💊 Homeostasis Module 5 – Pharmacology 📖 What is Pharmacology? Pharmacology is the study of how drugs interact with the body. Topics Covered Introduction to Pharmacology Drug Administration & Absorption Agonists & Antagonists Drug Distribution Metabolism & Elimination 👩‍⚕️ Why Pharmacology Matters for Nurses Nurses use pharmacology to: ✅ Give the right drug, right dose, to the right patient ⚠️ Recognise adverse effects early before they become serious 🧠 Understand why a drug works and how it produces its effects 💉 Administer medications safely and effectively ⭐ Two Fundamentals of Pharmacology 1. Pharmacokinetics (PK) "What the body does to the drug." The four stages (ADME): Absorption – Drug enters the bloodstream. Distribution – Drug travels to tissues and organs. Metabolism – Drug is broken down (mainly in the liver). Elimination – Drug leaves the body (mainly via the kidneys). 2. Pharmacodynamics (PD) "What the drug does to the body." Includes: Receptor binding Agonism Antagonism Dose-response relationship 🚶 The Drug Journey (ADME) 💊 Administration Drug enters the body. Examples: Oral, IV, topical. 🩸 Absorption Drug crosses into the bloodstream. 🚚 Distribution Drug travels through the blood to tissues and organs. 🏭 Metabolism Mainly occurs in the liver. The liver treats many drugs like foreign substances and breaks them down. 🚽 Elimination Mainly through the kidneys in urine. 💉 Drug Administration & Absorption The route of administration affects: ⏱️ How fast the drug works 🩸 How much reaches the bloodstream (bioavailability) ⚠️ Possible side effects Incorrect absorption can lead to: Too much drug → Toxicity (may be fatal) Too little drug → No therapeutic effect Too slow absorption → Delayed treatment 📋 Routes of Administration Route Example Key Point Oral (PO) Tablets, Capsules Convenient but lower bioavailability Sublingual (SL) Dissolves under tongue Bypasses liver → faster effect Intravenous (IV) Emergency drugs 100% bioavailability, fastest route Intramuscular (IM) Vaccines Absorbed from muscle Subcutaneous (SC) Insulin Injected into fatty tissue Topical Creams, Patches Local or slow systemic effect Inhalation Salbutamol Rapid absorption through lungs Rectal (PR) Suppositories Useful if oral route isn't possible 🌟 Bioavailability How much of the drug reaches the bloodstream Definition Bioavailability = the amount of drug that reaches the systemic circulation in its active form. 👉 It determines how much of the drug the body can actually use. Important Points IV drugs = 100% bioavailability Oral drugs = lowest bioavailability Oral doses are usually higher than IV doses because much of the drug is lost before reaching circulation. Small intestine is where majority of the absorption of the drug How it processes From small intestine it goes into the blood stream then to the liver From the liver detoxes or metabolises and that called first pass After liver detox it goes into the blood stream, then that is where the drug can be used How much free drug is in the bloodstream in its active form is the most important thing 🍽️ Oral Drug Absorption Most drug absorption occurs in the small intestine. Process Small Intestine ⬇️ Portal Vein ⬇️ Liver (First-Pass Metabolism) ⬇️ Systemic Bloodstream ⬇️ Target Tissues 🏭 First-Pass Metabolism Why some drugs can't be taken orally After absorption: Drug absorbed from gut wall travels through the portal vein directly to the liver. Liver enzymes metabolise (break down) part of the drug. Meaning you actually destroy potentially a very large portion of drug before you can actually use it. It needs to be in the blood steam for us to use it. Only the remaining active drug enters the bloodstream. Result Less active drugs reach circulation. Reduces bioavailability. Clinical example in depth Under the tongue you have blood vessels that directly drain into the vena cava, that goes straight into the heart and then around systematically completely bypassing the liver to avoid the enzyme destruction. This gives us a higher bioavailability. 👅 Why Sublingual Drugs Work Faster Drugs placed under the tongue: Absorb directly into blood vessels. Drain into the vena cava. Travel straight to the heart. Bypass the liver, avoiding first-pass metabolism. Result Faster onset Higher bioavailability 🚑 Why IV Drugs Are Fastest IV medications are injected directly into the bloodstream. Benefits: ✅ Immediate effect ✅ 100% bioavailability ✅ Precison dosing Ideal for emergencies Highest Risk Route: Once in you can’t take it back. IV drug administration requires double checking and monitoring 🧬 Drug Movement Across Cell Membranes The phospholipid bilayer controls whether drugs can enter cells. Lipid-Soluble Drugs Non-polar Small molecules Cross membranes easily Move by passive diffusion Usually absorbed well Water-Soluble Drugs Polar or charged Often larger molecules Cross membranes poorly May require chemical modification or transport mechanisms ⭐ Key Concept: Most drugs rely on passive diffusion. If a drug cannot cross the membrane, it cannot reach its target and will be ineffective. The concept of how drugs get in and out of the phospholipid bilayer explains all of pharmacology if it can't cross the membrane and get where it needs to go the drug is ineffective. drugs that are lipid soluble are non polar; small and easily cross with passive diffusion If a drug is water soluble so it is polar or