Pharmacokinetics: How Drugs Move Through the Body

Summary

This script delves into pharmacokinetics, detailing how drugs move within the body post-administration. It covers absorption through various routes, distribution to target cells, metabolism by enzymes, and eventual excretion. The importance of bioavailability, the influence of blood components and anatomical barriers, the first-pass effect in the liver, and the role of the kidneys in excretion are highlighted, offering a foundational understanding of drug movement and elimination.

Takeaways

• 💊 Pharmacokinetics is the study of how drugs move within, act upon, and are eliminated from the body.
• 🌐 The four main processes of pharmacokinetics are absorption, distribution, metabolism, and excretion.
• 🔄 Absorption involves the drug moving from its site of administration into the bloodstream, often across membranes.
• 🍽 Bioavailability refers to the proportion of a drug that is successfully absorbed into systemic circulation, which can be affected by food.
• 🚀 Distribution is the journey of the drug through the bloodstream to target cells and molecules, influenced by interactions with blood components like plasma proteins.
• 🛡️ Anatomical barriers such as the blood-brain barrier can limit the movement of drugs to certain areas of the body.
• 🛠️ Metabolism involves chemical reactions that modify the drug, often to inactivate it and prepare it for excretion, with the liver playing a key role in this process.
• 🌀 The first-pass effect describes the initial metabolism of a drug in the liver, which can significantly reduce the drug's bioavailability.
• 🧬 Different enzymes in the body perform metabolic functions, which are crucial for detoxifying small molecules that the immune system cannot handle.
• 🚰 Excretion is the final step where the drug or its metabolites are removed from the body, primarily through urination, defecation, sweating, or exhalation.
• 🧘‍♂️ Understanding pharmacokinetics helps in determining appropriate drug dosages and managing drug interactions with food, drink, and other substances.

Q & A

What is pharmacokinetics?

Pharmacokinetics is the study of how drugs move within the body, including their absorption, distribution, metabolism, and excretion.

What are the four main processes involved in pharmacokinetics?

The four main processes are absorption, distribution, metabolism, and excretion.

How does absorption occur when a drug is administered topically?

Absorption occurs by the drug moving through the skin or a mucous membrane and then through the walls of nearby blood vessels.

What is the impact of food on oral drug absorption?

The presence of food can slow down the absorption process of orally administered drugs through the lining of the stomach or intestines.

What is meant by 'bioavailability' in the context of drug absorption?

Bioavailability refers to the proportion of a drug that is successfully absorbed into systemic circulation.

How does distribution of a drug in the bloodstream relate to plasma proteins?

The drug's interaction with plasma proteins can influence its ability to reach target cells; if a drug binds too tightly to these proteins, it may not reach its intended destination.

What is the blood-brain barrier and how does it affect drug distribution?

The blood-brain barrier is an anatomical barrier that prevents certain substances from passing out of the bloodstream into brain tissue, thus affecting the distribution of some drugs.

What is the first-pass effect and how does it influence drug metabolism?

The first-pass effect refers to the initial pass of a drug through the liver, where it is often metabolized to some degree, which can greatly reduce the drug's bioavailability.

How do enzymes play a role in drug metabolism?

Enzymes in the body perform metabolic functions, often aimed at inactivating the drug and targeting it for excretion, which is crucial for detoxification of small molecules.

What are the typical pathways for drug excretion from the body?

Drugs or their remnants are typically excreted via exhalation, sweating, urination, or defecation, with the kidneys playing a significant role in this process.

How does the enterohepatic recirculation process contribute to drug excretion?

Enterohepatic recirculation is the process by which bile, secreted by the liver, is recirculated back to the liver and most of the drug is then excreted by the kidneys, with the remainder exiting in feces.

Outlines

00:00

💊 Introduction to Pharmacokinetics

Professor Dave introduces the concept of pharmacokinetics, explaining the journey drugs take within the body from absorption to excretion. He outlines the four main processes: absorption, distribution, metabolism, and excretion. The paragraph emphasizes the importance of understanding how drugs move through the body, interact with blood components, and are influenced by various factors such as food, drink, and other drugs. It also touches on bioavailability and the challenges drugs face in reaching target cells due to binding with plasma proteins or anatomical barriers like the blood-brain barrier.

05:03

🔁 Metabolism and Excretion of Drugs

This paragraph delves into the details of drug metabolism and excretion. It discusses the first-pass effect, where the liver significantly reduces a drug's bioavailability after oral administration. The role of enzymes in metabolizing drugs is highlighted, explaining their importance in detoxifying the body from small molecules. The paragraph also describes the excretion process, involving the kidneys in removing harmful substances and the various ways drugs can be expelled from the body, such as through exhalation, sweating, urination, and defecation. The enterohepatic recirculation is mentioned as a pathway for drug excretion, and the overall summary provides a comprehensive look at the final stages of a drug's lifecycle within the body.

