis fluid is put into the body cavity so that exchange can happen across the body’s own peritoneal membrane. The fluid must be drained and replaced. Kidney transplant is required to replace the damaged kidney and to reverse kidney failure symptoms, it is believed to be the better solution as it is a long term solution. Sometimes patients need to wait a long time for a suitable donor which needs to be of the same blood type and tissue type to minimise the risk of rejection. However, immunosuppressants still need to be taken by the patient to prevent rejection. In most cases a donor is a family member due to the degree of similarity, this is possible as only one kidney is required for survival.

Question 51

Haemodialysis.