Excretion as an example of homeostatic control

Question 1

What is excretion

Answer 1

this is the removal of metabolic wastes from the body

Question 2

What products must be excreted

Answer 2

• carbon dioxide from respiration
• nitrogen containing compounds such as urea
• other compounds such as the bile pigments found in faeces

Question 3

What are the excretory organs

Answer 3

• the lungs
• the live
• the kidney
• the skin

Question 4

How do the lungs excrete

Answer 4

• every living cell in the body produces carbon dioxide
• it is passed from living cells into the blood stream where it is transported mostly in the form of hydrogen carbonate ions to the lungs
• go into the alveoli and then they are excreted as you breathe out

Question 5

How does the liver excrete

Answer 5

• liver is involved in excretion
• many metabolic roles and some of the substances produced will be passed into the bile for excretion with the faeces
• it is involved in converting amino acids into urea, amino acids are broken down by the process of deamination, the nitrogen containing part of the molecule is then combined with carbon dioxide to make urea

Question 6

How do the kidneys excrete

Answer 6

• urea is passed into the blood stream into the kidneys, they are transported in solution and dissolved in the plasma
• in the kidneys the urea is removed from the blood plasma to become part i the urine, its stored in the bladder and then excreted from the body via the urethra

Question 7

How does the skin excrete

Answer 7

• skin is involved in excretion, not primary function of the skin
• sweat contains, salt, urea, water and uric acid
• loss of water is involved in maintaining body temperature and the water potential of the blood

Question 8

why is excretion important

Answer 8

• allowing the metabolism to build up could be fatal, carbon dioxide and ammonia are toxic and they interfere with cell processes by altering the pH therefore preventing normal metabolism
• other metabolic products may act as inhibitors and reduce the activity of essential enzymes

Question 9

What do the hydrogen ions do inside the red blood cells

Answer 9

• the hydrogen ions affect the pH of the cytoplasm in the red blood cells
• hydrogen ions interact with bonds within haemoglobin, and change its 3D shape, this reduces the affinity of haemoglobin for oxygen and affects oxygen transport
• hydrogen ions can then combine with the haemoglobin and form haemoglobinic acid
• carbon dioxide that is not converted to hydrogen carbonate ions combine directly with haemoglobin and produce carbaminohaemoglobin, they are unable to combine with oxygen as normal

Question 10

What do hydrogen ions do in blood plasma

Answer 10

• reduce the pH of the plasma

Question 11

Why is it important to maintain the pH of blood plasma

Answer 11

• if it changes that it can alter the structure of the many proteins in the blood that transport a wide range of substances around the body
• has proteins in the blood to reduce the change in pH

Question 12

What happens if the change in pH is small

Answer 12

• excess hydrogen ions are detected by the respiratory centre in the medulla oblongata of the brain which causes an increase in the breathing rate to help remove the excess carbon dioxide

Question 13

What happens if the change in pH is large

Answer 13

• headaches, drowsiness, restless, tremor and confusion
• rapid heart rate and change in blood pressure
• can be caused by diseases or conditions that affect the lungs themselves

Question 14

What happens to wasteful amino acids

Answer 14

• body cannot store excess amino acids
• amino acids contain almost as much energy as carbohydrates therefore wasteful just to excrete them
• they are transported to the liver where the toxic amino group is removed, then it is converted into ammonia which is converted into urea which is less toxic and soluble
• keto acid is used in respiration to release energy or may be converted to a carbohydrate for fat storage

Question 15

Deamination equation

Answer 15

amino acid + oxygen = Keto acid + ammonia

Question 16

formation of urea equation

Answer 16

ammonia + carbon dioxide = urea + water

Question 17

What does the liver do

Answer 17

• carries out metabolic processes
• important role in homeostasis
• therefore it has to have a good blood supply and the internal structure of the liver ensures as much blood as possible flows past liver cells to remove unwanted substances and makes sure concentrations are maintained

Question 18

What is the blood sources to and from the liver

Answer 18

the hepatic artery

the hepatic portal vein

Question 19

Describe the hepatic artery

Answer 19

• this is when oxygenated blood from the heart travels from the aorta via the hepatic artery into the liver
• supplies oxygen for aerobic respiration this is important as liver cells are very active as they carry many metabolic processes and many of these require energy in the form of ATP

