CARDIOVASCULAR SYSTEM: ANATOMY AND PHYSIOLOGY OF HEART — A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
The cardiovascular system includes the heart, blood vessels, and blood. It acts as a transport system of the body — carrying oxygen, nutrients, hormones, and removing waste products to maintain homeostasis (internal balance).
Dpharmguru’s exam insights:
In my years of teaching cardiovascular physiology, I have observed that students often get overwhelmed by the number of structures and terms. Remember: The heart is a double pump — the right side pumps to the lungs (pulmonary circulation), and the left side pumps to the body (systemic circulation). This is the most fundamental concept in cardiology!
MAIN FUNCTIONS OF THE CARDIOVASCULAR SYSTEM
- Carry oxygen (O₂) and nutrients to all cells and tissues of the body.
- Transport carbon dioxide (CO₂) and other waste materials from tissues to the lungs and excretory organs.
- Distribute water, electrolytes, and hormones throughout the body.
- Help regulate body temperature (thermoregulation).
ANATOMY OF THE HEART
The heart is a muscular organ found in all vertebrates. It continuously pumps blood throughout the body. In humans, the heart is about the size of a clenched fist. Average weight: 250–300 g in females, 300–350 g in males. Normal rate: about 70–72 beats per minute. Location: inside the thoracic cavity, between the lungs, in a space called the mediastinum, just above the diaphragm. It is tilted slightly to the left and has a cone shape.
Coverings of the Heart
The heart is covered by a serous membrane called the pericardial sac (pericardium).
- Fibrous Pericardium: Outer tough layer that connects the heart to large blood vessels (vena cava, aorta, pulmonary artery, and vein).
- Serous Pericardium: Thin, delicate inner membrane that forms the epicardium (outer wall of heart) and lines the large blood vessels.
Layers of the Heart Wall
- Epicardium (Visceral Pericardium): Outer thin, transparent layer made of mesothelium. Protects and lubricates the heart.
- Pericardial Cavity: Space between the epicardium and the serous pericardium. Filled with pericardial fluid, which reduces friction during heartbeats.
- Myocardium: Middle thick layer made of cardiac muscle tissue. Responsible for heart contraction.
- Endocardium: Inner thin layer made of endothelium and connective tissue. Lines the inner cavities and valves of the heart.
Chambers of the Heart
| Chamber | Main Features |
|---|---|
| Right Atrium (RA) | Thinnest of all chambers. Forms right border of the heart. Receives blood from superior vena cava, inferior vena cava, and coronary sinus. Sends blood to right ventricle through tricuspid valve. |
| Right Ventricle (RV) | Contains inlet (tricuspid valve) and outlet (pulmonary valve). Has muscular walls and conus outflow towards pulmonary artery. |
| Left Atrium (LA) | Forms the posterior surface (base) of the heart. Receives oxygenated blood from two right and two left pulmonary veins. Pumps blood to left ventricle through mitral valve. |
| Left Ventricle (LV) | Thickest and strongest chamber (3× thicker than RV). Has mitral valve inlet and aortic valve outlet. Pumps blood to aorta for systemic circulation. |
Dpharmguru’s exam insights:
The left ventricle has the thickest wall because it must pump blood to the entire body. The right ventricle pumps only to the lungs, so its wall is thinner. This is a very common exam question!
Valves of the Heart
Heart valves ensure one-way flow of blood and prevent backflow.
Atrioventricular (AV) Valves:
| Valve | Location | Cusps | Function |
|---|---|---|---|
| Tricuspid Valve | Between right atrium and right ventricle | 3 cusps | Prevents backflow into right atrium |
| Mitral (Bicuspid) Valve | Between left atrium and left ventricle | 2 cusps | Prevents backflow into left atrium |
Semilunar Valves:
| Valve | Location | Function |
|---|---|---|
| Pulmonary Valve | Between right ventricle and pulmonary artery | Prevents backflow from pulmonary artery |
| Aortic Valve | Between left ventricle and aorta | Prevents backflow from aorta |
Dpharmguru’s exam insights:
Remember the mnemonic for AV valves: “Try Before You Buy” — Tricuspid (Right), Bicuspid (Left). The chordae tendineae prevent the valves from inverting during ventricular contraction. This is frequently tested!