Keywords

💡Pharmacokinetics

Pharmacokinetics is the study of how drugs move within the body, including their absorption, distribution, metabolism, and excretion. It is central to the video's theme as it provides the framework for understanding the processes that drugs undergo once inside the body. The script discusses the four main processes of pharmacokinetics to explain how drugs are administered, how they reach their target, and how they are eventually removed from the body.

💡Absorption

Absorption refers to the process by which a drug moves from its site of administration into the bloodstream. It is a critical step in pharmacokinetics as it determines the bioavailability of the drug. In the script, absorption is described for different administration routes, such as topical application and oral ingestion, and the concept of bioavailability is introduced as a measure of how much of the drug enters systemic circulation.

💡Distribution

Distribution is the process through which a drug travels in the bloodstream to reach target cells and molecules. It is essential for the drug's efficacy and is influenced by factors such as drug-plasma protein binding. The script explains how distribution can be affected by the drug's interaction with blood components and anatomical barriers, such as the blood-brain barrier.

💡Metabolism

Metabolism in the context of pharmacokinetics refers to the chemical modifications that a drug undergoes, primarily aimed at rendering it inactive and preparing it for excretion. The script discusses the first-pass effect, where drugs taken orally are metabolized in the liver before entering general circulation, which can significantly impact the drug's bioavailability.

💡Excretion

Excretion is the final process in pharmacokinetics, where the drug or its metabolites are removed from the body. The script describes how excretion typically occurs through urine, feces, exhalation, or sweating, with the kidneys playing a significant role in removing harmful substances from the bloodstream.

💡Bioavailability

Bioavailability is a term used to describe the proportion of a drug that enters systemic circulation and is available to exert its therapeutic effect. It is mentioned in the script in the context of absorption, highlighting the importance of this concept in determining the effectiveness of a drug after administration.

💡Passive Transport

Passive transport is a mechanism by which drugs can cross cell membranes without the use of energy, as described in the script for drugs administered topically or orally. It is an essential concept in understanding how drugs can move through biological barriers and reach their target sites.

💡Active Transport

Active transport is a process that requires energy to move substances, including drugs, across cell membranes against a concentration gradient. The script briefly mentions this as one of the ways drugs can pass through membranes, emphasizing the complexity of drug absorption mechanisms.

💡Anatomical Barriers

Anatomical barriers are physical structures within the body that can impede the movement of drugs to certain areas. The script discusses several examples, such as the blood-brain barrier, blood-placental barrier, and blood-testicular barrier, illustrating how these barriers can affect drug distribution and efficacy.

💡Hepatic Portal System

The hepatic portal system is a network of blood vessels that carries blood from the gastrointestinal tract to the liver. The script explains its role in the first-pass effect, where orally administered drugs are metabolized in the liver before entering the general circulation, significantly affecting drug bioavailability.

💡Enterohepatic Recirculation

Enterohepatic recirculation is a process where drugs or their metabolites are excreted into the bile and then reabsorbed from the intestine back into the liver. The script mentions this as one of the pathways for drug excretion, highlighting the complex mechanisms by which drugs can be eliminated from the body.

Highlights

Introduction to pharmacokinetics, the study of how drugs move within the body and are eventually removed.

The four main processes of pharmacokinetics: absorption, distribution, metabolism, and excretion.

Absorption as the process of drug movement from its site of administration into the bloodstream.

Distribution describes how drugs travel through the bloodstream to reach target cells and molecules.

Metabolism involves the modification of drugs by enzymes, rendering them ineffective.

Excretion as the process by which drugs exit the body, typically through urine or feces.

Influence of food, drink, and other drugs on the pharmacokinetic processes.

Bioavailability as the proportion of a drug successfully absorbed into systemic circulation.

Passive and active transport mechanisms for drug movement through biological membranes.

The impact of plasma protein binding on drug distribution and reaching target cells.

Anatomical barriers like the blood-brain barrier and their role in drug distribution.

The blood-placental barrier and its regulation of substances passing into the fetus.

The blood-testicular barrier and its implications for treating testicular disorders.

First-pass effect and its impact on the bioavailability of orally administered drugs.

Role of the liver in drug metabolism and the potential for activation or inactivation of drugs.

Importance of metabolic enzymes in detoxifying the body from small molecules.

Excretion methods including exhalation, sweating, urination, and defecation.

The role of the kidneys in removing harmful substances from the bloodstream during excretion.

Enterohepatic recirculation and its influence on drug excretion via bile and the kidneys.

Glands that produce fluids like saliva and sweat in promoting drug excretion.

Transcripts

It’s Professor Dave, I wanna tell you about pharmacokinetics.