Question 20

Describe the hepatic portal vein

Answer 20

• deoxygenated blood from the digestive system enters the liver via the hepatic portal vein
• blood is rich in the products of digestion, and the concentrations are uncontrolled as they have just come from the intestines
• blood may also contain toxic compounds that have been absorbed from the intestine therefore it is important that substances do not continue to circulate around the body before concentrations are adjusted
• blood leaves the liver by the hepatic vein which rejoins the vena cava and the blood returns to the bodies normal circulation
• 4th vessel connected to the liver called the bile duct, secretion from the liver has functions in digestion an excretion and the bile duct carries bile to the gall bladder where it is stored until it needs to digest fats in the small intestine

Question 21

what else does bile produce

Answer 21

it produces belirubin which leaves the body with the faeces

Question 22

Describe the structure of the liver

Answer 22

• blood vessels arranged to make sure that there is the greatest possible contact between the blood and the liver cells
• liver divided into lobes which are divided into lobules these are cylindrical
• the hepatic artery and portal vein enter the liver into smaller vessels they run parallel to the lobules and are known as inter-lobular vessels
• at intervals branches from the hepatic artery and the hepatic portal vein enter the lobules
• blood is mixed and passes along a special chamber called the sinusoid which is in close contact with the liver cells, they are able to remove substances from the blood and return other substances to the blood
• specialised macrophages called kupffer cells move about within the sinusoids, these break down and recycle old red blood cells a product of this is bilirubin
• when blood reaches the end of the sinusoid the concentrations of many of its components have been modified and regulated
• at the centre of each lobule is a branch of the hepatic vein known as the intra-lobular vessel, the sinusodis empty into this vessel and the branches of the hepatic vein from different lobules join together to form the hepatic vein which gets rid of blood in the liver

Question 23

How is bile duct formed

Answer 23

• made in the liver, released into the bile canaliculi this joins together to form the bile duct

Question 24

Describe liver cells

Answer 24

• liver cells appear to be unspecialised, they have cubodial shape with many microvilli on their surface
• functions include protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts
• cytoplasm dense and specialised in the numbers of certain organelles that it has

Question 25

name the metabolic functions that the liver has to carry out

Answer 25

- control blood glucose levels, amino acids levels and lipid levels - synthesis of bile, plasma proteins, cholestrol - synthesis of red blood cells in the fetus - storage of vitamins A D and B12 and iron and glycogen - detoxification of alcohol and drugs - breakdown of hormones - destruction of red blood cells

Question 26

How is glycogen stored in the liver

Answer 26

- liver - stores 100-120g of glycogen - glycogen forms granules in the cytoplasm of the hepatocytes, glycogen can be broken down into glucose into the blood

Question 27

Name the enzymes in the liver cells that are involved in breaking substances down

Answer 27

- catalase | - cytochrome P450

Question 28

How can toxins be rendered harmless

Answer 28

- oxidation - reduction - methylation - combining with another molecule - liver cells contain enzymes that render the toxic molecules less toxic

Question 29

How is catalase broken down

Answer 29

- converts hydrogen peroxide to oxygen and water | - catalase has a particularly high turnover number

Question 30

How is cytochrome P450 broken down

Answer 30

- group of enzymes used to break down drugs including cocaine and various medicinal drugs - used in other metabolic reactions such as the electron transport during respiration

Question 31

How is alcohol detoxified

Answer 31

- alcohol is broken down in the hepatocytes by the action of the enzyme dehydrogenase forming ethanal - this is dehydrogenated further by the enzyme ethanal dehydrogenase - final compound that is produced is ethanoate which is combined with coenzyme A to form acetyl coenzyme A which enters the process of aerobic respiration - hydrogen atoms that are released from the alcohol are combined with another coenzyme called NAD to form reduce NAD

Question 32

What is NAD required to do

Answer 32

- NAD is required to oxidise and breakdown any fatty acids for use in respiration - if the liver has to detoxify to much alcohol it uses stores of NAD and has insufficient left to deal with the fatty acids, these fatty acids are then converted back to lipids and stored as fats in hepatocytes which cause the lipids to become enlarged

Question 33

How is urea formed

Answer 33

- consist of two processes these are deamination and ornithine Deamination - removes the amino group and produces ammonia - ammonia is soluble and toxic therefore it is not allowed to accumulate - produces keto acid which can enter respiration directly to release energy Orithine cycle - as ammonia is soluble and toxic it must be converted to a less toxic form quickly, - ammonia is combined with carbon dioxide to produce urea - ammonia and carbon dioxide combine with the amino acid to produce ornithine to produce citrulline which is converted into arginine by addition of further ammonia - arginine is then re-converted to ornithine by the removal of urea

