Excretion

Question 0

Define deamination

Answer 0

The removal of the amine group from an amino acid to produce ammonia

Question 1

What are the two main functions of the kidney?

Answer 1

– They are excretory organs, removing nitrogenous and other metabolic waste from the body

– They play an important part in homeostasis, by helping to regulate pH, water and sodium ion concentrations in the blood

Question 2

What is a sinusoid?

Answer 2

A blood filled space

NOT A BLOOD VESSEL

Question 3

What are the stages of the formation of urea?

Answer 3

Deamination —> removal of the amine group
—> keto acid + ammonia

Ornithine cycle —> ammonia -> urea + H2O
—> takes place in the liver

Question 4

Define excretion

Answer 4

The removal of metabolic waste

Question 5

Why is excretion necessary?

Answer 5

To prevent the build up of substances to toxic levels

Question 6

Explain the importance of removing metabolic wastes, including carbon dioxide and nitrogenous waste, from the body.

Answer 6

Carbon dioxide must be removed as, when it dissolves in water is produces hydrogencarbonate ions. These ions compete with oxygen for space on the haemoglobin.This causes a reduction in oxygen transport.

Carbon dioxide can also combine directly with haemoglobin to form carbaminohaemoglobin, which has a low affinity for oxygen.

Can cause respiratory acidosis; breathing difficulties, headaches, drowsiness, restlessness etc caused by Carbon Dioxide dissolving in the blood plasma and combining with water to produce carbonic acid, which dissociates to release hydrogen ions. This lowers the pH.

Nitrogenous wastes must be removed because the amino group is highly toxic, but proteins and amino acids are very high in energy, so it would be wasteful to excrete them.

In the orthinine cycle, the amine group is removed to form ammonia, which forms urea, water and a keto acid when added to oxygen and carbon dioxide.

The keto acid can be used in respiration and the urea is transported to the kidneys for excretion.

Question 7

Describe the histology and gross structure of the liver.

Answer 7

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

The hepatic vein takes deoxygenated blood away from the liver - which rejoins the vena cava and normal circulation will proceed.

Bile duct is where the substance bile is secreted, which is carried to the gall bladder where it is stored until it is required in the small intestines.

The hepatic portal vein brings blood from the small intestine, the blood is rich in the products of digestion, and this means that any harmful substances ingested will be broken down quickly by the liver cells (hepatocytes).

The liver is made up of lobules, which consists of cells called hepatocytes that are arranged in rows.

Each Lobule has a Central vein in the middle that connects to the hepatic vein.

Every single lobule has branches of the hepatic artery, hepatic portal vein and bile duct.

Hepatic artery and hepatic vein are connected to the central vein via blood filled spaces called sinusoids.

The blood flows past every hepatocytes via the sinusoid, this ensures that the harmful stuff are broken down quickly. Also the blood provides the liver cells with oxygen.

The central veins from all the lobules join up to form the hepatic vein.

Question 8

Describe the formation of urea in the liver, including an outline of the ornithine cycle.

Answer 8

Amino acid + Oxygen →Keto acid + Ammonia

Ammonia + Carbon dioxide → Urea + Water

2NH3+CO2→CO(NH2)2+H2O

Question 9

Describe the roles of the liver in detoxification.

Answer 9

Catalase can convert 5 million molecules of H2O2 into harmless substances in a minute.

Alcohol contains a lot of chemical potential energy which can be used in respiration.

Ethanol Dehydrogenase catalyses the detoxification of alcohol in hepatocytes.

Ethanol →Ethanal → Ethanoic Acid → Acetyl CoA

Ethanal and Ethanoic acid are dehydrogenated, and the hydrogen reduces NAD.

If too many NADs are busy detoxifying alcohol, there will be too few NAD to break down fatty acids for use in respiration, so the fatty acids are converted back to lipids, which are stored in hepatocytes, making the liver enlarged- Fatty liver.

Question 10

Describe the histology and gross structure of the kidney.

Answer 10

Supplied with blood from the renal artery and is drained by the renal vein.

The Kidney is surrounded by a tough capsule, the outer region is the cortex and the inner is the medulla.

The central region if the pelvis, which leads into the ureter.

Question 11

Describe the detailed structure of a nephron and its associated blood vessels.

Answer 11

The nephron starts in the cortex, where the capillaries form a knot called the glomerulus, surrounded by the Bowman’s capsule.

Fluid from the blood is pushed into the capsule by ultrafiltration.

The fluid leaves the capsule and flows through the nephron, starting with the proximal convoluted tubule, and then into the medulla for the loop of Henle, which is a hairpin counter current multiplier.

Here the composition of the fluid is altered by Selective reabsorbtion.

Substances are reabsorbed back into the tissue fluid and capillaries surrounding the nephron tubule.

The fluid then passes into the Distal convoluted tubule, and then into the collecting duct as urine.

Question 12

Describe and explain ultrafiltration

Answer 12

• Blood flows into the glomerulus via the afferent arteriole which is at a higher pressure than the blood that leaves through the efferent arteriole due to the difference in size of the diameters of the lumen
• Blood enters the glomerulus and must pass through 3 distinct layers in order to enter the Bowman’s capsule
• Endothelium of capillaries- contains gaps from which blood passes through as well as the substances dissolved in it
• Basement membrane- fine mesh of collagen fibres and glycoproteins that do not allow molecules with an RMM larger the 69000 to pass through (usually proteins)
• Epithelium of Bowman’s capsule- contain finger like projections (podocytes) that fluid from the glomerulus can pass through into the Bowman’s capsule

Question 13

Describe and explain selective reabsorption

Answer 13

• Na ions are actively transported out of the wall of the convoluted tubule and enter the surrounding tissue fluid
• Sodium is transported into the cell with Amino Acids or Glucose by facilitated diffusion.
• As the concentrations of Amino Acids or glucose rise, they diffuse into the tissue fluid; they may also be actively removed.
• They then diffuse into the blood and are carried away.

