Pharmacology: Drug Absorption and Membrane Physiology

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Questions and Answers

What is the primary route of drug administration emphasized in this chapter?

Transdermal route

Intranasal route

Intravenous route

Oral route (correct)

Which of the following factors does NOT affect drug absorption?

Route of administration

Physicochemical properties of the drug

Physiological factors

Patient's emotional state (correct)

What must drug molecules cross after oral administration to reach systemic circulation?

Intestinal epithelium (correct)

Blood-brain barrier

Cellular membranes in the liver

Skin layers

What is drug absorption primarily dependent on?

Physicochemical properties of the drug Signup and view all the answers

Which describes the focus of drug absorption in this chapter?

The transfer of drug from its administration site to the blood Signup and view all the answers

Which of the following best describes a significant aspect of membrane physiology related to drug absorption?

Drugs may cross cellular membranes by passive diffusion or transport. Signup and view all the answers

What is the significance of drug passages across the cell membrane in drug absorption?

It influences the rate and extent of drug absorption into the systemic circulation. Signup and view all the answers

Which combination of factors is critical for systemic drug absorption as discussed?

Physicochemical properties, drug product nature, and absorption site anatomy Signup and view all the answers

Which of the following veins is involved in the systemic absorption of paracetamol?

Inferior hemorrhoidal vein Signup and view all the answers

What is the typical pH level of the rectum?

8 Signup and view all the answers

Which route do most drugs take to enter the bloodstream?

Direct route through villi Signup and view all the answers

What happens to lipid-soluble substances in terms of absorption?

They are usually absorbed through the lacteals Signup and view all the answers

What is the function of the hepatic portal vein in drug absorption?

Transports nutrients and drugs to the liver Signup and view all the answers

How do the hemorrhoidal veins differ in their systemic and local absorption functions?

Different veins are responsible for local and systemic absorption Signup and view all the answers

Which part of the lymphatic system is primarily involved in the indirect route of drug absorption?

Lacteals Signup and view all the answers

What characterizes the absorption of drugs through the rectum?

Erratic absorption Signup and view all the answers

What role does the diffusion coefficient (D) play in passive diffusion?

It is related to the size and lipid solubility of the drug. Signup and view all the answers

How does membrane thickness affect the rate of diffusion?

Thinner membranes increase the rate of diffusion. Signup and view all the answers

What is the primary characteristic of passive diffusion?

It involves the spontaneous movement of molecules. Signup and view all the answers

Which factor does NOT affect the rate of passive diffusion?

Shape of the molecule Signup and view all the answers

What is the primary mechanism behind carrier-mediated transport?

Specialized transport proteins facilitate the movement. Signup and view all the answers

What factor enhances passive diffusion in the intestines?

Higher surface area due to villi and microvilli. Signup and view all the answers

Which parameter is NOT part of Fick's first law of diffusion?

Volume of the absorbing medium Signup and view all the answers

In terms of transport, what does the term 'selective actions' refer to?

The specific pathways that facilitate or inhibit molecule movement. Signup and view all the answers

What is the effect of high food viscosity on transit time?

Decreases transit time Signup and view all the answers

Which drug class is likely to increase transit time?

Domperidone Signup and view all the answers

In a stressed state, how does the transit time change?

Decreases to 14 hours Signup and view all the answers

What characterizes the digestive (fed) state in terms of motility patterns?

Irregular contractions for mixing contents Signup and view all the answers

How long is the Small Intestine Transit Time (SITT) in the fasted state?

4-8 hours Signup and view all the answers

What is the role of the Migrating Motor Complex (MMC) during the fasted state?

To empty the upper GIT to the cecum Signup and view all the answers

How does depression affect transit time?

Increases transit time to 49 hours Signup and view all the answers

Which statement about irregular contractions during the digestive state is true?

They aid in mixing contents and advancing streams Signup and view all the answers

What percentage of cardiac output is received by the splanchnic circulation?

28% Signup and view all the answers

What effect does food intake have on blood flow in the splanchnic area?

It increases blood flow. Signup and view all the answers

Which of the following is a consequence of decreased mesenteric blood flow during congestive heart failure?

Decreased rate of drug removal from the intestinal tract. Signup and view all the answers

Which pathway do lipophilic drugs use for absorption into systemic circulation?

Lacteal or lymphatic vessels. Signup and view all the answers

What is the primary benefit of drug absorption through the lymphatic system?

Bypasses the liver and avoids the first-pass effect. Signup and view all the answers

Which drug is NOT mentioned as being absorbed through the lymphatic system?