In the previous tutorial, we learned about all of the different ways that drugs can be

administered, or basically the ways that drugs can enter the body, whether in the mouth,

through the skin, directly into the bloodstream, or otherwise.

Now we want to talk about what happens next.

Once drugs are inside your body, how do they move around, and get to where they need to go?

Do they stay in the body indefinitely, or are they eventually removed from the body

somehow?

The study of pharmacokinetics deals with precisely these issues, and in learning about this,

we can highlight four main processes.

Those are absorption, distribution, metabolism, and excretion.

Absorption generally describes the way the drug moves from its site of administration

across one or more membranes, often into the bloodstream, if it was not administered there directly.

Distribution describes its journey through the bloodstream to target cells and specific

target molecules within.

Metabolism describes the ways that it eventually gets modified by enzymes and rendered ineffective,

and excretion describes the way it then exits the body, typically either through urine or feces.

We want to understand each of these processes, as well as the ways that interactions with

food, drink, other drugs, and additional factors can have an influence on them, so let’s

get a closer look at these now.

As we said, the first step is absorption.

If a drug is administered topically, this could mean moving through the skin or a mucous

membrane, and subsequently through the walls of nearby blood vessels.

If administered orally, this would mean being absorbed through the lining of the stomach

or intestines, a process which would be slowed by the presence of food.

Some drugs will pass through the cells that comprise these membranes by passive transport,

others by active transport, but one way or another they will make it through on the way

to their destination.

In describing this process, it will be appropriate to review the term “bioavailability”.

This term describes the proportion of a drug that is successfully absorbed into systemic

circulation.

Next, we mentioned distribution.

This is the way that drugs move through the bloodstream, after being absorbed into it,

or injected into it.

There are many factors that influence this, like the way that the drug interacts with

the components of blood, such as plasma proteins.

If the drug binds too tightly to these proteins, it will not be able to reach its target cells.

Sometimes, a second drug is administered in conjunction with the first that has a higher

affinity for these proteins than the first one does, thus serving the sole purpose of

displacing the primary drug once bound to the protein, allowing it to be delivered to

its destination.

Beyond these blood elements, there are other factors that may hinder the movement of a drug.

These are anatomical barriers found in certain organs.

We have already discussed the blood-brain barrier in the biopsychology series.

This prevents certain substances from passing out of the bloodstream into brain tissue.

Some drugs will not be able to surpass this barrier, while others will, such as psychotropic

drugs, or those affecting the mind.

There is also the blood-placental barrier, that regulates which substances can pass from

the bloodstream of a pregnant woman into the fetus.

There are, however, a number of substances that are able to pass through this barrier

that can still do harm to the fetus, such as alcohol and certain medications.

And then there is the blood-testicular barrier, which prevents many substances from reaching

the male testes, therefore making disorders of the testes difficult to treat.

After distribution, there is metabolism.

This describes any chemical reactions that the drug may participate in, often aimed at

inactivating it and targeting it for excretion.

If a drug is traveling through the bloodstream, it is highly likely to be metabolized to some degree.

For example, if a drug is taken orally, it will be absorbed through the intestinal wall,

and for this reason, the part of the circulatory system it enters is a collection of blood

vessels called the hepatic portal system.

These carry blood directly to the liver, where they will be metabolized in some manner.

This is called the first-pass effect, referring to the first pass of a drug through the liver,

and this will typically greatly reduce the bioavailability of a drug.

In certain cases, metabolism in the liver actually activates a drug, but this is less

common, and the first pass effect can inactivate over 90 percent of an orally administered

drug before it is able to reach general circulation.

This must be taken into account when determining appropriate dosage.

Of course, drugs eventually reach their target cells, but even then, after enough time elapses,

they will be metabolized.

There are many different enzymes in the body that perform these metabolic functions, which

are very important, because the immune system is only good at dealing with large biological

particles like viruses or bacteria.

It has no defense against small molecules, so this detoxification mechanism aimed specifically

at small molecules had to evolve for life to exist in a chemical world.

Finally, after metabolization there is excretion, where the drug or its remnants exit the body.

This is typically done via exhalation, sweating, urination, or defecation.

The kidneys are heavily involved in this process, as they must remove harmful substances from

the bloodstream.

Some drugs are metabolized into gaseous form, and are thus easily exhaled.

Some drugs are excreted through bile, a substance secreted by the liver to aid in digestion,

as bile is recirculated back to the liver via enterohepatic recirculation, whereby most

of the drug can then be excreted by the kidneys, and the rest will exit in the form of feces.

And as we said, glands that produce fluids such as saliva and sweat can also promote

excretion, though this method tends to be less effective.

And with that, we have traced the journey of a drug into the body, around the body,

and out of the body, which gives us a basic understanding of pharmacokinetics.