Before your EMT or Paramedic training, you need to understand why the body fails and what you're actually treating. This isn't a textbook — just what matters in the field.
The heart and blood vessels are your body's delivery network — constantly circulating oxygen, nutrients, and removing waste. When this system fails, everything else fails fast.
The heart has two sides. The right side receives deoxygenated blood from the body and sends it to the lungs. The left side receives oxygenated blood from the lungs and pumps it out to the body. Each side has an atrium (collects blood) and a ventricle (pumps blood).
The heart runs on electricity. The SA node (sinoatrial node) in the right atrium fires spontaneously at 60–100 times per minute — this is your heart rate. The signal travels through the AV node and down to the ventricles, causing them to contract and push blood out. When this electrical system is disrupted, you get arrhythmias.
Cardiac output = heart rate × stroke volume. Stroke volume is how much blood the heart pumps per beat (~70 mL at rest). At rest, your heart pumps about 5 liters per minute — roughly your entire blood volume. In shock, cardiac output drops and tissues starve.
Simplified cardiac anatomy — blue = deoxygenated blood, red = oxygenated blood
Blood pressure = cardiac output × vascular resistance. When someone is in shock, one or both of these are failing. Your job is to keep blood moving to the brain, heart, and lungs — everything else is secondary.
The heart stops pumping effectively. Two shockable causes: Ventricular Fibrillation (VF — chaotic quivering) and Pulseless VT. Two non-shockable: PEA and Asystole. Only VF and pulseless VT respond to defibrillation.
A coronary artery gets blocked — usually by a clot. The muscle downstream dies if not reperfused. STEMI = blockage large enough to see on a 12-lead ECG as ST elevation. Time = muscle. Get to PCI.
Inadequate perfusion to tissues. Three main types: hypovolemic (not enough blood), cardiogenic (heart can't pump), distributive (vessels too dilated — anaphylaxis, sepsis, neurogenic). Each has different treatment priorities.
The lungs move oxygen into the blood and carbon dioxide out. Simple in concept, critical in execution — airway management is the #1 priority in every emergency.
Nose/mouth → pharynx (throat) → larynx (voice box). The epiglottis guards the opening to the trachea, preventing food from entering. Most airway obstructions occur here.
Trachea splits into left and right mainstem bronchi at the carina. This is why an ET tube that goes too far causes breath sounds on only one side.
Tiny air sacs surrounded by capillaries. O₂ diffuses in, CO₂ diffuses out by concentration gradient. The average adult has ~480 million alveoli — total surface area of a tennis court.
Breathing is passive. The diaphragm contracts and flattens → thoracic cavity expands → negative pressure draws air in. When the diaphragm relaxes, air is pushed out. Injury that disrupts this is immediately life-threatening.
Pulse oximetry measures oxygen saturation of hemoglobin — not actual oxygen delivery. A patient in shock can have a normal SpO₂ while tissues are still starving if cardiac output is low. Treat the whole patient.
Chronic CO₂ retainers (COPD patients) may have a SpO₂ baseline of 88–92%. Don't over-oxygenate — target 88–92% in known COPD. High-flow O₂ can blunt their hypoxic drive.
Air trapped in the pleural space collapses the lung and shifts the mediastinum. Signs: absent breath sounds, hypotension, tracheal deviation (late), JVD. Life-threatening. Paramedic: needle decompression at 2nd ICS, MCL.
The body can't maintain adequate oxygenation or ventilation. Signs: SpO₂ <90%, accessory muscle use, tripod positioning, cyanosis, altered mental status. Treat with BVM or advanced airway. Don't wait.
Smooth muscle in the bronchi contracts, narrowing airways. Classic wheeze (expiratory). Treatment: albuterol (bronchodilator) via nebulizer or MDI. Severe: no wheeze = bad sign (no air movement at all).
The nervous system controls everything — heart rate, breathing, blood pressure, consciousness, and muscle movement. Understanding sympathetic vs. parasympathetic is crucial for understanding how medications work.
Activated under stress, fear, blood loss, pain. Effects: ↑ heart rate, ↑ blood pressure, dilated pupils, bronchodilation, peripheral vasoconstriction (pale/cool skin), ↑ blood glucose. This is why shocked patients are pale, tachycardic, and sweaty. Epinephrine mimics this system.
Normal resting state. Effects: ↓ heart rate, ↓ blood pressure, constricted pupils, bronchial constriction, vasodilation. Atropine blocks this system — used for symptomatic bradycardia. Opioids stimulate it (pinpoint pupils, respiratory depression).
