3.7 Homeostasis and the Kidney

Define homeostasis

The process of maintaining the body in a state of dynamic equilibrium - a constant internal state despite changes in the external environment

Define negative feedback

A change in a system produces a second change, which reverses the first change

Define set point

The norm at which the system operates

State the series of stages in negative feedback

1. A receptor detects the level of the factor and its deviation from the set point

2. The receptor sends instructions to a co-ordinator

3. The co-ordinator communicates with one or more effectors which make responses that are corrective procedures

4. The factor returns to normal

Define positive feedback

In which an effector increases a change

What are the two main functions of the urinary system

Osmoregulation
Excretion

Define excretion

The removal of nitrogenous metabolic waste from the body

Define osmoregulation

The mechanism by which the balance of water and dissolved solutes in the plasma is regulated

Describe the production of urea

Protein is digested into amino acids

Excess amino acids are deaminated

Amino group is converted to ammonia and then to urea

Contents of blood from the aorta to renal arteries

Oxygenated

High level of urea

Contents of blood from renal veins to vena cava

Deoxygenated

Lower level of urea

State the three main regions to the mammalian kidney

Cortex, medulla and renal pelvis

Where do the renal arteries divide into arterioles

Cortex

Where is the site of osmoregulation

medulla

Where is the origin of the ureter

renal pelvis

Structures in the cortex

-Glomerulus
-Proximal Convoluted Tubule
-Distal Convoluted Tubule

Structures in the medulla

Collecting duct
Loop of Henle

Advantages of ammonia

Little energy needed to produce it

Highly soluble

Disadvantages of ammonia

Highly toxic in concentrated solution

Needs a very high volume of water for its excretion

Advantages of Urea

Less toxic than ammonia

Less water needed for its excretion

Disadvantages of Urea

More energy needed to produce urea than ammonia

Advantages of uric acid

Very low toxicity

Low solubility

Low volume of water needed for its excretion

Disadvantages of uric acid

High energy required to produce uric acid

Habitats that require ammonia as a nitrogenous waste

water

Habitats that require urea as a nitrogenous waste

terrestrial in conditions with adequate water supply

Habitats that require uric acid as a nitrogenous waste

Terrestrial in arid conditions

Define ultrafiltration

Filtration under high pressure

How does the blood arriving in the capillaries of the glomerulus from the afferent arteriole high pressure

-The afferent arteriole ha a wider diameter than the efferent arteriole

• The heart's contraction

What are the three layers separating the blood entering the glomerulus from the inside of the Bowman's capsule

The wall of the capillary

The basement membrane

Podocytes

Describe the wall of the capillary

A single layer of endothelium cells with pores called fenestrations

Describe basement membrane

An extracellular layer of proteins, mainly collagen and glycoproteins

Describe podocytes

Squamous epithelial cells

Explain the role of the basement membrane

act as a selective barrier

Explain the role of podocytes

Pedicels wrap around a capillary pulling the podocyte closer to the basement membrane

Name the gaps between pedicles

filtration slits

How is the efficiency of filtration increased

Feet of podocytes increase surface area for filtration

The short distance between the podocytes and the capillary

Channels between the feet of the podocytes increase the concentration gradient between the tissue fluid

What is in the glomerular filtrate

water, glucose, salts, urea, amino acids

Describe the net filtration pressure

Hydrostatic pressure of the plasma is high

Osmotic pressure of the plasma is lower than that of the filtrate due to the presence of plasma proteins

Fluid pressure in the Bowman's capsule increases as the volume increases

Where does selective reabsorption mostly take place

Proximal Convoluted Tubule

State how the PCT is suited to selective reabsorption

• A large surface area because it is long

• Microvilli on cuboidal epithelial cells

• Basal channels

• Many mitochondria

• Close association with capillaries

• Large amount of RER and golgi body

Why does the PCT have many mitochondria

Provide ATP for active transport

How much glucose and amino acids are reabsorbed in PCT

ALL

Why does PCT have microvilli

Increase surface area

Why are the capillaries in closes contact with PCT

Reduce diffusion distance

Constant blood flow increases concentration gradient

Why does the PCT have basal channels

Increases surface area

Concentration increases in channels so increases concentration gradient

Why does the PCT have a large amount of RER and golgi bodies

Production of proteins for facilitated diffusion and active transport

How is water reabsorbed in the PCT

By osmosis from a high water potential gradient in the filtrate to a lower water potential in the cytoplasm of PCT cells. Water then moves into the tissue fluid and then the blood

How are glucose and amino acids reabsorbed

1. Co-transport

2. Glucose or amino acid and two Na+ ions bind to a carrier protein in the cuboidal epithelium cell membrane

3. They enter the cell by facilitated diffusion

4. Na+ ions are pumped into the capillary reducing their concentration in the epithelial cell below that of the lumen.

5. More Na+ enters the cell bringing in glucose or an amino acid molecule

6. Glucose or amino acid enters the capillary by facilitated diffusion

Why is Co-transport of glucose called secondary active transport

it is not using energy from ATP directly but uses energy from the electrochemical gradient of Na+ ions from the lumen into the cell which was generated by the active transport of Na+ into the capillary

