The Kidney and Osmoregulation 15.5+6

Question 1

What roles do the kidneys play in the body?

Answer 1

They take part in excretion and ultrafiltration. This means they remove nitrogenous waste products from the body which is urea and they also maintain the water and pH levels of the blood

Question 2

What artery supplies the kidneys with blood?

Answer 2

Renal artery

Question 3

What vein removes the blood from the kidneys?

Answer 3

Renal vein

Question 4

What is the kidney made up of and what function do these units have?

Answer 4

It is made up of millions of nephrons which are small filtering units.

Question 5

What do the kidneys excrete and where does it go?

Answer 5

The kidneys produce urine and this is excreted out of its ureters which is then collected in the bladder and released through the urethra

Question 6

What are the main structures of the kidney?

Answer 6

• the cortex
• the medulla
• the pelvis

Question 7

What is the cortex?

Answer 7

This is the dark outer layer that filters the blood so has a vast network of capillaries that branches out from the nephron

Question 8

What is a medulla?

Answer 8

This contains the tubules of the nephrons and these form pyramids which are the little petal shapes you see on the kidney.

Question 9

What is the pelvis?

Answer 9

The central chamber where urine is collected and passes down into the ureter

Question 10

What do nephrons do?

Answer 10

There are millions of nephrons that make up the medulla and pyramids. These filter the blood, remove the nitrogenous waste and balancing the water and mineral ions before returning it to the blood. They provide a lot of tubules which allows lots of kilometres for reabsorption

Question 11

List the 5 structures of the nephron

Answer 11

• Bowmans Capsule
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct

Question 12

What is the Bowman’s capsule and what does it do?

Answer 12

A cup-shaped structure that contains a tangle of capillaries known as the glomerulus. More blood will enter the glomerulus than will leave it due to ultrafiltration

Question 13

What is the Proximal convoluted tubule and what does it do?

Answer 13

This is the first coiled region after the Bowman’s capsule which is in the cortex, and this is where many of the substances needed for the body are reabsorbed into the blood.

Question 14

What is the Loop of Henle and what does it do?

Answer 14

This is a long loop of the tubule which is in the medulla and creates a very high solute concentration in the fluid. The loop of Henle then makes a hairpin bend and the ascending limb travels all the way up to the cortex

Question 15

What is the Distal convoluted tubule and what does it do?

Answer 15

This the second twisted loop in which the fine-tuning of water balance takes place. The permeability of this wall is dependent on the ADH levels in the blood. It also regulates the pH and ion concentrations

Question 16

What is the collecting duct and what does it do?

Answer 16

This is where urine passes down through a tubule, down the medulla and into the pelvis. More fine-tuning of the water balance takes place and the walls also depend on ADH levels

Question 17

What has happened to the blood when it leaves the kidney?

Answer 17

The network of capillaries surrounding the nephron leads to the renal vein. The blood in the renal vein has many reduced levels of urea and sometimes glucose depending on selective reabsorption, but substances like amino acids and other things the body needs are still the same as when the blood entered the kidney

Question 18

What is ultrafiltration?

Answer 18

Ultrafiltration is the first step to removing nitrogenous waste and osmoregulation of the blood

Question 19

What does ultrafiltration result in?

Answer 19

Tissue fluid in the capillary beds

Question 20

Describe the process of ultrafiltration

Answer 20

• The unfiltered blood enters the Bowman’s capsule through the wide efferent arteriole and the blood leaves through a smaller afferent arteriole
• Due to this there is a lot of pressure in the capillaries of the glomerulus and this forces blood out through the capillary wall.
• The blood becomes filtered into a filtrate as it passes through the basement membrane into the Bowman’s capsule

Question 21

What is the basement membrane and what does it do?

Answer 21

Part of the membrane of the Bowman’s capsule wall, which is made up of a network of collagen fibres and acts as a sieve, filtering out many things.

Question 22

Why do red blood cells not enter the Bowman’s capsule?

Answer 22

They are too big to pass through the basement membrane and some big proteins don’t pass either

Question 23

What are the extra cells that are part of the Bowman’s capsule wall called?

Answer 23

They are called podocytes and they act as an additional filter. They have extensions called pedicels that wrap around capillaries, forming slits to catch any cells, platelets or proteins that managed to get through the basement membrane

Question 24

What is the glomerular filtration rate?

Answer 24

The volume of blood filtered through the kidneys at a given time

Question 25

What is left in the filtrate that enters the Bowman’s capsule?

Answer 25

Glucose, salt, urea, ions. 20% of water and solutes have already been removed from the plasma.

Question 26

What is the purpose of reabsorption?

Answer 26

Ultrafiltration removed urea from the blood, however, lots of important components of blood plasma that the body needs were also filtered out, so must be reabsorbed back into the blood. An example of this is glucose which would never be excreted as it’s used for respiration

Question 27

What happens at the proximal convoluted tubule?

Answer 27

The glucose, vitamins, amino acids and hormones are moved back into the blood by active transport. Water and sodium ions are reabsorbed as well, sodium ions also undergo active transport, while chloride ions and water diffuse down their concentration gradients.

Question 28

What adaptations do the proximal convoluted tubule cells have?

Answer 28

• They are covered with microvilli to increase the surface area over which the substances can be reabsorbed and SA:V ratio for diffusion
• They have many mitochondria which provide a lot of ATP needed for active transport

Question 29

Where do the substances that have been reabsorbed go?

Answer 29

They diffuse into the extensive capillary network which surrounds the tubules.

Question 30

Where does the filtrate go after the convoluted tubule reabsorption has happened?

Answer 30

It reaches the loop of Henle and right at the end of the proximal convoluted tubule, the filtrate is isotonic (the same concentration) as the tissue fluid outside the nephron

Question 31

Why is the Loop of Henle so special?

