CH7 homeostasis & kidney

Question 1

how does negative feedback work in homeostasis?

Answer 1

• the set point for a factor is the norm it operates at.
• an input changes the system
• a receptor detects the level that the factor has deviated from the set point
• instructions are sent to a co-ordinator which stores information to communicate to effectors.
• effectors bring about change that return system to set point

Question 2

give 2 examples of negative feedback

Answer 2

• glucose concentration if above the set point will result in insulin being secreted to convert the glucose to glycogen and increasing the rate it is respired.
  if below the set point, glucagon is secreted.

-if temperature falls below set point, increased respiration generates heat and constriction of superficial blood vessels
if above set point superficial blood vessels dilate and heat radiates from the body

Question 3

what is positive feedback

Answer 3

when an effector increases the change

Question 4

give 2 examples of positive feedback

Answer 4

• oxytocin stimulates contractions which in turn stimulate the production of more oxytocin.
• the first stage of clot formation is that platelets arrive at the surface secreting signalling molecules which attract more platelets.

Question 5

What is excretion?

Answer 5

Excretion is the removal of metabolic waste made by the body.

Question 6

What is osmoregulation?

Answer 6

Osmoregulation is the control of the water potential of the body’s fluids by the regulation of the water content of the body.

Question 7

Name three ways by which water is released from the body.

Answer 7

Water is excreted as a metabolic waste product of respiration.
Water is secreted e.g. in tears and saliva.
Water is egested in faeces.

Question 8

What are the two main functions of the kidneys?

Answer 8

1. Osmoregulation

2. Excretion

Question 9

Explain how urea is produced and give the chemical equation of its production.

Answer 9

Dietary protein is digested into amino acids which are transported to the liver and then around the body where they are assimilated into proteins. Any excess amino acids are deaminated in the liver and the amine group is converted into urea.

amino acid —> a-keto acid + amonia —> urea

Question 10

What is deamination?

Answer 10

Deamination is the removal of an amine group from a molecule. Excess amino acids are deaminated in the liver and the amine group is converted into urea.

Question 11

Describe the structure of the kidney.

Answer 11

Humans have two kidneys, one on either side of the vertebral column. A though renal capsule covers each kidney. Each kidney receives blood from a renal artery and returns blood to the general circulation in a renal vein. The blood from the renal artery is filtered in the outer layer, the cortex, at the Bowman’s capsules. The medulla contains the loops of Henle and the collecting ducts which carry the urine to the pelvis. The pelvis empties the urine into the ureter and a ureter from each kidney carries urine to the bladder.

Question 12

State the structures within the cortex and the medulla.

Answer 12

The glomerulus and the proximal and distal convoluted tubules are in the cortex.
The loop of Henle and the collecting duct are in the medulla.

Question 13

What are kidneys made of?

Answer 13

Kidneys are made of tubes called nephrons. There are a million nephrons in the kidney, each of which is about 30mm long providing a large area for exchange.

Question 14

What is nephron?
Give an overview of the structure of a nephron. In your answer include:

Where the filtered blood is taken.

Answer 14

A nephron is an individual blood filtering unit.

An afferent arteriole which is a branch of the renal artery brings blood into the nephron and divides into about 50 parallel capillaries in the glomerulus, enclosed by the Bowman’s capsule. The filtered blood is then carried by an efferent arteriole to:

A capillary network surrounding the proximal and the distal convoluted tubules.

The vasa recta, a capillary network surrounding the loop of Henle.

Question 15

What happens to the blood filterate?

Answer 15

The blood filterate is diverted through the nephron and the collecting ducts of many nephrons join and carry the urine to the pelvis and ureter.

Question 16

What is ultrafilteration?

Answer 16

It is filteration under high pressure.

Question 17

During ultrafilteration blood arives in the glomerulus capillaries from the afferent arteriole and is under high perssure. Why?

Answer 17

1) Heart’s contraction increases the pressure of arterial blood.
2) The afferent arteriole has a wider diameter than the efferent arteriole.

Question 18

By how many layers is the blood entering the glomerulus, seperated from the space inside the Bowman’s capsule (Bowman’s space)?

Describe these layers.

Answer 18

The blood entering the glomerulus is separated from the space inside the Bowman’s capsule - the Bowman’s space by three layers:

1) The wall of the capillary which is a single layer of endothelium cells with pores called fenestrae which are about 80nm in diameter.
2) The basement membrane which is an extra-cellular layer of proteins, mainly collagen and glycoproteins. It is a molecular filter and is the selective barrier, acting like a sieve, between the blood and the nephron.
3) The wall of the Bowman’s capsule is made of squamous epithelial cells called podocytes. Processes from each podocyte called pedicels, wrap around a capillary, pulling it closer to the basement membrane. The gaps between the pedicels are called filtration slits.

Question 19

Explain how ultrafiltration occurs?