How are chloride ions reabsorbed in the PCT

They follow the sodium ions down an electrochemical gradient

What happens to the water potential as the filtrate travels along the PCT and why

it increases because ions and polar substances are reabsorbed into the blood

Why does the sodium ion concentration stay the same throughout the PCT

it is reabsorbed at the same rate as water

Why does concentration of urea increase in the PCT

It is not reabsorbed

Volume of filtrate decreases

State the permeability of the descending limb

Permeable to water

State what happens to the water in the descending limb

Leaves the filtrate into the tissue fluid of the medulla by osmosis and then it moves to the vasa recta

State what Na+ and Cl- do in the descending limb

diffuse into the descending limb from the medulla

When is the filtrate at its most concentrated in the loop of Henle

at the bottom of the hairpin

State the permeability of the ascending limb to water

Impermeable

How is the low water potential in the medulla maintained

sodium and chloride ions are actively transported out of the filtrate in the tubule to the tissue fluid in the medulla

What is the mechanism in the loop of Henle called

Counter-current multiplier

What does the vasa recta act as in water reabsorption

Counter current exchanger

What does a longer loop of Henle cause

more ions can be pumped into the medulla. This lowers the water potential of the medulla allowing more water to be reabsorbed into the bloodstream by osmosis

Describe the steps of osmoregulation when there is an increase in the water potential of the blood

Detector/ receptor - Osmoreceptors in the hypothalamus detect higher water potential of the blood and send a nervous impulse to the posterior lobe of the pituitary gland

Coordinator - Posterior lobe of the pituary gland releases less ADH into the blood.

Effector -Permeability of collecting duct and distal convuluted tubule to water decreases

Describe the steps of osmoregulation when there is a decreases in water potential of blood

Detector - Osmoreceptors in the hypothalamus detect low water potential of the blood and send a nervous impulse to the posterior lobe of the pituitary gland

Coordinator - Posterior lobe of the pituitary gland releases more ADH into the blood.

Effector - Permeability of collecting duct and distal convuluted tubule to water increases

State the result of less ADH being released

Fewer aquaporins are inserted into the membrane of cells in the collecting duct

Less water is reabsorbed from the collecting duct and distal convoluted tubule

Larger volume of less concentrated urine produced

State the result of more ADH being released

More aquaporins are inserted into the membrane of cells in the collecting duct

More water is reabsorbed from the collecting duct and distal convoluted tubule

Smaller volume of more concentrated urine produced

Where does ADH bind to

receptors on the membrane

What can a fall in water potential be caused by

Reduced water intake

Sweating

Intake of large amount of salts

Another name for ADH

vasopressin

What does ADH stand for

antidiuretic hormone

State common causes for kidney failure

diabetes

high blood pressure

autoimmune disease

infection

crushing injuries

Problem with a high protein diet

increased urea levels which can be converted into uric acid, this can crystalise and form kidney stones which can tear and damage tissues

Problem of high blood pressure

excessive filtration by the glomeruli and loss of nutrients

Leads to damage to the glomerulus which if severe can lead to cells and plasma proteins being lost in the urine

Define haemodialysis

using a dialysis machine to remove excess water, urea and ions from the plasma

Define peritoneal dialysis

uses selectively permeable membranes in the body to remove excess water, urea and ions from the plasma

Advantages of haemodialysis

Effective removal of waste products

Care given by trained professionals

Rapid correction of electrolyte imbalances

Treatment usually occurs only three times a week

Disadvantages of haemodialysis

Vascular access surgery required

Schedule inflexibility

Must travel to centre 3 times a week

Risk of bacteraemia

Cramping with unltrafiltration

Advantages of peritoneal dialysis

Schedule flexibility

Few risks of dialysis associated cramps

clinic visits limited to 1 - 2x a month

No need for vascular access

Disadvantages of peritoneal dialysis

Permanent external catheter

Risk of peritonitis

Must store dialysis equipment at home

Need for self monitoring care

Where there is complete failure of the kidneys what is the treatment

Kidney transplant

What can result in a rejection to a transplanted kidney

differences in blood groups and in the antigens on the surface of cells

Advantages of living donor

Shorter waiting time

Less risk of rejection

Lasts longer

Donor makes informed decision

Disadvantages of living donor

Pressure on potential donors

Donor only has one kidney

Risk to donor and recipient from surgery

Advantages of deceased donor

Feel healthier

Have more energy

Be able to work and travel

Deceased donors not harmed in surgery

Disadvantages of deceased donor

Long waiting times

Pain following surgery

Could reject kidney

Need to take immunosuppressive drugs for the rest of your life

Increased risk of infection

Lasts less time

A – Kidney

B – Ureter (transports urine to the bladder)

C – Bladder (stores urine)

D – Urethra (carries urine out of the body)

E – Renal vein (blood returns to the general circulation)

F – Renal artery (blood enters the kidney)

K – Medulla (reabsorption of water occurs here)

L – Cortex (ultrafiltration and selective reabsorption occurs in this region)

M – Pelvis (empties urine into the ureter)

N – Ureter (transports urine to the bladder)