Nitrogen Excretion

Question 1

what is positive nitrogen balance? What is negative nitrogen balance?

Answer 1

Question 2

under what circumstances do we have positive nitrogen balance?

Answer 2

during pregnancy/growth - when the body protein is increasing

Question 3

Under what circumstances do we have negative nitrogen balance?

Answer 3

in a ‘catabolic state’ when we’re using our body protein for energy (malnutrition, burns, cancer)

Question 4

nitrogen excretion is dependent on what factors?

Answer 4

• protein intake
• protein catabolism ( inadequate caloric intake, injury, cancer)
• renal function (85% of N excreted as urea, 5% as NH4+ and 10% as uric acid and creatine ) - nitrogen also lost in feces and skin exudates

Question 5

what are the 3 key stages of amino aicd catabolism?

Answer 5

1. transamination - transfer of amino group to alpha ketoglutarate - generates alpha-ketoacid and glutamate
2. oxidative deamination of glutamate - generates ammonia and alphaketoglutarate
3. urea cycle - converts toxic NH3 to urea

Question 6

describe the process of transamination

Answer 6

transamination is the process by which the amino portion of the amino acid is transfered to a keto acid - thus generating a different amino acid

in most cases, the recieving keto acid is alpha ketoglutarate and the amino acid thus formed is glutamate

Question 7

describe the process of oxidative deamination

Answer 7

oxidative deamination of glutamate by glutamate dehydrogenase releases an alpha-ketoglutare to be recycled and generates free amonia

• most amino aicd nitrogen can be released as ammonia in this pathway

Question 8

briefly describe the urea cycle

Answer 8

converst toxic NH3 to a safe form (urea) for excretion from the body

takes place in the liver using 5 enzymes

forms one molecule of free ammonia and one molecule of aspartate donate the two nitrogens which are combined with CO2 to form the urea molecule

Question 9

what makes up most of the solid component of urine?

Answer 9

Urea (50% of the solid component)

Question 10

urea transport in the nephron is driven by?

Answer 10

by passive diffusion - becoming progressively concentrated along the length of the tubule

Question 11

which part of the nephron is highly permeable to urea due to the presence of specific transporters?

Answer 11

the collecting duct is highly permeable = urea is reabsorbed into interstitial space

Question 12

amount of urea excreted in urine depends on what?

Answer 12

the presence/absense of ADH - upregulates urea transporters

Question 13

what percent of the osmolarity of the renal medullary interstitium is contributed by urea?

Answer 13

40-50%

Question 14

what are the nitrogenous components of urine?

Answer 14

Urea

creatinine

uric acid

ammonia

Question 15

what is creatine? what is its role?

Answer 15

creatine - main role is to faciliate ATP production in energy dependent tissues (muscle and brain)

Question 16

what is creatinine? What is its role?

Answer 16

Creatinine is formed spontaneoulsy from phosphocreatine - at a constant rate 2% of creatine levels/day

plasma level depends on muscle bulk - removed from the blood by glomerular filtration and tubular secretion

*if renal filtration is deficient, plasma creatinine increases*

measurement of creatinine in urine is used to measure kidney function

Question 18

what is uric acid? What is its role in the body?

Answer 18

it is the end product of purine metabolism - freely filtered in the glomerulus with 90% being reabsorbed in the PCT

hyperuricaemia can occur due to high purine intake (meat or seafood) or due to poor renal function - leads to deposition in the joints (gout) or kidneys (kidney stones)

Question 19

what is ammonia? What is its role in the body?

Answer 19

NH3+

the majority of renal ammonia excretion derives from intrarenal production not from glomerular filtration

produced by glutamate meatbolism in acid base balance - levels are normally low - but can increase due to diet, liver disease, or UTIs

Question 20

what is the normal pH of urine?

Answer 20

5-6

Question 21

how do the different nephron components adjust urine composition?

Answer 21

• proximal convoluted tuuble - active reabsorption of filtered ‘solutes’ this increases the interstitium osmolarity - enhancing absorption of water
• loop of henle- the thin descending limb has low permeability to ions and urea- while being highly permeable to water - concentrates filtrate
• loope of henle- the ascending limb is impermeable to water but is permeable to ions, contribues to the countercurrent multiplier system
• the distal convoluted tubule - renal acidification (H+ ions pumped in) and reabsorption of Na+ and water
• collecting duct - fine tuning of volume and urine concentration - regulated by ADH

Question 22

what is proteinuria?

Answer 22

increased levels of protein in the urine - often ‘foamy’ in appearance

3 main mechanisms that cause proteinuria =

• glomerular proteinuria - due to disease of glomerulus
• overload of prerenal proteinuria - persistent proteinuria without glomerular disease
• tubular proteinuria- due to low reaborption of filtered proteins in the PCT - can be inherited or due to ingestion of drugs or toxins