14.1 Homeostasis in mammals (Excretion)

Question 1

Excretion

Answer 1

Removal of waste products generated by metabolic reactions inside body cells. Some of these products are toxic, while others are simply in excess of requirements

Question 2

The kidneys help maintain a constant environment by:

Answer 2

• excreting waste products, particularly the nitrogenous waste product urea
• helping to control the quantity of water in body fluids

Question 3

Most common excretory products

Answer 3

• urea (main)

- carbon dioxide

Question 4

Deamination

Answer 4

• the breakdown of excess amino acids in the liver, by the removal of the amino (NH2) group which forms ammonia.
• ammonia is a very poison chemical and can denature proteins in the cytoplasm of cells. It is therefore quickly turned into another chemical called urea

Question 5

Why remove excess amino acids from the body?

Answer 5

• amino group (NH2) of the amino acid provides useful energy and so it stays in the body
• What left over is ammonia and is very soluble and highly toxic, so has to be removed in the form of urea out of the body

Question 6

Structure of kidney

Answer 6

• 2 kidneys
• Each kidney receives blood from a renal artery (oxygenated) and renal vein (deoxygenated)
• Each kidney has a narrow tube called the ureter which connects to the bladder
• the bladder connects to urethra, which carries urine to the outside of the body
• covered by a capsule
• beneath the capsule lies the cortex
• central area is made up of the medulla
• where the ureter joins is the area called the pelvis
• made up of thousands of tubes called nephrons

Question 7

Nephron structure

Answer 7

• afferent arteriole carries blood from renal artery to the glomerulus (tight network of capillries) in Bowman’s capsule
• blood leaves glomerulus through efferent arteriole
• tube from Bowman’s capsule runs through the proximal convoluted tubule
• then the loop of Henle - which goes into medulla
• then the distal convoluted tubule - which goes back to the cortex
• then the collecting duct - which goes through medulla that leads to the pelvis of the kidney

Question 8

The kidney makes urine in a 2 step process

Answer 8

1) Ultrafiltration

2) Selective absorption

Question 9

Ultrafiltration

Answer 9

separation of large solute molcules from small ones by passing blood plasma through the basement membrane of the renal capsule (Bowman’s)

Question 10

Selective absorption

Answer 10

The removal of particular substances from the glomerular filtrate, and their return to blood

Question 11

The blood in the glomerulus is separated from the space inside the renal capsule by:

Answer 11

• the capillary wall (endothelium) which is one cell thick and has pores in it
• the basement membrane in the wall of the renal capsule
• the layer of cells making up the wall of the renal capsule call podocytes which have slits in them

Question 12

Ultrafiltration process

Answer 12

• The blood in a glomerulus is at a relatively high pressure, because the efferent arteriole is narrower than the afferent arteriole.
• This forces molecules from the blood through these 3 structures, into the renal capsule.
• The pores in the capillary endothelium and the slits between the podocytes will let all molecules through. but the basement membrane acts as a filter and will only let small molecules through

Question 13

Function of basement membrane

Answer 13

• acts as a filter
• stops large protein molecules from getting through
• Blood cells and white blood cells are also too large to pass through the barrier and so stay in the blood

Question 14

glomerular filtration rate

Answer 14

rate at which fluid filters from the blood in the glomerular capillaries into the Bowman’s capsule
- affected by difference in water potential

Question 15

Some of the substances filtered into the renal capsule need to be retained in the body:

Answer 15

• much of the water
• all of the glucose
• some of the inorganic ions
  Therefore there is selective reabsoprtion in the walls of the proximal convoluted tubule

Question 16

Selective reabsorption process

Answer 16

• Sodium ions are actively pumped out of the outer surface of a cell in the wall of the proximal convoluted tubule into the blood (no protein carriers to return them into the PCT again)
• This lowers the concentration of Sodium ions in the cell so Sodium ions passively diffuse into the cells from the fluid inside the tubule through special co-transporter proteins
• As Sodium ions diffuse through, they each bring either a glucose or amino acid molecule
• Glucose and amino acids diffuse down their potential gradient into the blood
• The movement of Sodium ions and glucose into the blood decreases the water potential in the blood and so water moves by osmosis into the blood

