The amount of chemical to which a person is exposed is extremely important. The chemical acts at a certain site, called the active site, triggering a biological response in a target tissue. Because the biological effect is caused by the presence of the chemical at the active site, the higher the concentration of the chemical at the site, the greater the response. This is the case with all known poisons, a phenomenon called the dose–response relationship.

The dose–response curve is sigmoid, with the linear portion between approximately 16 percent and 84 percent. To compare the potency of chemicals causing similar responses, the dose that produces a biological response in 50 percent of the subject group is chosen, because it can be calculated with the least chance of error. If the biological response is mortality, the dose that kills 50 percent of the exposed population is known as the lethal dose 50, or LD50. Toxicity ratings for chemicals are based on their LD50s. The toxicity rating indicates the amount of chemical required to produce death, but it should be remembered that all chemicals can kill. Thus, all chemicals are toxic. More important than the toxicity of a chemical is its hazard or risk of usage, a concept that incorporates exposure to dosage. For example, botulinum toxin is not especially hazardous, even though it is supertoxic, because food is well-preserved, keeping the exposure or dose very low. In contrast, ethanol (alcohol) is hazardous even though it is not very toxic, because some people have a tendency to use it to excess.

Role of the lymphatics

After a chemical crosses the transport barrier at the portal of entry, it remains in the interstitial spaces, the spaces between cells that are filled with water and loose connective tissue. The absorbed chemical can gain entry into the bloodstream directly via the blood capillaries or indirectly via the lymphatic capillaries.

Lymphatic capillaries are minute vessels located in the interstitial spaces, with one end closed and the other end draining into larger lymphatic vessels. Just like blood capillaries, the walls of the lymphatic capillaries are composed of a thin layer of cells, the endothelial cells. Unlike the blood capillaries, however, the junctions between the endothelial cells of the lymphatic capillaries are much looser, and as a result lymphatic capillaries are much more porous than blood capillaries. Plasma proteins and excess fluid in the interstitial spaces from blood capillaries enter the lymphatic capillaries and eventually flow back to the heart via the lymphatic system. Insoluble aerosols that cross the alveolar wall by pinocytosis may be absorbed into the circulatory system after first entering the porous lymphatic capillaries.

Role of the blood

The chemical is distributed via the blood to the various tissues of the body, where the chemical is transported across blood capillary walls. There are four types of blood capillary walls: tight, continuous, fenestrated, and discontinuous.

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Tight capillary walls are characterized by tight junctions between the endothelial cells, which prevent the diffusion of large molecules and impede that of hydrophilic molecules. The capillaries in the brain are typical of this type of capillary and form part of the blood–brain barrier.

In a continuous capillary wall, channels about five nanometres wide exist between endothelial cells, allowing most small molecules to pass through. Capillaries of this type are found in the skeletal and smooth muscles, connective tissue, lungs, and fat. Chemicals given by intramuscular or subcutaneous injection are readily absorbed into the bloodstream, as are deposited aerosols that dissolve in the fluid lining the respiratory system and cross the alveolar wall.

In a fenestrated capillary wall, holes as large as 100 nanometres are found in the endothelial cells. Capillaries in the intestine and glomeruli in the kidney have fenestrated capillary walls, which account for the high permeability of blood capillaries for absorption by the intestine and for filtration of the blood by the kidney.

The discontinuous capillary wall, the most porous of all capillaries, contains large gaps between the cells through which large molecules and even blood cells pass. This type of capillary is found in the reticuloendothelial system (including the liver, spleen, and bone marrow), which assists in the removal of aged blood cells.

The porous nature of capillaries in most tissues or organs means that a chemical in the bloodstream can be distributed almost freely to most tissues, except for organs with a barrier. The molecules diffuse from the blood to the interstitial spaces of the tissue and finally into the cells by either diffusion or active transport.

Role of tissue blood flow

The rate at which a chemical accumulates in a particular tissue is influenced by the blood flow to that tissue. The well-perfused organs—i.e., organs that receive a rich blood supply relative to organ weight—include major organs like the liver, brain, and kidney. A middle group receives an intermediate blood supply and includes the skeletal muscle and skin. The poorly perfused group includes the fat and bone. As a chemical is distributed to the tissues by the bloodstream, the chemical concentrations in the well-perfused organs rapidly reach a steady state with the blood concentration while the concentrations of the chemical in the poorly perfused tissue lag behind.

Role of protein binding

The plasma contains many proteins, the most abundant being albumin. Some chemicals are known to bind to albumin. Because albumin is too large to cross the blood capillary wall, chemicals that are bound to this plasma protein are confined in the bloodstream and are not readily distributed to the tissues. Chemicals with a high affinity to bind with plasma proteins have lower concentrations in tissues than do chemicals that are not bound to plasma proteins.