charged or too big it will have a difficult time crossing the phospholipid bi layer. Thats where certain drugs are altered chemically to make it easier or harder to get through certain situations. Most drugs rely on passive diffusion so it needs to be able to cross the membrane 🎯 Agonists & Antagonists (Pharmacodynamics) Drugs work by binding to specific receptors. Receptors: Specialised proteins or inside cells Natural Ligands: Neurotransmitters, hormones - the body's own chemical messengers Drug action: Drugs mimic or block these natural chemicals at the receptor 🔑 Lock-and-Key Principle Drug = artificial Key Receptor = Lock Neurotransmitters = Natural keys If the drug doesn't fit the receptor, the desired effect won't occur. Binding depends on: Drug shape Chemical structure ✅ Agonists Agonists: Bind to receptors. Activate receptors. Produce a biological response mimicking the body's natural signaling molecules Characteristics High affinity Positive efficacy Types Full Agonist Produces the maximum possible response. Partial Agonist Produces a smaller response, even when all receptors are occupied. 🚫 Antagonists Antagonists: DRUGS THAT BLOCK RECEPTORS Bind to receptors. Do NOT activate them. Prevent natural chemicals (agonists) from binding. Characteristics High affinity Zero (or negligible) efficacy Effect They block receptor activation without producing a biological response themselves. 🔗 Affinity vs Efficacy Fundamental concepts in pharmacodynamics that describe how ligands (drugs) interact with receptors. Affinity How strongly a drug binds to a receptor. High affinity = drug binds easily. Determines the likelihood of binding. Efficacy The ability of the drug-receptor complex to produce a biological response. Determines how well the drug activates the receptor. In context of Agonist and Antagonist 1. Agonist Interactions Agonists bind to receptors and trigger a biological response. Affinity: Agonists have high affinity, allowing them to bind easily to receptors. Efficacy: Agonists have positive intrinsic efficacy (ranging from partial to full), meaning they turn the receptor "on" to initiate a response. Full vs. Partial: Full agonists possess maximum efficacy, while partial agonists have lower efficacy, producing a submaximal response even at 100% receptor occupancy. 2. Antagonist Interactions Antagonists bind to receptors but do not trigger a response, acting as blockers. Affinity: Antagonists have high affinity (often higher than agonists), allowing them to bind strongly to the receptor. Efficacy: Antagonists have zero or negligible efficacy; they bind but do not activate the receptor. Half-Life (t½) – Nursing Notes What is a half-life? A half-life is the amount of time it takes for 50% of a drug to be removed from the body. Think of it like this: 💊 You take 100 mg of a medication. If the medication has a 4-hour half-life: Time Amount left 0 hours 100 mg 4 hours 50 mg 8 hours 25 mg 12 hours 12.5 mg 16 hours 6.25 mg Notice it never loses the same number of milligrams—it loses half of what remains. Easy way to remember Half-life = Time needed for HALF the drug to disappear. Not all of it. Just half. Method 1: Repeatedly divide by 2 (the easiest) Every half-life, divide the amount remaining by 2. Example 1 A patient takes 160 mg of a drug. The half-life is 4 hours. After 4 hours: 160 ÷ 2 = 80 mg After 8 hours: 80 ÷ 2 = 40 mg After 12 hours: 40 ÷ 2 = 20 mg After 16 hours: 20 ÷ 2 = 10 mg Time Amount Remaining 0 hrs 160 mg 4 hrs 80 mg 8 hrs 40 mg 12 hrs 20 mg 16 hrs 10 mg Method 2: Count the number of half-lives First, work out how many half-lives have passed. Formula: Number of half-lives = Total time ÷ Half-life Example 2 A drug has a half-life of 6 hours. How much remains from 80 mg after 18 hours? Step 1 How many half-lives? 18 ÷ 6 = 3 half-lives Step 2 Halve the amount 3 times. 80 → 40 → 20 → 10 mg Answer: 10 mg remains. Example 3 A patient receives 200 mg. The half-life is 5 hours. How much remains after 15 hours? Step 1 15 ÷ 5 = 3 half-lives Step 2 200 → 100 → 50 → 25 mg Answer: 25 mg If they ask how much has been eliminated Be careful—sometimes the question asks how much is left, and sometimes how much has been eliminated. Example A patient takes 120 mg. Half-life = 8 hours After 16 hours, how much has been eliminated? Step 1: 16 ÷ 8 = 2 half-lives Step 2: 120 → 60 → 30 mg remaining Step 3: 120 − 30 = 90 mg eliminated ✔ Remaining = 30 mg ✔ Eliminated = 90 mg Step 1: Write down the question Example: A patient receives 80 mg of a medication. The half-life is 4 hours. How much remains after 8 hours? Step 2: Find the number of half-lives Ask yourself: How many 4-hour periods are in 8 hours? Do this calculation: 8 ÷ 4 = 2 ✅ 2 half-lives have passed. This is always the first thing you should do. Step 3: Write the starting amount Starting amount: 80 mg Step 4: Halve it once for each half-life Since 2 half-lives have passed, divide by 2 two times. First half-life 80 ÷ 2 = 40 mg Second half-life 40 ÷ 2 = 20 mg Step 5: Write your answer ✅ 20 mg remains. I think you've understood it! 🥳 The key is that you're now doing the same two steps every time: Time ÷ Half-life = Number of half-lives Divide the starting amount by 2 that many times Practice Questions Try these without looking at the answers first! 1. A patient receives 64 mg of a medication with a half-life of 2 hours. How much remains after 6 hours?