Question 17

After selective reabsorption, the fluid in the nephron has:

Answer 17

• no glucose or amino acid
• a lower concentration of sodium ions than originally
• less water than the filtrate originally had

Question 18

Adaptations of the Proximal Convoluted Tubule to increase rate of absorption

Answer 18

• microvilli to increase the surface area of the inner
  surface facing the lumen
• tight junctions that hold adjacent cells together so that
  fluid cannot pass between the cells (all substances that
  are reabsorbed must go through the cells)
• many mitochondria to provide energy for sodium–
  potassium (Na+–K+) pump proteins in the outer
  membranes of the cells
• co-transporter proteins in the membrane facing
  the lumen.

Question 19

Function of the Loop of Henle

Answer 19

• The function of these long loops is to create a very high concentration of sodium and chloride ions in the tissue fluid in the medulla.
• As you will see, this enables a lot of water to be reabsorbed from the fluid in the collecting duct, as it flows through the medulla.

Question 20

Reabsorption in the Loop of Henle

Answer 20

• Cells in ascending limb (impermeable) actively transport sodium and chloride ions into the tissue fluid.
• This decreases the water potential in the tissue fluid and increases the water potential inside the ascending limb
• The descending limb is permeable to water, but the ascending limb is not
• As water flows through the descending limb, water moves out of it by osmosis
• At the same time, sodium and chloride ions diffuse into the loop (descending limb) from ascending limb, down their concentration gradient
• By the time the fluid reaches the bottom of the loop, it has a much lower water potential than at the top of the loop - highly concentrated at the bottom of loop
• This creates a low water potential in the tissues of the medulla. The longer the loop the lower water potential that can be produced
• As the fluid goes up the ascending limb, losing sodium and chloride ions all the time, it becomes gradually less concentrated

Question 21

Counter-current multiplier

Answer 21

Having two limbs side by side like this (loop of Henle), enables the maximum concentration of solutes to be built up inside and outside the tube at the bottom of the loop. This is called a counter-current multiplier

Question 22

The Distal Convoluted Tubule and the Collecting Duct

Answer 22

• the fluid inside the tubule as it leaves the loop and Henle and moves into the collecting duct has lost a little more water and more sodium ions than it had when it entered the loop
• Because more water has been lost, the concentration of urea has increased
• Now, in the distal convoluted tubule, sodium ions are actively transported out of the blood
• the fluid then flows through the collecting duct
• this passes through the medulla, where a low water potential has been produced by the medulla
• as water flows through the collecting duct, water moves down the water potential gradient from the collecting duct and into the tissues of the medulla
• this further increases the concentration of urea in the tubule
• the fluid that finally leaves the collecting duct and flows into the ureter is urine

Question 23

Osmoregulation

Answer 23

the control of the water content of body fluids (part of homeostasis)
- it is important that cells are surrounded by tissue fluid of a similar water potential to their own contents, to avoid to much loss or gain, which could disrupt metabolism

Question 24

ADH (antidiuretic hormone)

Answer 24

• secreted from the posterior pituitary gland

- “don’t pee” hormone

Question 25

Osmoregulation Process

Answer 25

• There is a decrease in water potential
• This change is detected by osmoreceptors in the hypothalamus (receptors)
• This signals the hypothalamus to conserve water
• Pituitary gland releases more ADH (effectors)
• ADH moves into the blood to the collecting ducts in the kidney
• ADH binds to receptors on the wall cells of the collecting duct and this triggers a cascade of reactions which produces an active phosphorylase enzyme
• This phosphorylase enzyme causes vesicles containing aquaporins to fuse to the cell membrane of the wall cells of the collecting duct and become part of the membrane
• This increases permeability of collecting ducts
• Water moves into the blood by osmosis
• Urine becomes concentrated
• Normal water potential of blood

Question 26

At which process is water most reabsorbed?

Answer 26

Selective Reabsorption (in proximal convoluted tubule)