Answer 31

It allows the body to produce urine that is more concentrated than the blood because different areas of the loop of Henle have different permeabilities. It acts as a countercurrent multiplier because it uses energy to produce conc gradients so that substances can move from one area to another

Question 32

Describe what happens in the Loop of Henle

Answer 32

• The descending limb leads from the proximal tubule and here the filtrate is isotonic with the blood. The upper part of the descending limb is impermeable to water, but the lower is permeable and this runs down into the medulla
• Sodium and chloride ion concentration in the tissue fluid gets higher
• As the filtrate moves down the descending limb, it moves out of the limb into the tissue fluid by osmosis, it then moves down the concentration gradient into the blood in the capillaries surrounding it
• Sodium and chloride ions cannot move into the descending limb because it is impermeable to them. This causes the filtrate at the bottom of the Loop of Henle to be hypertonic (more concentrated) than the blood
• The first part of the ascending limb is very permeable to sodium and chloride ions so they move out of the ascending limb by diffusion, decreasing the concentration of ions in the filtrate.
• The second part of the ascending limb is slightly less permeable so the ions are actively transported out of the loop of Henle into the medulla tissue against their conc grad
• The upper part of the ascending limb is impermeable to water which means the filtrate becomes increasingly dilute while the tissue fluid in the medulla is very concentrated which is essential for the ions to move into the descending loop of Henle. This is why it’s a countercurrent multiplier system.
• At the top of the ascending limb, the filtrate is hypotonic to the blood again

Question 33

What happens in the distal convoluted tubule?

Answer 33

Here, the permeability of the walls varies with levels of ADH (anti-diuretic hormone). If the body lacks salt, the sodium ions will be actively pumped out and the chloride ions will diffuse down their electrochemical gradient. If the body has too much salt, water can leave the distal convoluted tubule by osmosis as it increases permeability.

Question 34

What happens in the collecting duct?

Answer 34

The filtrate passes down into the collecting duct and passes the concentrated tissue fluid of the medulla. Here, the volume and concentration of urine are determined and water moves out of the collecting duct all the way through it which produces very hypertonic urine when the body needs to conserve water. This is all determined by the ADH levels

Question 35

How is the kidney adapted to its function? (non-cellular)

Answer 35

• Layer of fat which protects it from damage
• Rich blood supply to maintain many different concentration gradients
• Efferent arteriole is wider than afferent arteriole which builds up glomerular pressure and causes fluid to be pushed out of the blood.

Question 36

What factors could affect the amount of urea in the urine?

Answer 36

• Amount of protein eaten
• Amount of exercise done
• How active the kidneys are

Question 37

What is osmoregulation?

Answer 37

The process of controlling the water potential of the blood within very narrow ranges

Question 38

Give examples of when the body might need to osmoregulate and why?

Answer 38

When you eat salty food, when you exercise, when you drink a lot of water, the water potential in your blood will change which puts osmotic stress on the body. The body osmoregulates to make sure the water potential in the tissue fluid is as stable as possible because if water moves out or in the cell too much it can cause a lot of damage

Question 39

What is ADH?

Answer 39

Anti-diuretic hormone and it controls the amount of water lost by urine. It is produced by the hypothalamus and secreted by the posterior pituitary gland.

Question 40

What exactly does ADH do to the kidneys?

Answer 40

It increases the permeability to water for the distal convoluted tubule and the collecting duct

Question 41

Describe how ADH works

Answer 41

ADH is released by the pituitary gland and carried in the blood to the cells of the collecting duct. Here, the hormone will bind to receptors on the ducts cells membranes which triggers the formation of cAMP as a secondary messenger inside the cell. The formation of cAMP triggers a cascade of events

Question 42

What is a secondary messenger?

Answer 42

A molecule which relays signals that it receives from receptors to molecules inside the cell

Question 43

What does cAMP do when it is produced?

Answer 43

• Vesicles in the cells lining the collecting duct will fuse with the cell surface membrane that is in contact with the tissue fluid in the medulla
• These vesicles contain protein bases water channels called aquaporins which make the vesicles permeable to water when they are inserted into the cell surface membrane
• Water can now move out of the tubule cells and into the medulla tissue fluid and blood capillaries by osmosis

Question 44

What happens if you have a lot of ADH?

Answer 44

More vesicles with the aquaporins are inserted into the cell membrane so more water can move out by osmosis and into the tissue fluid. This makes the filtrate inside the tubules very concentrated.

Question 45

What happens if you don’t have a lot of ADH?

Answer 45

cAMP levels fall and the vesicles containing aquaporins are removed from the tubule cells and enclosed in vesicles again. The permeable becomes impermeable to water so it can’t leave. This results in very dilute urine.

Question 46

What are osmoreceptors?

Answer 46

These are receptors in the hypothalamus which are sensitive to inorganic ion concentrations in the blood. They are closely linked to ADH release

Question 47

What happens when water is in short supply?

Answer 47

When water is in short supply, the water potential of the blood decreases. Osmoregulators detect this as there is a higher concentration of inorganic ions. The osmoregulators send impulses to the posterior pituitary gland which releases stored ADH into the blood. The permeability of the collecting duct and distal convoluted tubule will increase so more water will be reabsorbed into the blood and tissue fluid. This will cause the urine to be more concentrated and less output, but the blood water potential will rise back to homeostasis

Question 48

What happens when water is in excess?

Answer 48

When water is in excess, the water potential of the blood will increase and the osmoregulators will detect a drop in inorganic ion concentration. They will send impulses to the posterior pituitary gland which will reduce the amount of ADH released or even stop it. This means the collecting duct is impermeable to water, so not a lot of water will be reabsorbed into the blood. This causes the water potential in the blood to drop and the urine to become more dilute and more output.