Answer 19

The high blood pressure in the capillaries of the glomerulus, force water and solute through the fenestrae of the capillaries, through the basement membrane and through the filtration silts between the pedicels into the cavity of the of the Bowman’s capsule. This is ultrafiltration which is filtration under high pressure.

Question 20

The solutes and water forced into the Bowman’s capsule constitute the glomerular filtrate. State what the glomerular filtrate contains.

Answer 20

The glomerular filtrate contains:
Water
Glucose
Amino acids
Salts
Urea

Question 21

Explain why the glomerular filtrate resembles plasma?

Answer 21

During ultrafiltration, molecules with RMM (relative molecular mass) of less than 30000 pass through the basement membrane easily and molecules with RMM>68000 are too big to pass through. Blood cells, platelets and large proteins e.g. albumin and antibodies, therefore, remain in the blood. The glomerular filtrate consequently resembles plasma although it lacks the large proteins.

Question 22

Why does the blood that leaves the glomerulus through the efferent arteriole have a low water potential?

Answer 22

The blood leaving the glomerulus into the efferent arteriole has a low water potential because much of the water has been lost and there is a high protein concentration remaining.

Question 23

What is the glomerular filtration rate? What determines this rate?
Give the values for the rate at which kidneys of an adult receive blood and the rate at which kidneys of an adult produce glomerular filtrate.

Answer 23

Of the blood that leaves the heart about 20% goes directly to the kidneys. The rate at which fluid passes from the blood in the glomerular capillaries into the Bowman’s capsule is known as the glomerular filtration rate. It is determined by the difference of water potential between the two areas - it is the balance of their hydrostatic pressures and the solute potentials.

Together, the kidneys of an adult receive blood at a rate of 1.1 dm^3 min^-1 and produce glomerular filtrate at a rate of 125 cm^3 min^-1.

Question 24

Give the definition of selective reabsorption.

Answer 24

Selective reabsorption is the uptake of specific molecules and ions from the glomerular filtrate in the nephron back into the bloodstream.

Question 25

Why is selective reabsorption important?

Answer 25

The glomerular filtrate contains wastes that the body needs to eliminate but it also contains useful molecules and ions including glucose, amino acids, chloride and sodium ions. Selective reabsorption is the process by which the useful products are reabsorbed back into the blood as the filtrate flows through the nephron.

Question 26

Where does selective reabsorption take place? (Provide some details.)

Answer 26

Selective reabsorption takes place in the proximal convoluted tubule (PCT), which is the longest and widest part of the nephron. It carries the glomerular filtrate away from the Bowman’s capsule. The blood in the capillaries surrounding the PCT reabsorbs all the glucose and amino acids some of the urea, and most of the water and sodium and chloride ions from the filtrate in the PCT.

Question 27

State all the features of the PCT that make it useful for selective reabsorption.

Answer 27

It has a large surface area because it is long and there are a million nephrons in the kidney.

It has cuboidal epithelial cells in its walls. Their surface area is increased by:

a) The microvilli which are about 1 μm long and face the lumen.
b) The invaginations called basal channels on the surface facing the capillaries and the basement membrane.

It has a close association with the capillaries.

It has many mitochondria providing ATP for active transport.

It has tight junctions between the cells of the proximal convoluted tubule epithelium. These are multi-protein complexes that encircle a cell and attach it tightly to its neighbours. This prevents molecules from diffusing between adjacent cells and prevents them from diffusing back into the glomerular filtrate.

Question 28

Fully explain the selective reabsorption that takes place in the proximal convoluted tubule. In your answer include the percentages of the reabsorptions.

Answer 28

About 70% of the salts are reabsorbed back into the blood. Some reabsorption is passive but most use active transport by membrane pumps.

All of the glucose and amino acids are reabsorbed from the glomerular filtrate back into the blood, by Co-transport with sodium ions. A glucose molecule and two sodium ions bind to a transport protein in the cuboidal epithelium cell membrane. They enter the cell by facilitated diffusion, dissociate from the transporter and then diffuse across. Sodium is then pumped into the capillaries by active transport and glucose enters blood by facilitated diffusion. Co-transport is also referred to as secondary active transport. This is because the active transport of sodium ions from the cuboidal epithelium cells into the blood reduces their concentration within the cells enhancing the diffusion of sodium ions into the cell, carrying in the glucose.

About 90% of the water in the glomerular filtrate is reabsorbed to the blood passively, by osmosis, as reabsorbed ions lower the water potential of the blood.

About 50% of the urea and small proteins in the glomerular filtrate is reabsorbed back to the blood by diffusion. The filtrate has lost so much water that their concentration there is high and so they diffuse into the blood down their concentration gradient.

The filtrate has lost water, urea, salt, glucose and amino acids back to the blood. At the base of the proximal convoluted tubule, the filtrate is isotonic with the blood plasma.