*E1 Human Physiology - Regulation of water content

Question 1

Ultrafiltration

Answer 1

1. The diameter of the lumen of afferent arteriole entering the glomerulus is larger than that of the efferent arteriole leaving glomerulus
2. This builds a high blood pressure in glomerulus
3. Since the walls of the glomerular capillaries and the Bowman’s capsule is very thin, the great pressure forces some plasma except plasma proteins and blood cell through the thin wall of glomerulus and inner wall of Bowman’s capsule to the capsular space, forming glomerular filtrate.

Question 2

How heart failure affects urine production and urine formed?

Answer 2

Pumping action of heart decreases in strength, pressure built up in glomerulus decreases, ultrafiltration rate decreases, volume of glomerular filtrate formed per unit time decreases, urine volume decrease, urea concentration and water content in blood increases

Question 3

Reabsorption

Answer 3

Glucose / amino acids (100%)
- by diffusion and active transport
- Concentration decreases

Salts (80%)
- by diffusion and active transport
- concentration remains unchanged (proportional to reabsorption of water)

Water (99%)
- water potential of blood is much lower than the remaining fluid in kidney tubules
- by osmosis

Urea (50%)
- by diffusion
- concentration of urea in collecting duct is higher than that in proximal convoluted tubule (percentage of water reabsorbed is higher than that of urea)

Question 4

Caffeine causes dilation of afferent arteriole. How caffeine in blood increases rate of urine production?

Answer 4

• The flow rate of blood in afferent arteriole is higher and more blood flows to the afferent arteriole, the pressure in glomerulus becomes higher
• The rate of formation of glomerular filtrate increase, the rate of water reabsorption in kidney tubule remains relatively the same, the rate of urine formation increases

Question 5

Adaptive structures of kidneys for reabsorption of useful matrials

Answer 5

Microvilli
- Inner surface of tubular cells of proximal convoluted tubule facing the lumen consists of numerous microvilli, which increases the surface area for reabsorption of useful materials

Mitochondria
- Many mitochondria are present at the base of tubular cells of kidney, much energy can be produced for reabsorption of all glucose, amino acids and most of the salts by active transport

Question 6

Osmoregulation - losing too much water

Answer 6

• After heavy sweating or severe bleeding, blood becomes concentrated and its water potential decreases to a value lower than normal
• This is detected by osmoreceptors in the hypothalamus that stimulates the pituitary gland to release more ADH into the blood circulation
• The wall of the collecting duct becomes more permeable to water. A greater proportion of water is reabsorbed by osmosis from the collecting duct into the surrounding blood capillaries
• A small volume of concentrated urine is produced, raising the water potential of blood to normal

Question 7

Osmoregulation - gaining too much water

Answer 7

• After drinking a large amount of water, blood becomes diluted and its water potential increases to a value higher than normal
• This is detected by osmoreceptors in the hypothalamus that stimulates the pituitary gland to release less ADH into the blood circulation
• The wall of the collecting duct becomes less permeable to water. A smaller proportion of water is reabsorbed by osmosis from the collecting duct into the surrounding blood capillaries
• A large volume of dilute urine is produced, lowering water potential of blood back to normal

Question 8

Osmoregulation - excess salt (/glucose) in meal

Answer 8

• More salt is absorbed into the plasma
  -The glomerular filtrate formed will have a high concentration of salt
• Water potential in glomerular filtrate decreases
• A smaller proportion of water is reabsorbed along the kidney tubule
• Excess salt is excreted in urine
• A large volume of urine with high salt concentration is produced

Question 9

Unavoidable great water loss when doing exercise

Answer 9

• Large amount of heat is generated by vigorous muscular activities, sweating increases so that more heat is lost by evaporation of sweat
• The lining of air sacs is moist, water evaporates into the air of the air sacs. Breathing rate increases during exercise, a greater amount of air is exhaled per unit time, more water vapour is exhaled out

Question 10

Why a diabetic person urinates more frequently than a healthy person?

Answer 10

• Larger volume of urine is produced by diabetic person than a healthy person because the blood glucose level of a diabetic person is higher than a healthy person, when the glucose is filtered into the kidney tubule, diabetic person fails to reabsorb all glucose from the glomerular filtrate
• The glomerular filtrate of diabetic person has lower water potential that that of healthy person
• A smaller proportion of water is reabsorbed at the collecting duct and a larger volume of urine will be produced

Question 11

Kidney failure - decrease intake of beans

Answer 11

• If excess beans are eaten, the amount of urea excreted would increase because beans contain a lot of protein which is digested into amino acid and is absorbed in the form of amino acid
• Excess amino acids are broken down in the liver to form urea
• People with kidney failure cannot remove urea away the body properly, accumulation of urea is toxic

Question 12

Kidney machine

Answer 12

Biological principles
- Blood is passed into a dialysis tubing which is selectively permeable to small molecules like urea, the dialysis tubing helps remove urea from the blood while retaining large blood components
- Dialysis fluid has similar water potential as blood to prevent loss of water and contains similar concentration of glucose, amino acids and mineral salts as blood to prevent loss of essential nutrients from blood
- Dialysis fluid has no urea, urea from the blood diffuse to the dialysis fluid along the concentration gradient and removed
- Dialysis fluid has the same temperature as blood to prevent heat loss from blood (can be 40C to speed up diffusion of urea)

Limitations
- Infection
- Takes a long time (small volume of blood enters the kidney machine per unit time)
- Needs to be repeated regularly (urea is continuously produced)
- Expensive

Question 13

Peritoneal dialysis

Answer 13

Advantages
- Simple
- Less expensive

Limitations
- Infection

Question 14

Kidney transplant

Answer 14

Limitations
- Immunological rejection of transplanted kidney (blood groups and tissues of donor and recipient must be matched)
- Use of immunosuppressive drugs weakens immune response of patients
- Not many people are willing to donate kidney