Module 5: Section 2 - Excretion

Question 1

One of the liver’s most important roles is getting rid of excess amino acids produced by eating and digesting protein. Amino acids contain nitrogen in their amino group. Nitrogenous substances can’t usually be stored by the body. This means excess amino acids can be damaging to the body, so they must be used by he body (e.g. to make proteins) or be broken down and excreted. How does excess amino acids broken down in the liver?

Answer 1

1) first, the nitrogen-containing amino groups (-NH2) are removed from any excess amino acids, forming ammonia (NH3) 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’s combined with CO2 in the ornithine cycle to create urea
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 2

The liver also breaks down other harmful substances, like alcohol, drugs and unwanted hormones. They’re broken down into less harmful compounds that can be excreted from the body - this process is called detoxification. What three harmful products are broken down by the liver?

Answer 2

1) alcohol - a toxic substance that can damage cells. It’s broken down by the liver into ethanAL, which is then broken down into a less harmful substance called acetic acid. Excess alcohol over a long period can lead to cirrhosis of the liver - this is when the cells of the liver die and scar tissue blocks blood flow
2) paracetamol - a common painkiller that’s broken down by the liver. Excess paracetamol in the blood can lead to liver and kidney failure
3) insulin - a hormone that controls blood glucose concentration. Insulin is also broken down by the liver as excess insulin can cause problems with blood sugar levels

Question 3

What is the hepatic artery and what is its function?

Answer 3

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

Question 4

What is the hepatic vein and what is its function?

Answer 4

the hepatic vein takes deoxygenated blood away from the liver

Question 5

What is the hepatic portal vein and what is its function?

Answer 5

the hepatic portal vein brings blood from the duodenum and ileum (parts of the small intestine), so it’s rich in the products of digestion. This means any ingested harmful substances are filtered out and broken down straight away

Question 6

What is the bile duct and what is its function?

Answer 6

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

Question 7

The liver is made up of liver lobules - what is the structure of these lobules?

Answer 7

liver lobules - cylindrical structures made of cells called hepatocytes that are arranged in rows radiating out from the centre

each lobule has a central vein in the middle that connects to the hepatic vein. Many branches of the hepatic artery, hepatic portal vein and bile duct are also found connected to each lobule

Question 8

What is the structure of the hepatic artery and the hepatic portal vein / how does blood flow through the liver? How does the structure of the liver and how blood flows affect the breakdown of harmful substances?

Answer 8

1) the hepatic artery and the hepatic portal vein are connected to the central vein by capillaries called sinusoids
2) blood runs through the sinusoids, past the hepatocytes that remove harmful substances and oxygen from the blood
3) the harmful substances are broken down by the hepatocytes into less harmful substances that then re-enter the blood
4) the blood runs to the central vein, and the central veins from all the lobules connect up to form the hepatic vein
5) cells called kuppfer cells are also attached to the walls of the sinusoids. They remove bacteria and break down old red blood cells
6) the bile duct is connected to the central vein by tubes called canaliculi

Question 9

One of the main functions of the kidneys is to excrete water products and to regulate the water potential of blood. Please give an overview of how the kidneys excrete waste products

Answer 9

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 into long tubules that surround the capillaries. This process is called ultrafiltration
3) useful substances are reabsorbed back into the blood from the tubules in the medulla and cortex - this is called selective reabsorption
4) the remaining unwanted substances 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 kidneys through the renal vein

Question 10

The long tubules along with the bundle of capillaries where the blood is filtered are called nephrons - there are around one million nephrons in each kidney. Where / how does ultrafiltration occur?

Answer 10

1) blood from the renal artery enters smaller arterioles in the cortex
2) each arteriole splits into a structure called a glomerus (inside the Bowman’s capsule)
3) this is where ultrafiltration takes place
4) the arteriole that takes blood into each glomerulus is called the afferent arteriole, and the arteriole that takes the filtered blood away is called the efferent arteriole
5) the efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerus is under high pressure
6) the high pressure forces liquid and small molecules in the blood out of the capillary and into the Bowman’s capsule
7) the liquid and small molecules pass through three layers to get into the Bowman’s capsule and enter the nephron tubule - the capillary wall, a membrane (called the basement membrane) and the epithelium of the Bowman’s capsule. Larger molecules like proteins and blood cells can’t pass through and stay in blood
8) the liquid and small molecules, now called filtrate, pass along the rest of the nephron and useful substances are reabsorbed along the way
9) finally, the filtrate flows through the collecting duct and passes out of the kidney along the ureter

Question 11

Where does selective reabsorption take place and how does this happen?

Answer 11

1) selective reabsorption takes place as the filtrate flows along the proximal convoluted tubule (PCT), through the loop of Henle, and along the distal convoluted tubule (DCT)
2) useful substances leave the tubules of the nephrons and enter the capillary network that’s wrapped around them
3) the epithelium of the wall of the PCT has microvilli to provide a large SA for the reabsorption of useful materials from the filtrate (in the tubules) into the blood (into the capillaries)
4) useful solutes like glucose, amino acids, vitamins and some salts are reabsorbed along the PCT by active transport and facilitated diffusion
5) some urea is also reabsorbed by diffusion
6) 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
7) the filtrate that remains is urine, which passes along the ureter to the bladder

Question 12

The loop of Henle is made up of two limbs - the descending limb and the ascending limb. They help set up a mechanism called the countercurrent multiplier mechanism. It’s this mechanism that helps to reabsorb water back into the blood - how does it work?

