Basic Principles VI Flashcards
renal excretion
most important route of elimination of drug or metabolites
glomerular filtration
only free drug molecules are filtered. amount filtered depends on glomerular blood flow and free drug concentration
amount of drug excreted formula
amount of drug excreted = amt entering tubule - amt reabsorbed
active tubular secretion
active transport, one system for acids and one for bases. exists for endogenous compounds like uric acid or choline. inhibition is usually competitive. saturation can occur at therapeutic doses or with overdoses. changes first order processes to 0 order
first order equation
v = Vmax[D]/km+[D]
0 order equation
v = Vmax[D]/[D]
passive reabsorption
especially common in lipid soluble drugs. after free water absorption, these are concentrated in the loop of henle. reverse concentration gradient occurs
active reabsorption
active transport for endogenous compounds, works for some drugs
amount of drug excreted formula factoring in reabsorption
amt of drug excreted = (glomerular filtration + active tubular secretion) - (passive reabsorption + active reabsorption)
how to enhance renal excretion?
forced diuresis by increasing urine volume, or manipulate the pH of the urine, trapping the ionized drug. increase the pH of the urine!
biliary excretion
enter bile by secretion (active transport system) or passive diffusion. reabsorbed by passive diffusion and the original absorption mechanism. cycling can occur.
other routes of elimination
lung (exhaled air), sweat, saliva, tears, breast milk
elimination of drugs from site of action
decreases the concentration in the plasma
clearance
a combination of biotransformation and excretion. contribution of each process will vary for different drugs. a volume of plasma that is cleared of drug per unit of time (ml/min)
clearance total =
clearance metabolic + clearance renal. in hepatic diseases, metabolic clearance is reduced. in renal disease, renal clearance is reduced.
factors influencing clearance
body surface area, protein binding, cardiac output, renal function, hepatic function, blood flow to systemic organs
alpha part of the elimination curve
distribution
beta part of the elimination curve
elimination
what is Ke determined by?
the rate limiting step in clearance. either metabolic or renal
absorption kinetics
helps us understand how much dose to give. absorption rates are related to the transport processes involved, either zero order or first order. first order are sensitive to drug concentration, while 0 order are saturated
zero order absorption rate formula
Ct = k t
an amount asorbed per unit time. Ct = concentration of a drug. T is time, and k is constant
first order absorption rate formula
Ct = M/Vd (1-e^-kt)
M = amount of drug at site
elimination kinetics
helps us understand how often to give a dose
zero order elimination formula
Ct = Co -KeT
Ke is elimination constant, Co is initial conentration, Ct is concentration at time T
first order elimination formula
log Ct = log Co - KeT/2.303
when can something switch from a first order to a zero order?
when concentration is higher than the constant, it is a 0 order
what has half lives? which order reaction
first order only have half lives
formula for half lives
t1/2 = 0.693/Ke
clearance formula involving distribution and elimination
Cl = Vd x Ke
t1/2 = 0.693Vd / Cl
