Formation Of Urine

Question 0

Describe the process of ultrafiltration

Answer 0

• blood flows into the glomerulus via the afferent arteriole which is at a higher pressure then the blood that leaves the glomerulus through the efferent arteriole
- the difference in diameters ensure that the blood in the glomerulus is kept a high pressure
• the high pressure forces liquid and small molecules in the blood out of the capillary and into the renal capsule
• the liquid and small molecules pass through 3 layers to get into the bowman’s capsule and enter the nephron tubules
- the endothelium of capillaries –> contains gaps in which blood passes through as well as the substances dissolved in it
- the basement membrane –> fine mesh of collagen fibres and glycoproteins that do not allow large molecules like proteins to pass through
- epithelium of bowman’s capsule –> contains finger like projections called podocytes that fluid from the glomerulus can pass through into the bowman’s capsule

Question 1

What substances are filtered out during ultrafiltration?

Answer 1

• ions forced out
• glucose
• water
• urea
• small proteins

Question 2

Explain the stages of selective reabsorption

Answer 2

• the sodium potassium pumps remove sodium ions from the cells lining the PCT - this reduces the concentration of sodium ions in the cell cytoplasm
• sodium ions are then transported into the cell along with amino acids or glucose via facilitated diffusion
• as the glucose and amino acid concentrations rise inside the cell they are able to diffuse out of the opposite side of the cell into the tissue fluid, they may also be actively removed
• from the tissue fluid these substances then diffuse into the blood and are carried away
• the readsorption of salts, glucose or amino acids reduces the water potential of the cell and increases the water potential in the tubule fluid so water will enter the cells and be reabsorbed into the blood by osmosis
• larger molecules such as small proteins that may have entered the tubule will be reabsorbed by endocytosis

Question 2

How are the cells lining the PCT specialised for selective reabsorption

Answer 2

• microvilli - increase the surface area for reabsorption
• co transporter proteins - transports glucose or amino acids
• sodium potassium pumps - pump sodium ions out of the cell and potassium ions into the cell actively against concentration gradient
• many mitochondria - provides energy from ATP for reabsorption process