PHARMACOLOGY

biological membranes which limit the distribution of drugs.

Biological membranes play a crucial role in limiting the distribution of drugs in the body. These membranes act as barriers that drugs must pass through to reach their target sites of action. Here are the main biological membranes that limit drug distribution, along with an explanation of their functions explain widely:

1. **Cell Membrane:** The cell membrane, also known as the plasma membrane, surrounds the cell and separates its internal environment from the external environment. It is composed of a phospholipid bilayer embedded with proteins. The cell membrane acts as a barrier to the passage of most drugs, particularly large or hydrophilic molecules, which may require specific transport mechanisms to enter or exit cells.

2. **Blood-Brain Barrier (BBB):** The blood-brain barrier is a specialized barrier that separates the blood from the brain and central nervous system (CNS) tissues. It is composed of endothelial cells that are tightly joined together, along with astrocytes and pericytes. The BBB restricts the passage of many drugs and substances into the brain, limiting the effectiveness of systemic medications for CNS disorders.

3. **Blood-Placental Barrier:** During pregnancy, the placenta acts as a barrier between the maternal and fetal circulations. The placental barrier protects the fetus from harmful substances but also limits the distribution of drugs to the fetus. Some drugs can cross the placenta and affect the developing fetus, highlighting the importance of careful consideration when prescribing medications to pregnant women.

4. **Blood-Cerebrospinal Fluid Barrier:** This barrier separates the blood from the cerebrospinal fluid (CSF) in the brain's ventricles. It is less restrictive than the BBB but still limits the passage of certain drugs and substances into the CSF.

5. **Mucosal Membranes:** Mucosal membranes line various organs and cavities in the body, such as the gastrointestinal tract, respiratory tract, and genitourinary tract. These membranes act as barriers and can limit the absorption of drugs administered through these routes. For example, the gastrointestinal mucosa can limit the absorption of certain drugs due to factors such as gastric acidity, intestinal motility, and the presence of efflux transporters.

6. **Skin:** The skin is the body's largest organ and serves as a barrier to the external environment. The outermost layer of the skin, known as the stratum corneum, is composed of dead skin cells and lipids that restrict the passage of many drugs. Transdermal drug delivery systems are designed to overcome this barrier and deliver drugs through the skin.

These biological membranes play critical roles in limiting the distribution of drugs in the body and protecting the body from harmful substances. Understanding these barriers is essential for developing effective drug delivery strategies and optimizing therapeutic outcomes.