Section 10: Absorption
- Page ID
- 316743
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Learning Objectives
After completing this lesson, you will be able to:
- Explain absorption and its role in toxicokinetics.
- Describe the primary routes of exposure.
- Explain the role of cell membranes in absorption.
- Identify ways in which xenobiotics pass across cell membranes.
In this section...
Topics include:
Section 10: Key Points
What We've Covered
This section made the following main points:
- Absorption is the process by which toxicants gain entrance into the body.
- Ingested and inhaled materials are considered outside the body until they cross the cellular barriers of the gastrointestinal tract or respiratory system.
- The likelihood of absorption depends on the:
- Route of exposure.
- Concentration of the substance at the site of contact.
- Chemical and physical properties of the substance.
- Exposure routes include:
- Primary routes:
- Gastrointestinal (GI) tract
- Mouth and esophagus — poorly absorbed under normal conditions due to short exposure time (nicotine and nitroglycerin are notable exceptions).
- Stomach — significant site for absorption of weak organic acids, but weak bases are poorly absorbed.
- Intestine — greatest absorption of both weak bases and weak acids, particularly in the small intestine.
- Colon and rectum — very little absorption, unless administered via suppository.
- Respiratory tract
- Mucociliary escalator — movements of the cilia push mucus and anything contained within up and out into the throat to be swallowed or removed through the mouth.
- Pulmonary region — most important site for absorption with about 50 times the surface area of the skin and very thin membranes.
- Skin
- Epidermis and stratum corneum — the only layer important in regulating the penetration of a skin contaminant.
- Toxicants move across the stratum corneum by passive diffusion.
- If a toxicant penetrates through the stratum corneum, it enters lower layers of the epidermis, dermis, and subcutaneous tissue, which are far less resistant to further diffusion.
- Gastrointestinal (GI) tract
- Other exposure routes:
- Injections
- Implants
- Conjunctival instillations (eye drops)
- Suppositories
- Primary routes:
- Cell membranes surround all body cells and are made up of a phospholipid bilayer in which each molecule contains a:
- Polar (hydrophilic, or attracted to water) phosphate head
- Lipophilic (attracted to lipid-soluble substances) lipid tail
- Xenobiotics must pass across cell membranes to enter, move within, and leave the body. This movement can be either:
- Passive transfer (most common) — simple diffusion or osmotic filtration with no cellular energy or assistance required.
- Facilitated transport — similar to passive transport, but a carrier-mediated transport mechanism and thus faster and capable of moving larger molecules.
- Active transport — movement against the concentration gradient (from lower to higher concentrations), requiring cellular energy from ATP.
- Endocytosis — the cell surrounds the substance with a section of its cell wall, separating from the membrane and moving into the interior of the cell.