Module 5.2 - Excretion As An Example Of Homeostatic Control

Question 1

Define excretion

Answer 1

The removal of metabolic waste from the body

Question 2

Define metabolic waste

Answer 2

Waste substances that may be toxic or are produced in excess by the reactions inside cells

Question 3

Describe CO2 as a waste product

Answer 3

Dissolves in blood to form carbonic acid, dissociates to form hydrogencarbonate ions and H+ ions, making blood acidic
Chemoreceptors sense the change and the respiratory centre of the medulla oblongata causes an increase in heart and breathing rate to get rid of excess CO2

Question 4

Why is excess CO2 toxic?

Answer 4

Too much results in less oxygen carriage
If blood pH drops too low, respiratory acidosis can occur (caused by lung diseases such as asthma/airway blockage, causes slow breathing, headaches, confusion, rapid heart rate)

Question 5

Describe nitrogenous waste as a waste product

Answer 5

From excess amino acids which can’t be stored
Excreted as urea in urine
Transported dissolved in plasma
Kidneys remove it from blood and turn it into urine

Question 6

Why is nitrogenous waste toxic?

Answer 6

Amino acids turned into ammonia in liver, which is highly toxic and highly soluble
Ammonia is turned into urea in liver before travelling in blood to kidneys to be removed and put into urine in the bladder

Question 7

Functions of the liver

Answer 7

Formation of urea from excess amino acids
Synthesis of bile, plasma proteins (fibrinogen), cholesterol
Storage of vitamins A, D and B12, iron, glycogen
Detoxification
Breakdown of hormones
Destruction of red blood cells

Question 8

Adaptations of hepatocytes

Answer 8

Many mitochondria (very metabolically active)
Many ribosomes (protein synthesis)
Many/larger Golgi apparatuses

Question 9

Role of afferent arteriole

Answer 9

Brings blood to glomerulus

Question 10

Role of proximal convoluted tubule

Answer 10

Where selective reabsorption occurs

Question 11

Role of the loop of Henle

Answer 11

Reduce water potential of medulla tissue

Question 12

Role of distal convoluted tubule

Answer 12

Concentrations of some salts are adjusted by active transport

Question 13

Role of collecting duct

Answer 13

Water is reabsorbed into surrounding tissue fluid in medulla

Fluid is now urine and will be taken by the duct through the pelvis to the bladder via the ureter

Question 14

What is ultrafiltration

Answer 14

Filtration at molecular level
Smaller molecules (urea, water, glucose, amino acids, ions) are filtered out of blood into lumen of the Bowman’s capsule

Question 15

Points about basement membrane

Answer 15

Mesh of glycoproteins and collagen acts as molecular filter
Nothing with molecular mass over 69,000 can pass through (e.g. proteins and blood cells)
Allows plasma to escape containing dissolved molecules

Question 16

What are podocytes?

Answer 16

Allow filtrate to pass beneath them in ultrafiltration

Question 17

How are molecules ‘selected’ to be reabsorbed in the PCT?

Answer 17

Specific carrier proteins

Question 18

What is reabsorbed in selective reabsorption?

Answer 18

All glucose
All amino acids
Most water
Some ions

Question 19

Process of selective reabsorption

Answer 19

1) Sodium potassium pumps sodium out into blood and potassium into cells by an active process (the cells have lots of mitochondria)
2) Many co-transporter proteins attached to cell surface membrane for facilitated diffusion of glucose/amino acids with sodium ions
3) Movements of ions causes a lower water potential in the PCT cells. Water moves from lumen to cells via osmosis and will follow ions into the blood
4) Glucose and amino acids diffuse into tissue fluid and then the blood

Question 20

Process of water reabsorption in loop of Henle

Answer 20

Purpose is to cause a decrease in water potential in tissue fluid of medulla
Ions are actively transported out of ascending limb into medulla tissue fluid
Water potential of tissue fluid surrounding loop of Henle and collecting duct is lower than inside them
Descending limb and collecting duct are permeable to water
Water is absorbed from descending limb and collecting duct into tissue fluid and then into blood

Question 21

Process of ADH release

Answer 21

Osmoreceptors in hypothalamus monitor water potential of blood
Neurosecretory cell bodies in the hypothalamus produce anti-diuretic hormone (ADH)
ADH travels down axon of neurosecretory cells to the terminal bulb in posterior pituitary gland to be stored
If osmoreceptors sense blood water potential is too low, they lose waste by osmosis
Triggers action potential down axon
Stimulates release of ADH from terminal bulb in posterior pituitary gland