Answer 12

1) near the top of the ascending limb, Na+ and Cl- 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, because there’s a high concentration of ions
2) because there’s a 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 (the ions cannot diffuse out - 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 (the ascending limb is impermeable to water, so it stays in the tubule)
4) the first three stages massively increase the ion concentration in the medulla, which lowers the water potential. This causes water to move out of the collective duct by osmosis. As before the water in the medulla is reabsorbed into the blood through the capillary network

Question 13

Why do different animals have different lengths of loop of Henle?

Answer 13

different animals have different length loops of Henle. The longer an animal’s loop of Henle, the more water they can reabsorb from the filtrate. When there’s a longer ascending limb, more ions are actively pumped out into the medulla, which creates a really low water potential in the medulla. This means more water move out of the nephron and collecting duct into the capillaries, giving very concentrated urine. Animals that live in areas where there’s little water usually have long loops to save as much water as possible

Question 14

How is water potential of the blood controlled?

Answer 14

1) the water potential of the blood is monitored by cells called osmoreceptors in a part of the brain called the hypothalamus
2) when the osmoreceptors are stimulated by low water potential in the blood, the hypothalamus sends nerve impulses to the posterior pituitary gland to release a hormone called antidiuretic hormone (ADH) into the blood
3) ADH makes the walls of the DCT and collecting dust more permeable to water
4) this means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis. A small amount of concentrated urine is produced, which means less water is lost from the body

Question 15

Dehydration occurs when you lose water - how is the water content of blood increased?

Answer 15

1) the water content of the blood drops, so its water potential drops
2) this is detected by osmoreceptors in the hypothalamus
3) the posterior pituitary gland is stimulated to release more ADH into the blood
4) more ADH means that the DCT and collecting duct are more permeable, so more water is reabsorbed into the blood by osmosis
5) a small amount of highly concentrated urine is produced and less water is lost

Question 16

If you’re hydrated, the water content of blood needs to be reduced - how?

Answer 16

1) the water content of the blood rises, so its water potential rises
2) this is detected by the osmoreceptors in the hypothalamus
3) the posterior pituitary gland releases less ADH into the blood
4) less ADH means that the DCT and collecting duct are less permeable, so less water is reabsorbed into the blood by osmosis
5) a large amount of dilute urine is produced and more water is lost

Question 17

What are two things that can cause kidney failure?

Answer 17

1) kidney infections - these can cause inflammation of the kidneys, which can damage the cells. This interferes with filtering in the Bowman’s capsules, or with reabsorption in the other parts of the nephrons
2) high blood pressure - this can damage the glomeruli. the blood in the glomeruli is already under high pressure but the capillaries can be damaged if the blood pressure gets too high. this means larger molecules like proteins can get through the capillary walls and into the urine

Question 18

Give four problems kidney failure can cause

Answer 18

1) waste products that the kidneys would normally remove 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. this causes parts of the body to swell
3) the balance of electrolytes (ions) in the body becomes unbalanced. the blood may become too acidic, and an imbalance of calcium 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 Hb in the blood.

Question 19

What is renal dialysis and how does it work?

Answer 19

1) renal dialysis is where a machine is used to filter a patient’s blood
- the patient’s blood is passed through a dialysis machine - the blood flows on one side of a partially permeable membrane and dialysis fluid flows on the other side

• waste products and 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 20

What are some pros / cons of renal dialysis?

Answer 20

1) patients can feel increasingly unwell between dialysis sessions because waste products and fluid start to build up in their blood
2) each dialysis session takes three to five hours, and patients need two or three sessions a week, usually in hospital. Expensive and inconvenient
3) however, dialysis can keep a person alive until a transplant is available, and it’s a lot less risky than transplant surgery

Question 21

What’re the pros / cons of a kidney transplant?

Answer 21

Pros:

• it’s cheaper to give a person a transplant then keep them on dialysis for a long time
• it’s more convenient for a person rather than regular dialysis sessions
• patients don’t have the problem of feeling unwell between sessions

Cons:

• the operation is risky
• the immune system may reject the transplant, so the patient has to take drugs to suppress it

Question 22

Pregnancy tests detect the hormone human chorionic gonadotropin (hCG) that;s only found in the urine of pregnant women - but how do pregnancy tests work?

Answer 22

1) a stick is used with an application area that contains monoclonal antibiotics or hCG bound to a coloured bead (blue). monoclonal antibiotics are all identical to each other
2) when urine is applied to the application area any hCG will bind to the antibody on the beads
3) the urine moves up to 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 turns blue because the immobilised antibody binds to any hCG attached to the blue beads, concentrating the blue beads in that area. If no hCG is present, the beads will pass through the test area without binding to anything, and so it won’t go blue

Question 23

Athletes regularly have their urine tested for steroids by a technique called gas chromatography - how does this work?

Answer 23

in gas chromatography the urine sample is vaporised and passed through a column containing a polymer. Different substances move through the column at different speeds, so substances in the urine sample separate out. Once the substances have separated out, a mass spectrometer converts them into ions, then separates them out depending on their mass and charge. The results are analysed by a computer and by comparing them with the results of known substances it’s possible to tell which substances were in the urine sample

Question 24

take a break

Answer 24

you’re doing great!!