CPT1: Excretion Flashcards
What is excretion?
Removel from the body without chemical modification
following IV ubhections no metabolism takes place. ELimminatino is 100% via renal excretion
What are the processes involved in elimination by the kidney?
- glomerular filtrationL size constraints, protein binding, glomerular stucture
- Tubular reabsorption and secretion: acidification/ alkalation, active ransport (competition/ saturation), organic acid/ base, protein binding
Describe the structure of the nephron
- Blood enters via the afferent arteioles which feeds the glomerulus (compacted capillary bed)
- Small molecules are absorbed into the bowmans capsule, moving thruogh the fenestrations in the endotherlial wall of the glomerulus
- Filtered fluid passes through the proximal tubule into the descending then ascending loop of henele, the distil convolutred tubule and into the collecting ducts
- The tubule walls absorb important AA, glucose and salts. These are absorbed back into the capillary vessels (lipid sol drugs able to cross membrne and are reabsorbed)
- unecessary water and toxics expelled as urine
During glomerular filtration:
- what molecules are filtered.
- what volume of plasma is filtered
- how does glomerular filtration work?
- Small molceules are. Large or protein bound molecules aren’t filtered
- 20% of plasma volume is filtered
- It is a passive process which is pressure driven
- What is active secretion?
- Describe active secretion
- Movement of substances from the peritubular capillaries to the renal tubing
- It is saturable
- energy requiring
- can generate positive gradients
- 2 seperate mechanisms for acids and bases
- possible interactions
During active secretion what are some examples of drugs which may compete with each other. What are the consequences for this?
Probenecid and penecillins share the same mechanism for secfretion therefore they compete and penicilin clearance is reduced
Describe reneal excretion
- 99% of water reabsorbed
- lipid solubile drugs also reabsorbed
- only very water soluble drugs can be excreted effectively by the kidneys
Describe ion trapping in relation to a barbituate over dose
Barbituates are acids. Therefore by giving sodium bicarbonate you can make the urine more alkaline. This means the barbituate will be more ionised so unable to be reabsorbed back into the blood therefore more is excreted
What does high renal clearance mean
If renal clearance is greater than GFR then there must be some active secrtion occuring
What does low renal clearance mean?
If renal clearance is lower than GFR then it must mean there is:
- Extensive reabsorption occuring or
- not filtered
Decribe creatinine clearance
- Creatinine is a waste product from muscle break down
- It is filtered by the kidney
- almost no active secretion
- almost no reabsorption
- therefore almost equal to GFR
Can be used to estimate GFR by measuring its clearance
What are factors which influence creatinine serum conc
- Creatinine production rate
- Depends on body muscle mass whihc in turn depends on: body weight, age (%muscle declines with age), gender (men have more than women)
- Creatine clearance rate
What calculation is used to work out creatinine clearance rate?
- Men: CrCl = ((140- age) x wt)/ 72 - SrCr
- Women: CrCl = ((140- age) x wt x 0.85)/ 72 - SrCr
SrCr = serum creatine conc (mg/ dl)
What are routes of eliminatin from the body?
- Renal
- Salivary - drug monitoring
- mammary - delver to baby
- pulmoanry
- Bilary
Describe billary elimination
- Similar to renal. Lipid soluble drugs initally absorbed then reabsorbed along with bulk of water
- Active secretion mechanisms for acids and bases but only work if M.Wt high enough
- Molceular weight of most compounds generally to low for billary excretion. Conjugation with glucuronide often increases m.wt sufficent enough
