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DrX Whiz Niraj
HAP-I (BP101T) | Unit 5
Cardiovascular System: The Heart
Comprehensive exam notes on Heart Anatomy, Conduction System, Cardiac Cycle, Blood Pressure, ECG, and Cardiac Disorders.
1. Anatomy of the Heart
The Heart is a hollow, muscular organ that pumps blood throughout the body. It is located in the mediastinum (the space between the lungs), with about two-thirds of its mass lying to the left of the body's midline.
Layers of the Heart Wall
- 1. Epicardium: The thin, transparent outer layer (also known as the visceral layer of serous pericardium).
- 2. Myocardium: The thick middle layer composed of cardiac muscle tissue. It is responsible for the pumping action of the heart.
- 3. Endocardium: The smooth, innermost layer that lines the chambers and covers the heart valves.
Chambers & Valves
- Chambers: 2 Superior Atria (receive blood) and 2 Inferior Ventricles (pump blood out).
- Atrioventricular (AV) Valves: Prevent backflow from ventricles to atria. Tricuspid valve (Right) and Bicuspid/Mitral valve (Left).
- Semilunar (SL) Valves: Prevent backflow from arteries to ventricles. Pulmonary and Aortic valves.
2. Blood Vessels & Blood Circulation
Blood circulates through two main closed circuits: Systemic Circulation (left side of heart pumps oxygenated blood to the body) and Pulmonary Circulation (right side of heart pumps deoxygenated blood to the lungs).
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Function | Carry blood away from the heart. | Carry blood towards the heart. | Exchange of gases, nutrients, and wastes. |
| Wall Thickness | Thick, highly elastic and muscular walls. | Thinner walls, less muscle. | Only one cell thick (Endothelium only). |
| Valves | Absent (Blood flows under high pressure). | Present (To prevent backflow under low pressure). | Absent. |
| Oxygenation | Mostly oxygenated (Exception: Pulmonary Artery). | Mostly deoxygenated (Exception: Pulmonary Vein). | Mixed. |
Fig 1: Cross-Section Comparison of Artery vs Vein
3. Conduction System of the Heart
The heart has an intrinsic, specialized network of cardiac muscle fibers called the Conduction System, which generates and distributes electrical impulses to stimulate coordinated muscle contraction.
Fig 2: The Cardiac Conduction System Pathway
The Pathway of Conduction:
- Sinoatrial (SA) Node: Located in the right atrium. Known as the Natural Pacemaker because it initiates the electrical impulses (Approx 70-80 beats/min).
- Atrioventricular (AV) Node: Delays the impulse slightly to allow the atria to empty their blood into the ventricles before they contract.
- Bundle of His (AV Bundle): The only electrical connection between the atria and the ventricles.
- Purkinje Fibers: Rapidly conduct the action potential through the ventricles, causing them to contract strongly and pump blood out.
Regulation by Autonomic Nervous System (ANS)
- Sympathetic Nervous System: Releases Noradrenaline. Increases heart rate and force of contraction (Fight or flight).
- Parasympathetic Nervous System: via Vagus Nerve (CN X). Releases Acetylcholine. Decreases heart rate (Rest and digest).
4. Cardiac Output, BP & Electrocardiogram
Cardiac Output (CO)
The volume of blood ejected by the left ventricle into the aorta each minute.
CO = Stroke Volume (SV) × Heart Rate (HR)
Normal CO = 70 mL/beat × 75 beats/min ≈ 5.25 Liters/min.
Blood Pressure & Pulse
- Blood Pressure: The pressure exerted by blood on the walls of blood vessels. Regulated by Baroreceptors and the RAAS (Renin-Angiotensin) system.
- Pulse: The alternating expansion and recoil of an artery that occurs with each systole of the left ventricle. Normal: 70-80 bpm.
Electrocardiogram (ECG / EKG)
An ECG is a recording of the electrical signals produced by the heart during each cardiac cycle.
Fig 3: Normal ECG Waveform Components
Clinical Correlation: Disorders of the Heart
- Hypertension: Persistently high blood pressure (140/90 mmHg or higher). Major cause of heart failure and kidney disease.
- Angina Pectoris: Severe chest pain caused by reduced blood flow (ischemia) to the myocardium. Warning sign of a heart attack.
- Myocardial Infarction (MI): Heart attack. Complete obstruction of blood flow in a coronary artery, leading to death (infarction) of cardiac muscle tissue.
- Arrhythmia: Irregular heart rhythm due to a defect in the conduction system (SA/AV node issues).
- Congestive Heart Failure (CHF): The heart is a failing pump. It pumps blood less effectively, leading to fluid accumulation in lungs (pulmonary edema) or body.
Exam Corner (Test Yourself)
1. Which structure is known as the "Natural Pacemaker" of the human heart?
A. AV Node
B. Purkinje Fibers
C. SA Node
D. Bundle of His
2. In a normal Electrocardiogram (ECG), the QRS complex represents:
A. Atrial Depolarization
B. Ventricular Depolarization
C. Ventricular Repolarization
D. Atrial Repolarization
3. Which of the following is TRUE regarding veins?
A. They carry blood away from the heart
B. They have thick, highly muscular walls
C. They contain valves to prevent backflow
D. Blood flows through them under high pressure