Biology: 3.7 Homeostasis and the kidney

Question 1

Define Homeostasis

Answer 1

It is a stable equilibrium between interdependent elements, especially as maintained by physiological processes in the body.

Question 2

What are the two functions of the kindey?

Answer 2

1: Removal of nitrogenous metabolic waste (urea) from the body - excretion.
2: Osmoregulation, the mechanism by which the balance of water and dissolved solutes is regulated.

Question 3

How is urea produced?

Answer 3

It is a poisonous chemical made by the liver. Produced when amino acids, which make up protein, are deaminated - NH2 groups are removed.

Question 4

What is the Bowman’s capsule responsible for?

Answer 4

It is the site of ultrafiltration. Its blood supply enters as an afferent arteriole which enters the glomerulus and leaves as the efferent arteriole. This is narrower than the afferent arteriole.

Question 5

Where does the efferent arteriole carry blood to?

Answer 5

1: A capillary network serving the proximal and distal convoluted tubules.
2: A capillary network running beside the loop of Henle known as the vasa recta.

Question 6

How does the glomerulus speed up the rate of filtration?

Answer 6

Endothelium of the capillaries has pores between the cells to speed up the process of filtration.

Question 7

What are podocytes?

Answer 7

Podocytes are the cells of the inner wall of the Bowman’s capsule. They have many processes which are wrapped around the capillaries of the glomerulus.
Large gaps between these processes allow free passage of the filtrate from the blood into the lumen of the renal capsule.

Question 8

What is the basement membrane?

Answer 8

It acts as a selective barrier allowing water and small molecules to pass from the blood of the nephron.

Question 9

Where does ultrafiltration occur?

Answer 9

In the glomerulus.

Question 10

How does ultrafiltration work?

Answer 10

Filtration under pressure that forces small soluble molecules from the plasma. Basement membrane of the capillary forms the selective barrier between the blood and the nephron and acts as a molecular sieve. Small molecules can pass through, but the plasma proteins and erythrocytes remain in the blood due to their larger size.

Question 11

What causes the high pressure causing filtration?

Answer 11

Hydrostatic pressure of the blood in the glomerular capillaries which is increased due to efferent arterioles being smaller than afferent arterioles and also by water potential of blood.

Question 12

What is selective reabsorption?

Answer 12

Selective as only certain molecules are removed from the glomerular filtrate. Re-absorption as all the molecules have already been absorbed once from the intestines.

Question 13

What does the proximal convoluted tubule reabsorb?

Answer 13

All the glucose and most of the water and salt.

Question 14

What does the distal convoluted tubule reabsorb?

Answer 14

Small amounts of water and salt for fine regulation of blood ion concentration.

Question 15

Where is most of the water reabsorbed from?

Answer 15

The collecting duct.

Question 16

How are glucose and salts reabsorbed?

Answer 16

Active transport.

Question 17

How is water reabsorbed?

Answer 17

Passively by osmosis following the transport of salt.

Question 18

What are the adaptations of the proximal convoluted tubule to re-absorption?

Answer 18

1: Microvilli increase surface area for re-absorption.
2: Capillaries are in close contact with the tubule.
3: Intercellular spaces increase surface area for movement of water.
4: Large numbers of mitochondria provide ATP needed for active transport.

Question 19

What happens if the plasma pH falls (i.e. an increase in H+ ions)?

Answer 19

Hydrogen ions are excreted by the kidney.

Question 20

What happens if the plasma pH rises?

Answer 20

Hydrogen carbonate ions are secreted.

Question 21

Explain the process of negative feedback.

Answer 21

It maintains the normal concentration of blood if it becomes too dilute or becomes more concentrated.

Question 22

What is ADH?

Answer 22

Antidiuretic hormone.

Question 23

Where is ADH secreted?

Answer 23

The posterior pituitary gland.

Question 24

What is osmoregulation?

Answer 24

The homeostatic function that maintains concentrations of enzymes and metabolites, so that reactions within cells occur at a constant and appropriate rate.

Question 25

What may a fall in water potential of the blood be caused by?

Answer 25

1: Reduced water intake
2: Sweating
3: Intake of large amounts of salt

Question 26

How is reduced water potential combatted?

Answer 26

Reduced water potential is detected by osmoreceptors in the hypothalamus of the brain. Secretory granules carry ADH along axons from the hypothalamus to the posterior lobe of the pituitary gland, from where ADH is secreted into the blood stream.

Question 27

What happens when ADH is carried to the kidneys?

Answer 27

1: ADH increases the permeability of the walls of the distal convoluted tubule and the collecting duct to water.
2: More water is reabsorbed from there into the region of high solute concentration, low water potential in the medulla.
3: More water is reabsorbed from the medulla into the blood in the capillaries.

Question 28

What are aquaporins?

Answer 28

Intrinsic membrane proteins with a pore through which water molecules move.

Question 29

How are aquaporins formed?

Answer 29

1: ADH binds to membrane receptors.
2: Adenyl cyclase catalyses the production of cyclic AMP, the second messenger.
3: Cytoplasmic vesicles containing aquaporins move to and fuse with the cell membrane.
4: Aquaporins are incorporated into the membrane.
5: Water molecules move in through their pores into the cell, down a water potential gradient.