Module 5.2 - Excretion

Question 1

Define excretion?

Answer 1

The removal of waste products of metabolism from the body.

Question 2

What is your metabolism?

Answer 2

All the chemical reactions that happen in your cells make up your metabolism. Your metabolism produces waste products.

Question 3

Give an example of excretion in respiration?

Answer 3

Carbon dioxide is a waste product of respiration and too much of CO2 in the blood is toxic, so it’s removed from the body by the lungs (e.g mammals) or gills (e.g.) fish. The lungs and gills act as excretory organs.

Question 4

Why is it so important that the liver breaks down excess amino acids?

Answer 4

The liver has to get rid of excess amino acids produced from eating and digesting proteins for amino acids contain N in the amino groups. Nitrogenous substances can’t usually be stored by the body meaning they can be damaging so need to be used or broken down and then excreted.

Question 5

Describe how excess amino acids are broken down by the liver?

Answer 5

1) The NH2 groups are removed from excess amino acids forming ammonia and organic acids - this process is called deamination.
2) The organic acids can be respired to give ATP or converted to carbohydrate and stored as glycogen.
3) Ammonia is too toxic for mammals to excrete directly, so it is combined with CO2 in the ornithine cycle to create urea. This partly happens in the mitochondria and partly in the cytoplasm.
4) The urea is released from the liver into the blood. The kidneys then filter the blood and remove the urea as urine which is excreted from the body.

Question 6

What are a few of the harmful substances the liver breaks down by detoxification?

Answer 6

> Alcohol (ethanol).
Paracetamol
Insulin

Question 7

Describe how the liver detoxifies alcohol in the blood?

Answer 7

Alcohol is broken down by the hepatocytes by the action of the enzyme ethanol dehydrogenase into ethanal and then into ethanoate (acetate). This acetate is combined with coenzyme A to form acetyl coenzyme A which enters the process of aerobic respiration.

Question 8

What can happen to the liver if it’s exposed to excess alcohol over a long period?

Answer 8

Can lead to cirrhosis (when the cells of the liver die and scar tissue blocks blood flow) of the liver or hepatitis due to fat in the hepatocytes causing the liver to become enlarged.

Question 9

What are the 4 different vessels connected to the liver?

Answer 9

> Hepatic artery
Hepatic vein
Hepatic portal vein
Bile duct

Question 10

What is the function of the hepatic artery?

Answer 10

Supplies the liver with oxygenated blood from the heart, so the liver has a good supply of oxygen for respiration, providing plenty of energy.

Question 11

What is the function of the hepatic vein?

Answer 11

Takes the deoxygenated blood away from the liver.

Question 12

What is the function of the hepatic portal vein?

Answer 12

Brings blood from the duodenum and ileum, so it’s rich in the products of digestion. This means any ingested harmful substances are filtered out and broken down straight away.

Question 13

What is the function of the bile duct?

Answer 13

Takes bile (a substance produced by the liver to emulsify fats) to the gall bladder to be stored.

Question 14

What are the duodenum and ileum?

Answer 14

Parts of the small intestine.

Question 15

What is the structure of the lobules that make up the liver?

Answer 15

Liver lobules are cylindrical structures made of cells called hepatocytes that are arranged in rows radiating out of the centre.
>Each lobule has a central vein in the middle that connects to the hepatic vein. Many branches of the hepatic artery/portal vein/bile duct are also found connected to each lobule.

Question 16

How are the hepatic artery and the hepatic portal vein connected to the vein?

Answer 16

By capillaries called sinusoids.

Question 17

Describe how the sinusoids work?

Answer 17

Blood runs through the sinusoids past the hepatocytes that remove harmful substances and oxygen from the blood. The harmful substances are broken down by the hepatocytes into less harmful substances that re-enter the blood. The blood runs to the central vein and the central veins from all the lobules connect up to form the hepatic vein.

Question 18

Where are the Kupffer cells and what are their function?

Answer 18

They are attached to the walls of the sinusoids and remove bacteria and break down old red blood cells.

Question 19

What is the canaliculi?

Answer 19

How the bile duct is connected to the central vein by tubes called the canaliculi.

Question 20

Give an overview of how the kidneys excrete waste products?

