Lecture 4 - Why the kidney works backwards

Question 1

What is total body water in an average male?

Answer 1

60% of body weight (70kg man) => ~42L

Question 2

What is total body water in an average female?

Answer 2

50% of body weight (60kg woman) => ~30L

Question 3

Why do females have less total body water?

Answer 3

Females have more adipose tissue than males, which have a low affinity for water

Question 4

How much water is lost per day by just breathing?

Answer 4

0.5L/day

Question 5

How much salt is lost in sweat?

Answer 5

Not much

30 mmol/L

Question 6

Approx how much water is gained by food and drink per day?

Answer 6

2.2L/day

Question 7

Approx how much water is produced by metabolism per day?

Answer 7

0.3L/day

Question 8

Approx how much water is lost insensibly via skin and lungs per day?

Answer 8

0.9L/day

Question 9

approx how much water is lost in urine per day?

Answer 9

1.5L/day

Question 10

approx how much water is lost in the faeces per day?

Answer 10

0.1L/day

Question 11

how much water do we turn over in a gentle environment?

Answer 11

2.5/30L which is ~5-8% of water ever day

we turn over more in hot/dry environments

Question 12

How much is the ECF volume regulated to?

Answer 12

~15% or more

Question 13

How much water and salt do we typically consume more than we need per day?

Answer 13

20%

Question 14

Why doesnt the bladder -> tube concept of the kidney work?

Answer 14

There are no water or urea pumps, so those substances can’t enter the kidney

Question 15

How do we get past without water and urea pumps?

Answer 15

1. separate out large volume of ECF from plasma
2. reabsorb ~99% of this back into the plasma.
   Water, salts, urea which we dont want to keep is left behind
3. Achieving the ideal kidney but backwards

Question 16

What is the aim of filtration?

Answer 16

To make a large volume of protein-free plasma (aka standard, ordinary extracellular fluid)

Question 17

What is the normal GFR rate?

Answer 17

125ml per min per 1.73m^2

Question 18

What is the average GFR rate for a male? for a female?

Answer 18

male = 7.5L/h => 180L/day
female = 6L/H = 144L/day

Question 19

What is the osmolarity of ECF?

Answer 19

~300mosmol/L

more like 290mosmol/L though!

Question 20

What happens to the fluid after filtration?

Answer 20

1. it passes through a tubule that is not very permeable to urea
2. salt and water are reabsorbed which leaves behind most of the urea and a small amount of remaining water

Question 21

What happens in the reabsorption step?

Answer 21

1. salt is actively pumped out of the tubule using ATP as fuel
2. Water follows salts, amino acids and glucose
3. Fluid which is removed from the tube is approx = to ECF concentration (~300mosmol/L )

Question 22

How much of the fluid has not been reabsorbed so far?

Answer 22

~20%

Question 23

what happens if the concentration of glucose is over the transport maximum?

Answer 23

the renal losses of glucose are linear

Question 24

What is the transport maximum of glucose?

Answer 24

10-11 and 16.6mmol/L plasma glucose.

some glucose is lost at 10+ mmol/L

Question 25

When does glucose loss become linear?

Answer 25

at 16+mmol/L

Question 26

What are the two mechanisms of sodium reabsorption

Answer 26

1. sodium reabsorption primarily by cotransport: with bicarbonate, glucose, amino acids
   this occurs in the early segment
2. Sodium reabsorbed mainly with chloride once other substances have been reabsorbed
   this occurs in the second segment

Question 27

What are the concentrations along the proximal tubule?

Answer 27

the amount of Na decreases, but there is no change in the concentration of Na

HCO3, glucose, amino acids also decrease

Cl increases along the tube

Water follows salt by paracellular osmosis so the osmolarity is stable and equivalent to ECF concentration

Question 28

Describe the composition of the end of the proximal tubule

Answer 28

• Tubular fluid has decreased from 6L/h to 1.2L/h
• approximately 80% of GFR is reabsorbed
• there is no change in Sodium concentration
• There are no glucose or amino acids here
• The fluid is iso osmotic as the missing osmoles are made up by urea
• Urea concentration is ~2.4x higher than it was in the original glomerular filtrate and ECF