Toxic chemicals are excreted from a biological system through various routes. Kidney is a very important organ for excretion. More toxic agents are excreted by this route than through any other routes. In addition to kidney, liver, lungs, gastro-intestinal track, sweat and saliva etc. constitute different routes through which foreign chemicals are eliminated from a biological system.
1. Excretion through Kidney:
Excretion of toxicants through kidney involves passive glomerular filtration, passive tubular diffusion and active tubular secretion. The mechanism is essentially the same which is employed to remove the end products of the normal metabolism inside a living being. Kidney receives about 25% of the total cardiac output and about 20% of the blood supply received is filtered by glomerular which have large pores, around 40 A in diameters.
Smaller molecules can easily pass through these pores while macromolecules with molecular weight greater than 70,000 cannot be filtered through these pores. As many toxic agents are smaller they are excreted through these pores. The degree up to which the toxic agents are bound to protein molecules affects their filtration through glomerular pores considerably.
After filtration at glomeruli both useful as well as useless materials are present in the lumen and it is the process of diffusion and active absorption by the blood capillaries around the tubular region by which useful substances are taken up. As most of toxic agents are rendered strongly hydrophilic within a biological system, they are not taken up in the blood supply again. However, lipid soluble substances may enter again into the blood stream by passive diffusion. Active secretion mechanism may also be employed to excrete organic cations and anions. In contrast to passive filtration protein bound toxicants are also available for the process of active secretion.
By manipulations in the process of excretion, the rate of elimination of various substances may be reduced or enhanced. Generally basic substances are excreted through urine at a much faster rate if the urine is acidic whereas the excretion of acidic compounds is quicker in alkaline urine. By bringing about changes in the pH therefore, the rate of elimination of some substances through urine can be stepped up.
Similarly, in cases, where a substance is excreted through an active mechanism, the period of retention of a drug within a biological system can be enhanced by introducing another compound which competes for attachment sites on carrier-molecule with the chemical whose retention is desired. These practices constitute an important means in the practice of therapeutic medicine and treatment of cases of poisoning.
2. Excretion through Biliary Secretions:
Toxicants which a biological system absorbs from the gastro intestinal track are carried to the liver first by means of the hepatic portal system, where they are metabolised. The products of their metabolism are directly excreted through biliary excretions into the gastro-intestinal track.
The excretion of toxic agents from the liver into the bile involves passive diffusion and active transport mechanism, the latter being more important mechanism, for the elimination of various toxic chemicals. If the toxicants or their metabolites are absorbed again from the intestines, they may be secreted again and in this way an entero-hepatic cycle may come into operation. The excretion of foreign chemicals through biliary secretions constitutes an important means by which toxicants are either removed or rendered harmless before they are able to reach the blood stream.
3. Excretion through Gastro-intestinal Track:
Some toxic agents may be excreted in the gastro-intestinal track along with the fluid secreted in it. In an average healthy man about 3 litres of fluid is secreted in the intestines per day. It appears that no specialized system is available, passive diffusion being the only process involved in such secretions.
4. Excretion through Lungs:
Toxic agents which enter a biological system through respiratory track may also be excreted through it. Many gases as carbon monoxide as well as alcohols and other volatile substances take this route of excretion.
5. Excretion through Milk:
Though a secretion of toxic agent along with milk is not a major route of excretion yet it is important due to fact that toxic agents may be passed on to the nursing child straight from the affectionate and caring mother. Toxic chemicals are excreted into the milk by the process of simple diffusion. As milk is usually more acidic than blood plasma basic compounds may get more concentrated in it than acidic ones. From catties the toxic chemical may come straight to human beings through milk and milk products which are considered rather more nutritive.
6. Excretion Along with Sweat and Saliva:
Excretion through sweat and saliva constitute rather an unimportant route of elimination of toxic agents quantitatively. Through skin the secretion is dependent on the process of simple diffusion involving non-ionized lipid solube part of the toxicant. The chemicals excreted through saliva in the mouth are usually swallowed and thus are made available to absorption in the gastro-intestinal track.