• The reabsorbtion of salts, glucose or amino acids reduces the water potential of the cell and increases the water potential in the tubule fluid, so water will enter the cells and be reabsorbed
into the blood by osmosis.

Question 14

Describe the structure of the cells of the proximal convoluted tubule

Answer 14

Microvilli - increase the surface area for re-absorption.

Co-transporter proteins - contained in the cell surface membrane that is in contact with the tubule fluid. Transports glucose or amino acids.

Na/K pumps - contained in the cell surface membrane opposite to the fluid tubule. Actively transports Na+ and K+ against their concentration gradient.

Many Mitochondria - provides the energy needed to drive the selective re-absorption process. Many mitochondria= a lot of ATP.

Question 15

Explain, using water potential terminology, the control of the water content of the blood, with reference to the roles of the kidney, osmoreceptors in the hypothalamus and the posterior pituitary gland.

Answer 15

In the loop of Henle, salts are transferred from the ascending limb to the descending limb. This means that the tissue fluid in the medulla has a very negative water potential, as so water is lost by osmosis, particularly in the collection duct.

1. The water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain
2. When the water potential is very low, they shrink, and stimulate neurosecretory cells in the hypothalamus.
3. These produce and release anti diuretic hormone which flows down the axon to the posterior pituitary gland where it is stored until needed.
4. When the neurosecretory glands are stimulated thy send action potentials down their axons and cause the release of ADH
5. It enters the capillaries running through the posterior pituitary gland. It is transported around the body and acts on the cells of the collecting ducts.
6. When it binds to the receptors, it causes a chain of enzyme catalysed reactions, the end result of which is the insertion of vesicles containing water-permeable channels (aquaporins) in the walls of the cells, so they are more permeable to water.
7. More water is reabsorbed, by osmosis, into the blood
8. Less urine, with a lower water potential is release.
9. Less ADH is released when the water potential rises again
10. The ADH is slowly broken down and the collecting duct receives less stimulus

Question 16

Outline the problems that arise from kidney failure

Answer 16

• Unable to remove excess water & waste products from the body
o E.g. urea & excess salts

• Inability to regulate urea and salt levels
• Death

Question 17

Discuss the use of renal dialysis for treatment of kidney failure

Answer 17

Waste, excess fluids and salts are removed from the body by passing the blood over a dialysis membrane.

This allows the exchange of substances between the blood and the dialysis fluid, which has the same concentration of substances as blood plasma.

Substances diffuse from both sides to create the correct concentration of substances.

Haemodialysis:
• Blood is passed through a machine that contains an artificial dialysis membrane. Heparin is used to avoid clotting. Thrice weekly trips to hospital lasting several hours.

Peritoneal dialysis
• The body’s own abdominal membrane is used as a filter.

Question 18

Discuss the use of transplants for treatment of kidney failure

Answer 18

Advantages
            o No dialysis
            o Less limited diet
            o Better physical feeling
            o Better quality of life
            o No longer ‘chronically ill’

Disadvantages
o Need immunosuppressants for life of kidney
o Major surgery
o Risk of infection
o Need frequent checks in case of rejection
o Side effects of medication

Question 19

Describe how urine samples can be used to test for pregnancy

Answer 19

• A human embryo secrets human chorionic gonadotrophin (hGC) as soon as it is implanted on the uterine lining. The hormone can be detected in the mother’s urine after as few as 6 days.
• Pregnancy tests contain monoclonal antibodies which are tagged with a blue bead and bind only to hCG.
• The hCG-antibody complex moves along the strip until it sticks to a band of immobilised antibodies, so forms a blue line
• One blue line is a control, so two means pregnancy

Question 20

Describe how urine samples can be used to detect misuse of anabolic steroids.

Answer 20

• Urine samples are tested using gas chromatography

1. The sample is vaporised in the presence of a gaseous solvent
2. It is passed down a long tube lined with an absorbing agent.
3. Each substance dissolves differently in the gas and stays there for a unique, specific time- the retention time
4. Eventually, the substance leaves the gas and is absorbed by the lining
5. It is then analysed to make a chromatogram
6. Standard samples of drugs and urine samples are run so drugs can be identified and quantified in the chromatogram.

Question 21

What is osmoregulation?

Answer 21

The maintenance of the optimum water potential of the blood

Question 22

Put the parts of the kidney nephron in the correct order

Answer 22

Bowman's capsule 
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct

Question 23

What are the three main processes involved in excretion?

Answer 23

Ultrafiltration

Selective reabsorption

Removal of nitrogenous waste (urine)

Question 24

Explain the role of the loop of Henle in the production of urine

Answer 24

• The loop of Henle causes a decrease in water potential in the
  medulla
• The ascending limb actively transports solute ions out of the loop
  of Henle
• Water potential of the tissues surrounding the collecting duct is
  lower than the fluid inside it
• Water is removed from the urine in the collecting duct

Question 25

What part of the nephron has walls impermeable to water?

Answer 25

The ascending limb of the loop of Henle

Question 26

What part of the nephron is glucose reabsorbed into the blood?

Answer 26

The proximal convoluted tubule

Question 27

What part of the nephron does ADH act on the walls?

Answer 27

Distal convoluted tubule / collecting duct

Question 28

What part of the nephron contains podocytes?

Answer 28

Bowman’s capsule

Question 29

What part of the nephron is the most water reabsorbed into the blood?

Answer 29

Proximal convoluted tubule