3.7 biology a level wjec

What is homeostasis?

the maintenance of a constant internal environment despite changes in the external environment.

What is negative feedback?

a change in a system produces another change which reverses the first change.

What is excretion?

The removal of nitrogenous waste made by the body.

The kidney has two main functions:

• Excretion

• Osmoregulation - the control of water potential of the body's fluids by regulating the water content and therefore the solute concentration.

How is urea produced?

• Dietary PROTEIN is digested into AMINO ACIDS which are transported into the LIVER and then around the body where they are ASSIMILATED into proteins. Any excess amino acids are DEAMINATED in the liver and the amino group is converted to UREA. The urea is carried in the PLASMA to the kidneys and excreted in URINE. 

What is deamination?

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

What is ultrafiltration?

Filtration under high pressure.

Blood arrives in the capillaries of the glomerulus from the afferent arteriole. It has high pressure because…

The afferent arteriole has a wider diameter than the efferent arteriole, the heart's contraction increases the pressure of arterial blood.

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

• The wall of the CAPILLARY is a single layer of ENDOTHELIUM cells with pores called FENESTRAE about 80nm diameter.

• The BASEMENT MEMBRANE is an extra-cellular layer of proteins, mainly collagen and glycoproteins. It is a selective barrier between the capillary wall and the nephron.

• The wall of the BOWMAN’S CAPSULE is made of squamous epithelial cells called PODOCYTES. Processes from each podocyte, PEDICELS, wrap around a capillary and pull the podocyte closer to the basement membrane. The gaps between the pedicels are called filtration slits.

What happens during ultrafiltration?

The high blood pressure in the capillaries of the glomerulus forces solutes and water through the fenestrae of the capillaries through the basement membrane and through the filtration slits between pedicels into the cavity of the bowman’s capsule.

What constitutes the glomerular filtrate?

The solutes and water forced into the Bowman’s capsule constitute the glomerular filtrate:

• Water

• Glucose 

• Salts

• Urea

• Amino acids

What remains in the blood?

Blood cells, platelets, and large proteins are too large to pass through the basement membrane and so remain in the blood.

What is the water potential at this stage (after ultrafiltration)?

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

What is selective reabsorption?

The uptake of specific (useful) molecules and ions from the glomerular filtrate in the nephron back into the bloodstream.

What does the proximal convoluted tubule do?

Carries filtrate away from Bowman’s capsule

What is reabsorbed?

The blood in the capillaries around 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. 

The PCT has…

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

• Cuboidal epithelial cells in its walls. Their surface area is increased by microvilli facing the lumen, and basal channels in the surface facing the basement membrane and capillary. 

• Many mitochondria, providing atp for active transport.

• Close association to capillaries - short diffusion distance

How does selective reabsorption of glucose and amino acids occur?

• All the glucose and amino acids are reabsorbed into the blood by co-transport with sodium ions. 

• The glucose or amino acid and two Na+ ions bind to a transporter protein in the cuboidal epithelium cell membrane.

• They enter the cell by facilitated diffusion, dissociate from the transporter and diffuse across. The glucose or amino acid enters the capillary by facilitative diffusion.

• Na+ ions are pumped into the capillary, reducing their concentration in the epithelial cell below that of the lumen. More Na+ enters the cell, bringing in glucose or amino acid molecules. 

• This is secondary active transport. Secondary because it is not using ATP directly buy uses energy from the electrochemical gradient of Na+ ions from the lumen into the cell, generated by the active transport of Na+ into the capillary.

How are other molecules reabsorbed?

• Water in the filtrate is reabsorbed into the blood by osmosis as reabsorbed ions lowered the water potential of the blood.

• 50% of the urea and small proteins in the filtrate is reabsorbed into the blood by diffusion.

Summary of selective reabsorption

In summary, the filtrate loses salts, water, urea, glucose and amino acids back into the blood. At the base of the PCT, the filtrate is isotonic with the blood plasma.

What is secondary active transport?

the coupling of movement e.g. Na+ ions, down their electrochemical gradient, with movement of another molecule, e.g. glucose.

The mechanism for water reabsorption

• The walls of the ascending limb are impermeable to water. They actively transport sodium and chloride ions out of the filtrate in the tubule into the tissue fluid in the medulla. The loop of Henle collectively concentrates salts in the tissue fluid, which therefore has a low water potential. As the filtrate climbs the ascending limb, it contains progressively fewer salts/ions. It becomes increasingly dilute and its water potential increases.

• The walls of the descending limb are permeable to water and slightly permeable to sodium and chloride ions. As filtrate flows down the descending limb, water diffuses out by osmosis into the tissue fluid of the medulla which has low water potential. From there it moves into the vasa recta, the capillaries surrounding the loop of Henle.

