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Intradermal injections are made directly into the skin, just under the stratum corneum. Tissue reactions are minimal and absorption is usually slow. If the injection is beneath the skin, the route is referred to as a subcutaneous injection. Since the subcutaneous tissue is quite vascular, absorption into the systemic circulation is generally rapid. Tissue sensitivity is also high and thus irritating substances may induce pain and an inflammatory reaction.
Many pharmaceuticals, especially antibiotics and vaccines are administered directly into muscle tissue (the intramuscular route). It is an easy procedure and the muscle tissue is less likely to become inflamed compared to subcutaneous tissue. Absorption from muscle is about the same as from subcutaneous tissue.
Substances may be made directly into large blood vessels when they are irritating or when an immediate action is desired, such as anesthesia. These are known as intravenous or intraarterial routes depending on whether the vessel is a vein or artery.
Parenteral injections may also be made directly into body cavities, rarely in humans but frequently in laboratory animal studies. Injection into the abdominal cavity is known as intraperitoneal injection. If it is injected directly into the chest cavity, it is referred to as an intrapleural injection. Since the pleura and peritoneum has minimal blood vessels, irritation is usually minimal and absorption is relatively slow.
Implantation is another route of exposure of increasing concern. A large number of pharmaceuticals and medical devices are now implanted in various areas of the body. Implants may be used to allow slow, time-release of a substance (e.g., hormones). In other cases, no absorption is desired, such as for implanted medical devices and materials (e.g., artificial lens, tendons and joints, and cosmetic reconstruction).
Some materials enter the body via skin penetration as the result of accidents or violence (weapons, etc.). The absorption in these cases is highly dependent on the nature of the substance. Metallic objects (such as bullets) may be poorly absorbed whereas more soluble materials that are thrust through the skin and into the body from accidents may be absorbed rapidly into the circulation.
Novel methods of introducing substances into specific areas of the body are often used in medicine. For example, conjunctival instillations (eye drops) are used for treatment of ocular conditions where high concentrations are needed on the outer surface of the eye, not possible by other routes.
Therapy for certain conditions require that a substance be deposited in body openings where high concentrations and slow release may be needed while keeping systemic absorption to a minimum. For these substances, the pharmaceutical agent is suspended in a poorly absorbed material such as beeswax with the material known as a suppository. The usual locations for use of suppositories are the rectum and vagina.
Distribution is the process whereby an absorbed chemical moves away from the site of absorption to other areas of the body. In this section we will answer the following questions:
When a chemical is absorbed it passes through cell linings of the absorbing organ (skin, lung, or gastrointestinal tract) into the interstitial fluid (fluid surrounding cells) of that organ. Interstitial fluid represents about 15% of the total body weight. The other body fluids are the intracellular fluid (fluid inside cells), about 40% of the total body weight and blood plasma which accounts for about 8% of the body weight. However, the body fluids are not isolated but represent one large pool. The interstitial and intracellular fluids, in contrast to fast moving blood, remain in place with certain components (e.g., water and electrolytes) moving slowly into and out of cells. A chemical, while immersed in the interstitial fluid, is not mechanically transported as it is in blood.
A toxicant can leave the interstitial fluid by:
If the toxicant gains entrance into the blood plasma, it travels along with the blood, either in a bound or unbound form. Blood moves rapidly through the body via the cardiovascular circulatory system. In contrast, lymph moves slowly through the lymphatic system. The major distribution of an absorbed chemical is by blood with only minor distribution by lymph. Since virtually all tissues have a blood supply, all organs and tissues of the body are potentially exposed to the absorbed chemical.
Distribution of a chemical to body cells and tissues requires that the toxicant penetrate a series of cell membranes. It must first penetrate the cells of the capillaries (small blood vessels) and later the cells of the target organs. The factors previously described pertaining to passage across membranes apply to these other cell membranes as well. For example, concentration gradient, molecular weight, lipid solubility, and polarity are important, with the smaller, nonpolar toxicants, in high concentrations, most likely to gain entrance.
The distribution of a xenobiotic is greatly affected by whether it binds to plasma protein. Some toxicants may bind to these plasma proteins (especially albumin), which "removes" the toxicant from potential cell interaction. Within the circulating blood, the non-bound (free) portion is in equilibrium with the bound portion. However, only the free substance is available to pass through the capillary membranes. Thus, those substances that are extensively bound are limited in terms of equilibrium and distribution throughout the body. Protein-binding in the plasma greatly affects distribution, prolongs the half-life within the body, and affects the dose threshold for toxicity.
The plasma level of a xenobiotic is important since it generally reflects the concentration of the toxicant at the site of action. The passive diffusion of the toxicant into or out of these body fluids will be determined mainly by the toxicant's concentration gradient. The total volume of body fluids in which a toxicant is distributed is known as the apparent volume of distribution (VD ). The VD is expressed in liters.