Conduction System of the Heart
The conduction system controls the rhythmic beating of the heart through electrical impulses.
- Sinoatrial (SA) Node: Located in the right atrium near superior vena cava. Known as the pacemaker of the heart. Generates electrical impulses that start each heartbeat.
- Internodal Pathways: Carry impulses from SA node to AV node. Three tracts: Bachmann’s (Anterior), Wenckebach’s (Middle), Thorel’s (Posterior).
- Atrioventricular (AV) Node: Located at the lower part of right atrium, near interatrial septum. Acts as pace-setter if SA node fails.
- Bundle of His: Arises from AV node, passes into interventricular septum, and divides into right and left branches.
- Bundle Branches: Right and left branches spread into ventricular walls and carry impulses to Purkinje fibres.
- Purkinje Fibres: Fine network of fibres beneath endocardium. Spread electrical impulses rapidly through ventricles.
Pathway of Impulse: SA Node → Internodal Pathways → AV Node → Bundle of His → Right & Left Bundle Branches → Purkinje Fibres
Blood Vessels
Blood vessels form a closed network of tubes that transport blood to and from the heart.
Types of Blood Vessels
- Arteries: Carry blood away from the heart under high pressure. Examples: Aorta, carotid, subclavian, renal, and iliac arteries.
- Arterioles: Smallest branches of arteries. Controlled by sympathetic nerves.
- Capillaries: Smallest blood vessels that connect arterioles and venules. Allow exchange of gases, nutrients, and waste.
- Venules: Smallest veins that collect blood from capillaries.
- Veins: Carry blood toward the heart under low pressure. Example: Jugular, subclavian, renal, iliac veins, and vena cava.
Dpharmguru’s exam insights:
Remember: Arteries carry blood away from the heart (A = Away), and veins carry blood toward the heart. Also, the pulmonary artery is the only artery that carries deoxygenated blood, and the pulmonary veins are the only veins that carry oxygenated blood — this is a very common trick question!
BLOOD CIRCULATION THROUGH THE HEART
With every heartbeat, blood is pumped through two closed circuits — Systemic circulation and Pulmonary circulation. These two circuits are connected in a series. Additionally, the body has Portal circulation and Coronary circulation.
1. Systemic Circulation
The left side of the heart is responsible for systemic circulation. It receives oxygenated blood from the lungs. The left ventricle pumps this oxygenated blood into the aorta. The aorta divides into smaller systemic arteries that carry oxygen-rich blood to all body tissues. The now deoxygenated blood moves into systemic venules, which unite to form systemic veins. These veins carry deoxygenated blood back to the heart through the superior vena cava, inferior vena cava, and coronary sinus, all opening into the right atrium.
2. Pulmonary Circulation
Pulmonary circulation ensures that blood is oxygenated in the lungs before it is sent back to the body. Deoxygenated blood from the body enters the right atrium through the superior and inferior vena cava. The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle. The right ventricle contracts and sends this blood through the pulmonary valve into the pulmonary artery — the only artery that carries deoxygenated blood. The pulmonary artery takes blood to the lungs, where carbon dioxide diffuses out and oxygen diffuses in. The now oxygenated blood returns to the heart via four pulmonary veins, opening into the left atrium.
3. Portal Circulation
Portal circulation refers to the flow of blood through two sets of capillaries before returning to the heart. The most important is the hepatic portal circulation. Blood reaches the liver from the hepatic artery (oxygenated) and the hepatic portal vein (nutrient-rich, deoxygenated). After detoxification and nutrient regulation, blood leaves the liver through hepatic veins, which open into the inferior vena cava.