• At the same time, some sodium ions and chloride ions diffuse into the descending limb. As the filtrate flows down the descending limb, it contains progressively less water and more ions and so, at the apex, the filtrate is most concentrated with the lowest water potential.

• The collecting duct runs back down into the medulla, passing through the region of low water potential. Water therefore diffuses out of the CD by osmosis, down a water potential gradient. The longer the loop of Henle, the lower the water potential in the medulla and the more water leaves the CD by osmosis. The filtrate becomes more concentrated than the blood (hypertonic) and by the time it reaches the base of the CD, it is urine. The water is reabsorbed back into the vasa recta into the general circulation.

Why is this considered a counter-current multiplier?

Having two limbs of  the loop running side by side with the fluid going in opposite directions allows the maximum concentration to be built up at the apex of the loop. This is a counter current multiplier. Because flow in the two limbs is in opposite directions (counter-current) and the solute concentration is increased (multiplied).

What is osmoregulation?

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

What are the steps of osmoregulation when there is lowered water potential in the blood?

Receptor -  Osmoreceptors in the hypothalamus detect this reduced water potential.

Co-ordinator -  Secretory vesicles carry ADH along axons in the hypothalamus to the posterior pituitary gland.

Effector - ADH is secreted into bloodstream by PPG. ADH increases the permeability of the walls of the DCT and the CD to water. So:

• More water is reabsorbed from there into the region of high solute concentration and low water potential in the medulla.

• More water is reabsorbed into the blood in the capillaries from the medulla.

• The water potential in the blood is restored to normal.

• The small volume of urine produced is concentrated. 

What are the steps of osmoregulation when there is heightened water potential in the blood?

Receptor - Osmoreceptors in the hypothalamus detect this heightened water potential.

Co-ordinator - hypothalamus sends a nervous impulse to the posterior pituitary gland.

Effector- Less ADH is released by the PPG and so the permeability of CD and DCT walls to water decreased, so:

• Less water is reabsorbed into the blood.

• Water potential is restored to normal.

• Body produces larger volume of more dilute urine.

What are aquaporins?

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

What is the ADH mechanism?

• ADH binds to membrane receptors

• Adenyl cyclase catalyses the production of cyclic AMP, the second messenger.

• Cytoplasmic vesicles containing aquaporins move and fuse with the cell membrane.

• Aquaporins are inserted into the membrane.

• Water molecules move in single file to their pores into the cell, down a water potential gradient.

What is the result of less ADH being released (due to heightened WP)?

• Fewer aquaporins are inserted into the membrane of cells in the CD

• Less water is reabsorbed from the CD and DCT

• Larger volume of dilute urine produced

What is the result of more ADH being released (due to lowered WP)?

• More aquaporins inserted into the membrane of cells in the CD.

• More water is reabsorbed from the CD and DCT.

• Smaller volume of more concentrated urine produced.

Common causes for kidney failure

• diabetes

• high blood pressure

• autoimmune disease

• infection

• crushing injuries

Problems with a high protein diet

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

Problems of high blood pressure

Excessive filtration by the glomeruli and loss of nutrients leads to damage in the glomerulus which can lead to cells and plasma proteins being lost in the urine

What is haemodialysis?

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

What is 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 only 3 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 ultrafiltration

Advantages of peritoneal dialysis

• Schedule flexbility

• Few risks of dialysis associated cramps

• Clinic visits limited to once or twice 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

What can result in rejection of a transplanted kidney?

Differences in ABO blood groups and in HLA antigens.

Advantages of a living donor

• shorter waiting time

• less risk of rejection

• lasts longer

• donor makes informed decision

Disadvantages of a 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

• More likely to reject

• Need to take immunosuppressants for the rest of your life'

• Lasts less time

Why do aquatic organisms excrete ammonia?

• highly toxic but extremely soluble in water

• large surface area of fish gills and amoeba allows ammonia to diffuse out rapidly and is immediately diluted below toxic concentrations.

Why do birds, reptiles and insects excrete uric acid?

• almost insoluble in water and non-toxic

• large energy cost to production but little water is needed for its excretion, important in conserving water and allowing a lifestyle where they must be light enough for flight.

why do mammals and some fish excrete urea?

• production requires energy

• less toxic than ammonia so tissues and body fluids dilute it below a toxic concentration and can tolerate it briefly.

What is metabolic water?

Water produced from the oxidation of food reserves (breakdown of food and its respiration). Some animals rely on metabolic water as they live in arid conditions.