If a toxicant is distributed only in the plasma fluid, a high VD results; however, if a toxicant is distributed in all sites (blood plasma, interstitial and intracellular fluids) there is greater dilution and a lower VD will result. Binding in effect reduces the concentration of "free" toxicants in the plasma or VD. The VD can be further affected by toxicants that undergo rapid storage, biotransformation, or elimination. Toxicologists determine the VD of a toxicant in order to know how extensively a toxicant is distributed in the body fluids. The volume of distribution can be calculated by the formula:
The volume of distribution may provide useful estimates as to how extensive the toxicant is distributed in the body. For example, a very high apparent VD may indicate that the toxicant has distributed to a particular tissue or storage area such as adipose tissue. In addition, the body burden for a toxicant can be estimated from knowledge of the VD by using the formula:
Once a chemical is in the blood stream it may be:
Most chemicals undergo some biotransformation. The degree with which various chemicals are biotransformed and the degree with which the parent chemical and its' metabolites are stored or excreted varies with the nature of the exposure (dose level, frequency and route of exposure).
Influence of Route of Exposure
The route of exposure is an important factor that can affect the concentration of the toxicant (or its' metabolites) at any specific location within the blood or lymph. This can be important since the degree of biotransformation, storage, and elimination (and thus toxicity) can be influenced by the time course and path taken by the chemical as it moves through the body. For example, if a chemical goes to the liver before going to other parts of the body, much of it may be biotransformed quickly. In this case, the blood levels of the toxicant "downstream" may be diminished or eliminated. This can dramatically affect its potential toxicity.
This is exactly what often happens with toxicants that are absorbed through the gastrointestinal tract. The absorbed toxicants that enter the vascular system of the gastrointestinal tract are carried by the blood directly to the liver by the portal system. This is also true for those rare drugs that are administered by intraperitoneal injection. Blood from most of the peritoneum also enters the portal system and goes immediately to the liver. Blood from the liver then flows to the heart and then on to the lung, before going to other organs. Thus, toxicants entering from the GI tract or peritoneum are immediately subject to biotransformation or excretion by the liver and elimination by the lung. This is often referred to as the "first pass effect." For example, first-pass biotransformation of the drug propranolol (cardiac depressant) is about 70% when given orally. Thus, the blood level is only about 30% of that of a comparable dose administered intravenously.
Toxicants that are absorbed through the lung or skin enter the blood and go directly to the heart and systemic circulation. The toxicant is thus distributed to various organs of the body before it goes to the liver, and not subject to this first-pass effect. The same is true for intravenously or intramuscularly injected drugs.
A toxicant that enters the lymph of the intestinal tract will not go to the liver first but will slowly enter the systemic circulation. The proportion of a toxicant moving by lymph is much smaller than that transported by the blood.
The blood level of a toxicant not only depends on the site of absorption but also the rate of biotransformation and excretion. Some chemicals are rapidly biotransformed and excreted whereas others are slowly biotransformed and excreted.
Disposition is the term often used to integrate all the processes of distribution, biotransformation, and elimination. Disposition models have been derived to describe how a toxicant moves within the body with time (also known as kinetic models). The disposition models are named for the number of areas of the body (known as compartments) that the chemical may go to. For example, blood is a compartment. Fat (adipose) tissue, bone, liver, kidneys, and brain are other major compartments.
Kinetic models may be a one-compartment open model, a two-compartment open model, or a multiple compartment model. The one-compartment open model describes the disposition of a substance that is introduced and distributed instantaneously and evenly in the body, and eliminated at a rate and amount that is proportional to the amount left in the body. This is known as a "first-order" rate, and represented as the logarithm of concentration in blood as a linear function of time.
The half-life of the chemical that follows a one-compartment model is simply the time required for half the chemical to be lost from the plasma. Only a few chemicals actually follow the simple, first-order, one compartment model.
For most chemicals, it is necessary to describe the kinetics in terms of at least a two-compartment model. In the two-compartment open model, the chemical enters and distributes in the first compartment, which is normally blood. It is then distributed to another compartment from which it can be eliminated or it may return to the first compartment. Concentration in the first compartment declines smoothly with time. Concentration in the second compartment rises, peaks, and subsequently declines as the chemical is eliminated from the body.
A half-life for a chemical whose kinetic behavior fits a two-compartment model is often referred to as the "biological half-life." This is the most commonly used measure of the kinetic behavior of a xenobiotic.
Frequently the kinetics of a chemical within the body can not be adequately described by either of these models since there may be several peripheral body compartments that the chemical may go to, including long-term storage. In addition, biotransformation and elimination of a chemical may not be simple processes but subject to different rates as the blood levels change.
Structural Barriers to Distribution
Organs or tissues differ in the amount of a chemical that they receive or to which they are exposed. This is primarily due to two factors, the volume of blood flowing through a specific tissue and the presence of special "barriers" to slow down toxicant entrance. Organs that receive larger blood volumes can potentially accumulate more of a given toxicant. Body regions that receive a large percentage of the total cardiac output include the liver (28%), kidneys (23%), heart muscle, and brain. Bone and adipose tissues have relatively low blood flow, even though they serve as primary storage sites for many toxicants. This is especially true for those that are fat soluble and those that readily associate (or complex) with minerals commonly found in bone.
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