Systemic Total Clearance and Its Components
The elimination of any drug from the body consists of two main processes: biotransformation (metabolism) and excretion. Accordingly, total systemic clearance ($Cl_t$) is the sum of two components:
- Metabolic clearance ($Cl_{met}$)
- Excretory clearance ($Cl_{excr}$)
The additivity formula is as follows: $Cl_{met} + Cl_{excr} = Cl_t$. This parameter is directly related to the volume of distribution ($V_d$) and the elimination rate constant ($k_{el}$): $Cl_t = V_d \times k_{el}$.
Relationship Between Clearance and Drug Concentration
From a mathematical perspective, clearance is the proportionality coefficient between the rate of drug elimination and its current concentration in a biological fluid. The calculation formula is:
$$Cl_t = \frac{\text{Elimination Rate}}{C}$$
An important rule follows from this relationship: the higher the drug concentration in the blood, the higher the absolute rate of its elimination (assuming clearance remains constant).
Calculation of Renal Clearance
Renal clearance ($Cl_{ren}$) indicates the volume of plasma that the kidneys clear of a drug per unit of time. Its physiological basis is the combined action of glomerular filtration, tubular secretion, and tubular reabsorption.
To calculate renal clearance, a constant drug concentration in blood plasma must be maintained. The formula is:
$$Cl_{ren} = \frac{C_u \cdot V_u}{C_p}$$
Where:
- $C_u$ is the drug concentration in urine (urina);
- $V_u$ is the urine flow rate (volume of urine produced per unit of time);
- $C_p$ is the drug concentration in blood plasma (plasma).
Hepatic Clearance and the Impact of Pathologies
Hepatic clearance ($Cl_{hep}$) reflects the liver's ability to metabolize xenobiotics and excrete unchanged substances into the bile. The magnitude of this parameter depends directly on hepatic blood flow.
In clinical practice, this is of immense importance. For example, in congestive heart failure, hepatic blood flow drops, thereby reducing the clearance of drugs that undergo extensive metabolism in this organ (such as lidocaine). Therefore, when prescribing therapy, the functional state of elimination organs in renal or hepatic failure must always be considered.