Answer 20

1) Blood enters the kidney through the renal artery and then passes through capillaries in the cortex of the kidneys.
2) As the blood passes through the capillaries, substances are filtered out of the blood and into long tubules that surround the capillaries. This process is called ultrafiltration.
3) Useful substances (e.g. glucose) are reabsorbed back into the blood from the tubules in the medulla and cortex - this is called selective reabsorption.
4) The remaining unwanted substances (e.g. urea) pass along the tubules, then along the ureter to the bladder, where they’re expelled as urine.
5) The filtered blood passes out of the kidney through the renal vein.

Question 21

Describe the process of ultrafiltration?

Answer 21

1) Blood from the renal artery enters small arterioles in the cortex.
2) Each arteriole splits into a structure called a glomeruus.
3) The high pressure in the afferent arteriole forces liquid and all molecules into the blood out of the capillary and into the Bowman’s capsule.
4) The liquid and small molecules pass through 3 layers to get into the Bowman’s capsule and enter the nephron tubule.
5) The liquid and small molecules now called filtrate, pass along the rest of the nephron and useful substances are reabsorbed.
6) Finally the filtrate flows through the collecting duct an d passes out of the kidney along the ureter.

Question 22

What is the glomerulus?

Answer 22

A bundle of capillaries looped inside a hollow ball called the Bowman’s capsule.

Question 23

What arteriole takes the blood into and out of the glomerulus and why?

Answer 23

> The afferent arteriole takes the blood into the glomerulus.
The efferent arteriole takes the filtered blood away from the glomerulus and is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure.

Question 24

What happens to proteins and red blood cells in the blood?

Answer 24

They stay in the blood for they are larger molecules.

Question 25

Describe how useful substances are reabsorbed along the nephron tubules?

Answer 25

> Selective reabsorption takes place as the filtrate flows along the proximal convoluted tubule (PCT), through the loop of Henle and along the distal convoluted tube (DCT).

1) Useful substances leave the tubules of the nephrons and enter the capillary network that’s wrapped around them.
2) Useful solutes are reabsorbed along the PCT by active transport and facilitated diffusion.
3) Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate. Water is reabsorbed from the loop of Henle, DCT and the collecting duct.
4) The filtrate that remains is urine, which passes along the ureter to the bladder.

Question 26

How is the epithelium of the wall of the PCT adapted for reabsorption of materials?

Answer 26

It has microvilli to provide a large surface area for the reabsorption of useful materials from the filtrate (in the tubules) into the blood (in the capillaries).

Question 27

What are some of the useful solutes that would be reabsorbed along the PCT and how?

Answer 27

> Glucose
Amino acids
Some salts
By active transport and facilitated diffusion.
Some urea is also reabsorbed by diffusion.

Question 28

What are some substances urine usually doesn’t contain?

Answer 28

> Proteins and blood cells - they’re too big to be filtered out of the blood.
Glucose, amino acids and vitamins - they’re actively reabsorbed back into the blood.

Question 29

What happens if the water potential of the blood is too low (the body is dehydrated)?

Answer 29

More water is reabsorbed by osmosis into the blood from the tubules of the nephrons, making the urine more concentrated and less water is lost during excretion.

Question 30

Where in the nephrons does the regulation of water potential occur and how?

Answer 30

In the middle and last parts of the nephron - the loop of Henle, DCT and the collecting duct. The volume of water reabsorbed is controlled by hormones.

Question 31

What mechanism helps to reabsorb water in the loop of Henle?

Answer 31

The Countercurrent multiplier mechanism.

Question 32

Describe the 4 key stages of the countercurrent multiplier mechanism?

Answer 32

1) Near the top of the ascending limb, Na+ and CI- ions are actively pumped out into the medulla. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla, due to the high conc. of ions.
2) Due to the lower water potential in the medulla than in the descending limb, water moves out of the descending limb into the medulla by osmosis. This makes the filtrate more concentrated of ions and they can’t diffuse out for the descending limb isn’t permeable to them. The water in the medulla is reabsorbed into the blood through the capillary network.
3) Near the bottom of the ascending limb Na+ and Cl- ions diffuse out into the medulla, further lowering the water potential in the medulla. (Water remains in tubule for its impermeable to water).
4) The first 3 stages massively increase the ion conc. in the medulla, lowering the water potential. This causes water to move out of the collecting duct by osmosis. The water in the medulla is reabsorbed into the blood.

Question 33

Why does the volume of the water reabsorbed from the collecting duct vary?

Answer 33

Controlled by the changing permeability of the collecting duct.

