Cardiovascular System
1. Oxygen-Poor Blood Returns to the Heart
• From the body: Blood lacking oxygen enters the right atrium via:
o Superior vena cava (from the upper body).
o Inferior vena cava (from the lower body).
• Right atrium contracts, pushing blood through the tricuspid valve into the right
ventricle.
2. Blood Travels to the Lungs (Pulmonary Circulation)
• The right ventricle contracts, sending blood through the pulmonary valve into the
pulmonary artery.
• The pulmonary artery carries oxygen-poor blood to the lungs, where it picks up oxygen
and releases carbon dioxide.
3. Oxygen-Rich Blood Returns to the Heart
• Freshly oxygenated blood returns to the left atrium via the pulmonary veins.
• The left atrium contracts, pushing blood through the mitral (bicuspid) valve into the
left ventricle.
4. Blood is Pumped to the Body (Systemic Circulation)
• The left ventricle (strongest chamber) contracts, forcing blood through the aortic valve
into the aorta.
• The aorta distributes oxygen-rich blood to the entire body.
Heart’s electrical activity
1. Sinoatrial (SA) Node – The Heart’s Pacemaker
• Located in the right atrium.
• Generates electrical impulses (60-100 beats per minute).
• This impulse spreads across both atria, causing atrial contraction (P-wave on ECG).
2. Atrioventricular (AV) Node – The Signal Delay Center
• Located between the atria and ventricles.
• Slows down the impulse to allow the ventricles time to fill with blood (PR interval on
ECG).
• If the AV node is blocked, the ventricles may not receive signals properly (heart block).
3. Bundle of His – The Signal Highway
• Carries the impulse from the AV node to the ventricles.
• Splits into the right and left bundle branches, ensuring both ventricles contract
together.
4. Purkinje Fibers – The Final Push
• Spread throughout the ventricular walls.
• Causes a strong, coordinated ventricular contraction (QRS complex on ECG), pumping
blood to the lungs and body.
5. Repolarization – Resetting the Heart
• After contraction, the ventricles relax and reset for the next beat.
• This is seen as the T-wave on the ECG.
ECG interpretation
1. Atrial Activity (P-wave)
• The atria are the top chambers of the heart.
• They contract first to push blood into the ventricles.
• This electrical activity creates the P-wave on an ECG.
• If the P-wave is missing or irregular, it means something is wrong with atrial activity
(e.g., atrial fibrillation).
2. AV Node (PR Interval)
• The AV node (Atrioventricular Node) sits between the atria and ventricles.
• It slows down the electrical signal slightly to allow the ventricles time to fill with blood.
• This delay is represented by the PR interval on an ECG.
• If it’s too long, there may be a heart block (signal delay). If too short, there may be an
extra pathway (like in Wolff-Parkinson-White syndrome).
3. Ventricular Depolarization (QRS Complex)
• The ventricles (bottom chambers of the heart) pump blood to the lungs and body.
• Their contraction is the strongest electrical event in the heart, creating the QRS complex
on an ECG.
• A wide QRS means the signal is taking longer to travel (could be a bundle branch block
or ventricular rhythm).
4. ST Segment (Heart Attack or Ischemia?)
• After contraction, the heart needs time to reset (repolarize).
• The ST segment shows if the heart muscle is getting enough oxygen.
• If it’s elevated, it may mean a heart attack (STEMI).
• If it’s depressed, it may suggest ischemia (low oxygen supply).
5. Ventricular Repolarization (T-wave)
• The ventricles relax and reset after pumping.
• This is shown by the T-wave on the ECG.
• If the T-wave is inverted, it could mean ischemia.
• If it’s tall and peaked, it may suggest high potassium (hyperkalemia).
6. QT Interval (Risk of Dangerous Arrhythmias)
• The QT interval includes both the QRS complex (ventricular contraction) and the T-wave
(ventricular relaxation).
• It shows how long the ventricles take to contract and reset for the next beat.
• If the QT interval is too long, it increases the risk of life-threatening arrhythmias like
Torsades de Pointes.
• Causes of prolonged QT:
o Some medications (e.g., certain antibiotics, antidepressants).
o Low potassium, calcium, or magnesium.
o Congenital conditions (Long QT Syndrome).