Question 34

What is the role of the kidney

Answer 34

- the role of kidneys is excretion, they remove waste products from the blood and produce urine, urine passes out of the kidney down the ureter to the bladder where it can be stored until its released

Question 35

Describe the structure of the kidney

Answer 35

- outer region is called the cortex - inner region is called the medulla - the centre is the pelvis which leads to the ureter

Question 36

Describe the fine structure of the kidney

Answer 36

- bulk is made out of tiny tubules called nephrons - nephrons start in the cortex at the bowmans capsule - the remainder of the nephron is coiled tubule that passes through the cortex which forms a loop down into the medulla and back to its cortex before joining a collecting duct that goes back to the medulla

Question 37

how is blood filtering done in the kidney

Answer 37

- renal artery splits to form many arterioles which lead to a knot of capillaries called the glomerulus - blood from the glomerulus continue into a efferent arteriole which carries the blood to more capillaries surrounding the rest of teh tubule which flow together into the renal vein - each glomerulus is surrounded by the Bowmans capsule - fluid from the blood is pushed into the Bowman's capsule by the process of ultrafiltration

Question 38

What are the three parts of the tubule

Answer 38

- proximal convoluted tubule - loop of henle - distal convoluted tubule

Question 39

What does the barrier consist of

Answer 39

- the endothelium of the capillary - the basement membrane - the epithelial cells of the Bowman's capsule

Question 40

describe the endothelium of the capillary

Answer 40

- narrow gaps between the cells of the endothelium of the capillary wall - the cells of the endothelium also contain pores called fenestrations - gaps allow blood plasma and substances dissolved in it to pass out of the capillary

Question 41

describe the basement membrane

Answer 41

- membrane consists of fine mesh of collagen fibres and glycoproteins - mesh acts as a filter to prevent the passage of molecules with a relative molecular mass greater than 60,000 - meas that most proteins are held in the capillaries of the glomerulus

Question 42

describe the epithelial cells of the Bowman's capsule

Answer 42

- these cells are called podocytes which have specialsied shape, have finger life projections called major processes - On each major process are minor processes that hold the cells away from the endothelium - ensure that there are gaps between the cells, therefore fluid from the blood in the glomerulus can pass between these cells into the lumen of the Bowman's capsule

Question 43

what does the Bowman's capsule lead to

Answer 43

it leads to the rest of the tubules, this has three parts - the proximal convoluted tubule - loop of henle - distal convoluted tubule

Question 44

What is ultrafiltration

Answer 44

this is the filtering of blood at molecular level

Question 45

How does ultrafiltration work

Answer 45

- blood flows into the glomerulus through a different arteriole this is wider than the efferent arteriole that carries the blood away from the glomerulus - difference in diameter ensures the glomerulus has a higher pressure than the pressure in the Bowman's capsule - pressure differences pushes fluid from the blood into the Bowman's capsule that surrounds the glomerulus

Question 46

What is filtered out of the blood

Answer 46

- water - amino acids - glucose - urea - inorganic mineral ions

Question 47

What does the concentrations of dissolved substances depend on

Answer 47

it depends on the water balance in the organism and is therefore variable

Question 48

what is left in the blood and does not go into the kidney

Answer 48

- blood cells and proteins are left in the capillary - proteins means that the blood has a low water potential making some of the fluid remain in the blood this contains some of the dissolved substances

Question 49

Why is low water potential in the kidney important

Answer 49

- helps absorption at a later stage

Question 50

What is the function of the nephrons

Answer 50

- when fluid enters the Bowman's capsule it passes along the negative tubule and its composition is altered by selective reabsorption

Question 51

Describe how the nephrons work

Answer 51

- in the proxmial convoluted tubule the fluid is altered by the reabsorption of all sugars, most mineral ions and some water, the cells here have a high surface area due to a highly folded surface - in the descending limb of the loop of henle the water potential of the fluid is decreased by the addition of mineral ions and the removal of water - in the ascending limb of the loop of henle the water potential is increased as mineral ions are removed by water potential - in the collecting duct the water potential is decreased again by th removal of water and the final product is urine