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Biological Barriers

Barrierae biologicae

For medical students2 min readUpdated 2026-10-10

Biological barriers are specialized cellular and tissue structures that form a border between the extracellular (spatium extracellulare) and intracellular (spatium intracellulare) spaces, as well as between the blood and tissue fluids of organs. Their primary function is the selective transport of substances, maintenance of homeostasis, and protection of the internal environment from circulating xenobiotics.

Barrier FoundationMembranes consist of a phospholipid bilayer that is impermeable to polar substances.
Brain ProtectionEndothelial tight junctions form the blood-brain barrier (BBB), which is impermeable to hydrophilic toxins.
Transporter ProteinP-glycoprotein mediates efflux—the active elimination of xenobiotics back out of the cell.

Cell Membranes and Transport Mechanisms

The structural basis of any cellular barrier is the phospholipid bilayer. Its organization is strictly ordered: the hydrophilic heads of the molecules face outward (toward aqueous environments), while the hydrophobic tails are tucked inside the membrane. This hydrophobic core creates a reliable barrier against polar compounds.

Drugs and other substances cross membranes via two main pathways:

Epithelial and Endothelial Barriers

Tissue barriers play a critical role in the distribution of substances throughout the body.

The epithelial barrier serves as a significant obstacle to hydrophilic compounds. For example, it accounts for the poor absorption of such substances from the lung surface and the absence of their reabsorption in the renal tubules. Lipophilic compounds generally cross the epithelium freely, but the body employs a protective efflux mechanism: specialized membrane transporter proteins (such as P-glycoprotein) can capture molecules that have entered the cell and actively pump them back out (e.g., from an enterocyte back into the intestinal lumen).

The endothelial barrier (capillary wall) varies depending on the tissue type:

  1. Continuous capillaries (skeletal muscle, subcutaneous tissue, internal organs). Large intercellular gaps (fenestrae) about 2 nm or more in size exist between endothelial cells (cellulae endotheliales). Hydrophilic substances easily penetrate tissues via filtration through these clefts, ensuring rapid exchange between blood and tissue fluid.
  2. Capillaries of the brain (CNS). Fenestrae are completely absent, and cells are tightly apposed. Transport of hydrophilic substances is prevented.

Histohematic Barriers

Histohematic barriers are specialized separating structures between the blood and the tissue fluid of specific organs. Their main function is to protect the vulnerable internal environment from toxins and xenobiotics circulating in the bloodstream.

Major types include:

Mnemonic

To easily remember capillary permeability, imagine a sieve: in muscles, it is a coarse sieve (with 2 nm pores) letting "water" (hydrophilic substances via filtration) pass through. In the brain (BBB), it is a monolithic wall of tight junctions through which only "fat" (lipophilic substances via diffusion) can quietly "seep."

Frequently asked questions

What transport mechanisms of drug molecules across cell membranes are distinguished in pharmacology?

Pharmacology distinguishes several primary mechanisms for drug transport across cell membranes. These include:

  • Passive transport — movement of substances along a concentration gradient without energy expenditure (includes passive diffusion, diffusio passiva).
  • Active transport — movement against a concentration gradient or via carriers, requiring energy expenditure (ATP).
  • Facilitated diffusion — transport involving specific systems for substances with chemical similarity to endogenous substrates.
  • Pinocytosis and receptor-mediated endocytosis — uptake of fluid or specific macromolecules by the cell.
What types of histohematic barriers exist in the human body?

Specialized histohematic barriers exist in the human body to protect the internal environment of organs from xenobiotics. The following types of barriers are distinguished:

  • Blood-Brain Barrier (barriera haematoencephalica) — protects the brain and maintains CNS homeostasis.
  • Blood-Ophthalmic Barrier — protects ocular fluids and is impermeable to hydrophilic polar substances.
  • Blood-Testis Barrier — located between the blood and the contents of the seminiferous tubules.
  • Blood-Follicular Barrier (blood-ovarian) — protects the ovaries.
  • Blood-Placental Barrier (placental) — separates maternal and fetal blood flow, functioning as a selective transport system.
What cellular and tissue structures form the blood-brain barrier?

The blood-brain barrier is formed by three levels of cellular and tissue structures. It consists of:

  • Brain capillary endothelial cells — form a dual-membrane layer without intercellular clefts, connected by tight junctions that block paracellular transport.
  • Basement membrane — contains fibrillar components and specialized cells.
  • Pericytes (pericytae) — located within the basement membrane.
  • Astrocytes (astrocyti) — glial cells whose processes form the glial limiting membrane (glial foot processes) covering 85–90% of the capillary surface.
Which transporter proteins mediate the active efflux of xenobiotics from cells?

Specialized membrane transporter proteins mediate the active efflux (pumping out) of xenobiotics and drugs from cells. Major efflux systems include:

  • P-glycoprotein (P-glycoproteinum / MDR1) — a key protein functioning as a membrane "pump" that actively eliminates substances from the cell.
  • ABC transporters (ATP-binding cassette) — a family of ATP-dependent cassette proteins that actively eliminate substances using energy.
  • MRP1 and BCRP — alternative efflux systems also associated with cellular drug resistance.
What physicochemical properties of drugs determine their ability to cross the placental barrier?

The ability of drugs to cross the placental barrier (trophoblast membrane, trophoblastus) is determined by their lipophilicity, size, and degree of ionization. Key physicochemical properties include:

  • Lipophilicity — lipophilic compounds readily cross via passive diffusion, whereas hydrophilic ones require carrier proteins.
  • Molecular weight (MW) — substances with MW ≤ 500 Daltons diffuse easily, those from 500 to 1000 Da cross at a low rate, and those > 1000 Da practically do not cross the barrier.
  • Degree of ionization — depends on the pH gradient; weak bases ionize in the more acidic fetal plasma, lose lipophilicity, and accumulate (the "ion trapping" phenomenon).
What methods exist to artificially bypass the blood-brain barrier for targeted drug delivery?

Pharmacological and technological strategies are used to artificially bypass the blood-brain barrier for targeted drug delivery to the central nervous system. Main methods include:

  • Use of prodrugs (Prodrugs) — administration of an inactive compound that binds to specific membrane carriers or undergoes structural modification (increased lipophilicity) to cross the membrane, and is subsequently activated within brain tissue.
  • Inhibition of efflux transporters — combining the target drug with a P-glycoprotein inhibitor (e.g., verapamil), which prevents the retrograde efflux of the drug from the brain into the blood.
  • Nanotechnology — use of biodegradable nanoparticles (10–1000 nm in size) in which the drug is encapsulated, covalently bound, or surface-adsorbed.
Why do hydrophilic polar substances not penetrate the brain?

In brain capillaries, endothelial cells are tightly apposed with no intercellular clefts. Because hydrophilic molecules cannot dissolve in the lipid membrane and no filtration pathways exist, they cannot cross the blood-brain barrier.

How do lipophilic substances cross cellular barriers?

Lipophilic compounds move across the cellular phospholipid bilayer via passive diffusion (diffusio passiva). This mechanism operates in all capillary types and cell membranes because it requires neither energy expenditure nor pores.

How does the blood-testis barrier protect tissues from lipophilic toxins?

Because lipophilic substances can freely enter tissues via passive diffusion, protection is provided by the efflux mechanism. ATP-dependent proteins (such as P-glycoprotein) recognize foreign molecules (like cyclosporine) and actively pump them back into the blood.

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