4. Coronary Circulation
The heart muscle itself needs its own blood supply for oxygen and nutrients. This supply is provided by the coronary circulation. The right and left coronary arteries arise from the base of the aorta. The cardiac veins collect deoxygenated blood from the heart muscle and merge into the coronary sinus, which opens into the right atrium.
CARDIAC CYCLE AND HEART SOUNDS
A cardiac cycle refers to one complete heartbeat, including contraction (systole) and relaxation (diastole) of both atria and ventricles. Duration of one cardiac cycle: 0.88 seconds.
Stages of the Cardiac Cycle
- Atrial Systole: Triggered by the SA node. The atria contract and push blood into the ventricles.
- Ventricular Systole: Triggered by the AV node. The ventricles contract, closing the AV valves (producing the first heart sound “lub”). Blood is pumped into the aorta and pulmonary artery.
- Ventricular Diastole: Ventricles relax, causing closure of semilunar valves (producing the second heart sound “dub”).
- Joint Diastole: A short resting period when both atria and ventricles are relaxed. Blood flows passively from vena cava to atria to ventricles.
Heart Sounds
| Sound | Cause |
|---|---|
| “Lub” | Closure of tricuspid and mitral (bicuspid) valves at the beginning of ventricular systole |
| “Dub” | Closure of aortic and pulmonary valves at the end of ventricular systole |
Dpharmguru’s exam insights:
The first heart sound “lub” is caused by the closure of the AV valves, and the second heart sound “dub” is caused by the closure of the semilunar valves. This is a classic exam question — remember which valves close for each sound!
BASICS OF ECG (ELECTROCARDIOGRAM)
ECG is a record of the electrical activity produced by the heart muscles during each heartbeat. A normal ECG has three main waves: P Wave, QRS Complex, and T Wave.
| Wave | What It Represents | Appearance / Features |
|---|---|---|
| P Wave | Atrial depolarisation (when both atria contract) | Small upward deflection |
| QRS Complex | Ventricular depolarisation (when ventricles contract) | Large upright triangular wave, starts and ends with downward deflection |
| T Wave | Ventricular repolarisation (when ventricles relax) | Dome-shaped, smaller and wider than QRS |
Dpharmguru’s exam insights:
Remember: The P wave represents atrial depolarisation, the QRS complex represents ventricular depolarisation, and the T wave represents ventricular repolarisation. A common exam question is: “Which wave represents ventricular depolarisation?” The answer is the QRS complex.
BLOOD PRESSURE AND ITS REGULATION
Blood Pressure (BP) is the force exerted by the flowing blood on the walls of the arteries. It is measured in millimetres of mercury (mmHg). In a normal adult, BP = 120 / 80 mmHg.
Types of Arterial Blood Pressure
- Systolic Pressure: Maximum pressure during heart contraction. Normal value: 120 mmHg.
- Diastolic Pressure: Minimum pressure during heart relaxation. Normal value: 80 mmHg.
- Pulse Pressure: Difference between systolic and diastolic pressures. Example: 120 – 80 = 40 mmHg.
- Mean Arterial Pressure (MAP): Average pressure during a full cardiac cycle.
Disorders Related to Blood Pressure
- Hypertension (High Blood Pressure): When the arterial pressure is consistently higher than normal. Types: Primary (Essential) Hypertension (90–95% cases) and Secondary Hypertension.
- Hypotension (Low Blood Pressure): When the blood pressure is lower than normal, leading to reduced blood flow to organs. Symptoms include dizziness, fainting, weakness, and fatigue.
Dpharmguru’s exam insights:
The cardiovascular system is the lifeline of the body. Understanding the heart’s anatomy, conduction system, and circulation is essential for understanding how the body maintains homeostasis. In exams, focus on the conduction pathway (SA node → AV node → Bundle of His → Purkinje fibres), the difference between systemic and pulmonary circulation, and the interpretation of ECG waves. These are almost guaranteed to appear!
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
- Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- National Institutes of Health (NIH). (2022). Cardiovascular System Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