Question 34

Why do animals that live in places such as deserts where there is low availability of water have long loop of Henles?

Answer 34

The longer an animal’s loop of Henle the more water they can reabsorb from the filtrate. Have a longer ascending limb where more ions are actively pumped out into the medulla creating a very low water potential in the medulla. This means more water moves out of the nephron and collecting duct into the capillaries, giving very concentrated urine.

Question 35

How is the water potential of the blood monitored?

Answer 35

By cells called osmoreceptors in the hypothalamus.

Question 36

What is ADH and what is its function?

Answer 36

It is antidiuretic hormone and makes the walls of the DCT and collecting duct more permeable to water.

Question 37

Describe ADH’s response when you’re dehydrated?

Answer 37

> When the water content/potential of the blood drops it is detected by osmoreceptors in the hypothalamus.
The posterior pituitary gland is stimulated to release more ADH into the blood.
More ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis.
A small amount of highly concentrated urine is produced and less water is lost.

Question 38

How can kidney failure be detected?

Answer 38

By measuring the glomerular filtration rate (GFR) - this is the rate at which blood is filtered from the glomerulus into the Bowman’s capsule. A rate lower than the normal range indicates the kidneys aren’t working properly.

Question 39

Give some examples of what kidney failure can be caused by?

Answer 39

> Kidney infections - can cause inflammation which can damage the cells and interfere with filtering in the Bowman’s capsule or with absorption in the other parts of the nephron.
High blood pressure - Can damage the glomeruli, if capillaries are damaged larger molecules such as proteins can get through the capillary walls and into the urine.

Question 40

Give 4 problems that can occur from kidney failure?

Answer 40

1)Waste products that the kidneys would normally remove (e.g. urea) begin to build up in the blood. Too much urea in the blood causes weight loss and vomiting.
2)Fluid starts to accumulate in the tissues because the kidneys can’t remove excess water from the blood, causes parts of the body to swell.
3)The balance of electrolytes (ions) in the body becomes unbalanced, the blood may become too acidic or an imbalance of Ca and phosphate can lead to brittle bones. Salt build-up may cause more water retention.
4)Long-term kidney failure causes anaemia - a lack of haemoglobin
If kidney failure can’t be controlled it can lead to death.

Question 41

What are the two types of dialysis?

Answer 41

> Renal dialysis

>Peritoneal dialysis - Uses the lining of your abdomen to filter your blood.

Question 42

Describe how renal dialysis works?

Answer 42

> The patient’s blood is passed through a dialysis machine where the blood flows on one side of a partially permeable membrane and dialysis fluid on the other side.
Waste products/excess water and ions diffuse across the membrane into the dialysis fluid, removing them from the blood. >Blood cells and larger molecules like proteins are prevented from leaving the blood.

Question 43

What are some of the disadvantages of renal dialysis?

Answer 43

> Patients can feel increasingly unwell between dialysis sessions because waste products and fluid start to build up in their blood.
Each session takes 3-5 hours and patients need 2-3 sessions a week, usually in a hospital. This is expensive and inconvenient for a patient’s lifestyle.

Question 44

What are the advantages of having a kidney transplant over dialysis?

Answer 44

> Cheaper
More convenient for the patient.
Patients don’t have the problem of feeling unwell between sessions.

Question 45

What are the disadvantages of having a kidney transplant?

Answer 45

> Major, risky operation.

>The immune system may reject the transplant, so the patient has to take drugs to suppress it.

Question 46

Describe how urine is used to determine whether or not someone is pregnant?

Answer 46

1) A stick is used with an application are that contains monoclonal antibodies for hCG bound to a coloured bead.
2) When urine is applied to the area any hCG will bind to the antibody on the beads.
3) The urine moves up the test strip, carrying the beads with it.
4) The test strip has antibodies to hCG stuck in place (immobilised)
5) If there is hCG present the test strip changes colour for the immobilised antibody binds to any hCG attached to the beads, concentrating the colour in that area.

Question 47

Why do athletes sometimes need to have their urine tested and how is it done?

Answer 47

Test for anabolic steroids (drugs that help build up muscle tissue).
Uses gas chromatography/mass spectrometry.

Question 48

Recreational drugs can also be tested for using GC and MS, what is an alternative testing method?

Answer 48

Test strips which contain antibodies that the drug being tested for will bind to. A colour change will occur indicating a positive result.