Aspirin Signup and view all the answers

What happens to the absorption of dietary lipids in the presence of lymphatics?

It may be partially facilitated. Signup and view all the answers

Which condition would most likely lead to increased drug bioavailability from the intestinal tract?

Increased mesenteric blood flow. Signup and view all the answers

Study Notes

Drug Absorption

Drug absorption is the process by which a drug enters the bloodstream.
The most common route of drug administration is oral, meaning the drug must pass through the gastrointestinal tract.
A drug's absorption depends on the physicochemical properties of the drug, the formulation of the drug product, and the anatomy and physiology of the absorption site.

Membrane Physiology

Membrane proteins play critical roles in cell function, acting as structural anchors, receptors, ion channels, or transporters.

Transport Processes Across Membranes

Passive diffusion is the primary mechanism for drug absorption.
- It involves the movement of a drug across a membrane from a region of higher concentration to lower concentration.
- It doesn't require energy.
- The rate of passive diffusion is influenced by:
  - Concentration gradient
  - Lipid solubility
  - Surface area of the membrane
  - Diffusion coefficient
  - Membrane thickness
Carrier-mediated transport involves specific protein carriers that bind to a drug molecule and facilitate its movement across the membrane.

Factors Affecting Drug Absorption in the GIT

Route of administration: Determines the site of absorption and affects the rate and extent of absorption.
Physicochemical properties of the drug:
- Lipid solubility: Lipophilic drugs are more easily absorbed across cell membranes.
- Molecular weight: Smaller molecules diffuse more readily.
- Ionization: Ionized drugs are less likely to be absorbed.
Nature of the drug product: Factors like formulation, particle size, and dissolution rate can impact absorption.
Anatomy & physiology of the absorption site:
- Surface area: The small intestine has a large surface area due to villi and microvilli, facilitating absorption.
- Blood flow: The splanchnic circulation receives about 28% of the cardiac output, supporting efficient drug absorption.
- Gastrointestinal motility: Transit time affects absorption. Factors that alter motility, like food viscosity, drugs, and stress, can influence absorption.
- pH gradient: The pH of the GI tract can impact the ionization state of drugs.
First-pass effect: Drugs absorbed orally pass through the liver before reaching systemic circulation, where they may be metabolized to a significant extent.

Direct Route

Most drugs (along with nutrients) enter the small intestinal villi, then small blood capillaries, transported through the superior and inferior mesenteric veins, hepatic portal vein, liver, inferior vena cava, and heart, finally reaching systemic circulation.

Indirect Route (via the lacteals)

This route is primarily used for fat absorption.
Fat absorbed via lacteals travels through the thoracic duct to the left subclavian vein near the heart.

Intestinal Transit Time (ITT)

The rate of gastrointestinal motility influences how quickly the stomach contents are emptied.
Factors affecting ITT:
- Food viscosity: Increases in viscosity decrease ITT, leading to a decrease in drug dissolution and diffusion.
- Drugs: Anticholinergics, antihistamines, and narcotics can decrease ITT while domperidone and metoclopramide increase ITT.
- Physiological conditions: Stress can shorten ITT, while depression can lengthen it.

Migrating Motor Complex (MMC)

Fasted/Interdigestive state: MMC acts as the propulsive force emptying the upper GIT to the cecum.
Digestive/Fed state: MMC is replaced by irregular contractions for mixing and advancing the intestinal contents towards the colon in short segments.

Perfusion of the Gastrointestinal Tract

The blood flow to the GI tract is crucial to transporting absorbed drugs into systemic circulation.
The high perfusion of the intestinal lining ensures a concentration gradient favoring drug absorption.
Factors affecting splanchnic blood flow:
- Food intake: Increases blood flow.
- Physical work: Decreases blood flow.

Absorption through the Lymphatic System

Lipophilic drugs are absorbed through the lymphatic system.
This route bypasses the liver, avoiding the first-pass effect.
Lymphatics are involved in lipid absorption and may play a role in absorbing lipophilic drugs.
Some poorly water-soluble drugs are soluble in oil and lipids, absorbed through the lymphatic system.
Examples: Bleomycin, aclarubicin, halofantrine, some testosterone derivatives, temarotene, ontazolast, vitamin D-3, and DDT.

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Description

This quiz covers the essential concepts of drug absorption, focusing on the oral administration route and the factors influencing absorption. It also delves into membrane physiology, highlighting the role of membrane proteins and transport processes such as passive diffusion. Test your understanding of these critical pharmacological principles.