The brain uses 20% of your oxygen at rest but has no energy stores — it starts dying within 4–6 minutes of cardiac arrest without CPR. The brainstem controls breathing and heart rate automatically. Increased intracranial pressure (head injury) causes the Cushing's Triad: hypertension, bradycardia, irregular respirations.
Most EMS drugs target the autonomic nervous system. Epinephrine is a full agonist (activates everything). Beta-blockers slow heart rate by blocking sympathetic receptors. Atropine speeds heart rate by blocking parasympathetic signals. Understanding this helps you predict effects and side effects.
Quick neuro assessment used in the field:
A — Alert and oriented
V — Responds to Voice
P — Responds to Pain only
U — Unresponsive
V, P, or U = altered mental status requiring immediate attention.
Blood supply to part of the brain is cut off (ischemic) or a vessel ruptures (hemorrhagic). FAST: Facial droop, Arm weakness, Speech difficulty, Time. Time to tPA window: 3–4.5 hours. Field role: rapid transport to stroke center.
Abnormal electrical activity in the brain. Tonic-clonic (grand mal): full-body convulsions, 1–3 min. Status epilepticus: >5 min — life-threatening. Field role: protect from injury, airway management, oxygen. Paramedics: benzodiazepines for status.
Could be anything: hypoglycemia (most common), hypoxia, head trauma, stroke, overdose, sepsis. Use AEIOU-TIPS mnemonic to remember causes. Always check glucose. Always give O₂. Never assume intoxication without ruling out medical causes.
Blood is the delivery system. Perfusion is the actual delivery of oxygen to cells. Your entire job in trauma — and much of medical care — is preserving perfusion to vital organs.
Carry oxygen via hemoglobin. Each hemoglobin molecule carries 4 O₂ molecules. SpO₂ measures what percentage of hemoglobin is saturated. Anemia = fewer RBCs = less O₂ capacity even with normal SpO₂.
Platelets form the initial plug at a wound. Clotting factors (proteins) build the fibrin mesh that makes a stable clot. Massive hemorrhage depletes both — this is why IV fluids alone aren't enough for severe bleeding. TXA (tranexamic acid) prevents clot breakdown.
The liquid portion (~55% of blood). Contains proteins, electrolytes, glucose, hormones. Normal saline (NS) and Lactated Ringer's (LR) are crystalloid fluids that replace plasma volume but dilute clotting factors if given in large amounts.
The body compensates for blood loss through tachycardia and vasoconstriction — you can lose up to 1,500 mL (30% of blood volume) before seeing a significant drop in blood pressure. Tachycardia and pale/cool skin are your early warning signs. Hypotension means they've already lost a lot.
In hemorrhagic shock, large fluid boluses dilute clotting factors and can actually worsen bleeding. Target systolic BP 80–90 mmHg in penetrating trauma until surgical hemorrhage control — keep the pressure just high enough to perfuse the brain.
Pathophysiology is what happens when the body breaks down. Understanding the cascade helps you recognize emergencies earlier and understand why you're doing what you're doing.
Blood loss, heart failure, vasodilation, or sepsis reduces cardiac output or oxygen delivery.
Sympathetic activation: ↑ heart rate, ↑ vasoconstriction, shunting blood to vital organs. Patient is tachycardic, pale, cool. BP may still be normal.
Compensatory mechanisms fail. BP drops. Brain and heart begin to ischemia. This is when the patient starts to deteriorate rapidly. Aggressive intervention required.
Cell death, multi-organ failure. This is why you treat shock aggressively before the patient decompensates — not after.
Hypoxia (low O₂ to tissues) triggers a dangerous spiral:
1. Cells switch to anaerobic metabolism → lactic acid builds up → acidosis
2. Acidosis impairs cardiac muscle → ↓ cardiac output → worse perfusion
3. Worse perfusion → more hypoxia → more acidosis
4. Cardiac arrest
Almost everything you do as an EMT or Paramedic comes down to three things: maintain oxygenation (airway, breathing, O₂), maintain perfusion (hemorrhage control, fluids, vasopressors), and treat the underlying cause (defibrillate, give epi, transport to definitive care).
Seeing 2+ of these together = something serious is happening. Trust your assessment over any single vital sign.
You now understand the essential physiology behind every emergency you'll encounter. Pick your certification track and start your modules.