Answer

6 ÷ 2 = 3 half-lives 64 → 32 → 16 → 8 mg

2. A patient receives 240 mg of a medication with a half-life of 4 hours. How much remains after 12 hours?

Answer

12 ÷ 4 = 3 half-lives 240 → 120 → 60 → 30 mg

3. A patient receives 100 mg of a medication with a half-life of 5 hours. How much has been eliminated after 10 hours?

Answer

10 ÷ 5 = 2 half-lives 100 → 50 → 25 mg remaining 100 − 25 = 75 mg eliminated

💡 Exam tip: Most half-life questions follow the same pattern: Calculate the number of half-lives: Time ÷ Half-life. Divide the starting amount by 2 that many times. Read the question carefully to see whether it asks for the amount remaining or the amount eliminated. 📝 Exam Tips Stage Key concept Key organ Administration Route determines onset and bioavailability - Absorption Must cross membranes; first pass effect GI Tract Distribution Protein binding, BBB, Volume of distribution Blood, Tissues Metabolism Phase 1 and 2; drug interaction Liver Elimination Half-Life; Renal function matters Kidneys ⭐ Remember ADME A = Absorption D = Distribution M = Metabolism E = Elimination ⭐ Pharmacokinetics = What the body does to the drug. ⭐ Pharmacodynamics = What the drug does to the body. ⭐ Bioavailability = Amount of active drug reaching systemic circulation. ⭐ First-pass metabolism occurs in the liver and reduces oral drug bioavailability. ⭐ IV = Fastest route + 100% bioavailability. ⭐ Sublingual drugs bypass the liver, giving a faster effect and higher bioavailability. ⭐ Agonists activate receptors; antagonists block receptors. ⭐ Affinity = Binding. Efficacy = Response.