Role of distribution barriers

There are barriers in certain organs that limit the distribution of some molecules. The blood–brain barrier consists of tight capillary walls with glial cells wrapped around the capillaries in the brain. Molecules must diffuse through two barriers to get from blood to the nerve cells of the brain. Despite the barrier, water, most lipid-soluble molecules, oxygen, and carbon dioxide can diffuse through it readily. It is slightly permeable to the ions of electrolytes, such as sodium, potassium, and chloride, but is poorly permeable to large molecules, such as proteins and most water-soluble chemicals. The blood–brain barrier is the reason the ions of some highly water-soluble metals, such as mercury and lead, are nontoxic to the brain of an adult. Children, however, are more sensitive to the toxicity of lead because the blood–brain barrier is less well developed in children.

The second distribution barrier is the blood–testis barrier, which limits the passage of large molecules (like proteins and polysaccharides), medium-sized molecules (like galactose), and some water-soluble molecules from blood into the seminiferous tubules of the testis. Water and very small water-soluble molecules, like urea, however, can pass through the barrier. The lumen of the seminiferous tubules is where sperm cells of more advanced stages develop. It is thought that the barrier protects the sperm cells.

The placental barrier between mother and fetus is the “leakiest” barrier and is a very poor block to chemicals. The placenta is composed of several layers of cells acting as a barrier for the diffusion of substances between the maternal and fetal circulatory systems. Lipid-soluble molecules, however, can cross readily, while the transfer of large-molecular-weight molecules is limited.

Excretion

An organism can minimize the potential damage of absorbed toxins by excreting the chemical or by changing the chemical into a different chemical (biotransformation), or by both methods. The body can excrete exogenous chemicals in the urine, bile, sweat, or milk; the lungs can excrete gases such as carbon monoxide.

Urinary excretion, the most common excretory pathway, takes place in the kidney, where the functional units are the glomerulus (a filter) and the renal tubule. The artery entering the glomerulus divides into capillaries, with fenestrated walls encased in the Bowman’s capsule. Twenty percent of the blood is filtered through the holes in the capillary walls; molecules smaller than 60,000 molecular weight end up in the filtrate, while red blood cells, large proteins, and chemicals bound to plasma proteins are not filtered.

Chemical exchange can also take place along the renal tubule. As the filtrate flows down the renal tubule, essential molecules, such as amino acids and glucose, are reabsorbed by active transport in the first portion of the tubule (the proximal tubule). Chemicals in the filtrate are also reabsorbed by active transport if they structurally resemble these essential molecules. Unlike glomerular filtration, tubular resorption of a chemical is not influenced by whether or not it is bound to plasma proteins.

As the fluid flows down the renal tubule, water and some chemicals are reabsorbed from the tubular fluid into the blood by diffusion. The tubular fluid emerges from the kidney and is collected in the urinary bladder. Lipid-soluble chemicals are readily reabsorbed in the renal tubule, and only water-soluble chemicals are excreted in the urine to a significant extent.

The second major excretory route is the bile, which is formed in the liver and flows into the intestinal tract. The liver does not filter chemicals as does the kidney, but the liver does secrete chemicals into bile. Chemicals excreted in the bile are eventually eliminated in the feces.

Biliary excretion of a chemical does not necessarily result in the elimination of the chemical from the body. Bile is dumped into the small intestine; there is a chance that chemicals in the bile may be reabsorbed by the intestine and in turn reenter the liver via the portal vein. This cycling of a chemical, known as the enterohepatic cycle, can continue for a long time, keeping the chemical in the body.

During inhalation exposure, absorption of the gas continues until the partial pressure of the gas in the tissues is equal to that of the inspired gases in the lungs. As soon as the concentration of inspired gases decreases or the exposure terminates, respiratory excretion of the gas occurs. Because the partial pressure of the inspired gas is lower in the lungs than in blood, the blood releases some gas molecules into the alveolar space and these molecules are exhaled. The tissues lose gas molecules to the blood, which carries them to the lungs to be excreted.

The composition of sweat is similar to that of plasma except that sweat does not contain proteins. After secretion, the fluid moves through the sweat duct, where salt and water are reabsorbed. The exact mechanism of sweat secretion is not known. It appears that sweat is a filtrate of plasma that contains electrolytes (such as potassium, sodium, and chloride) and metabolic wastes (like urea and lactic acid). Because sweat resembles a filtrate of plasma, water-soluble chemicals, like some drugs and metal ions, are found in sweat. Sweat is not a major route of excretion of chemicals, however.

Milk is a potential, albeit minor, route of chemical excretion, but more importantly it is a potential means of chemical exposure for breast-fed infants.

Most chemicals enter milk by diffusion. Therefore, only the nonionized, lipid-soluble forms of organic chemicals are found to a significant extent in milk. Chemicals with a molecular weight less than 200 and that are present in plasma not bound to proteins are more likely to be found in milk. Because the lipid content of milk is higher than that of plasma, highly lipid-soluble chemicals can exist in a more concentrated level in milk than in plasma. Therefore, milk can be a significant route of excretion for highly lipid-soluble chemicals in lactating women.