❤️ Heart and Circulation
Learn heart anatomy, cardiac cycle and double circulation for CUET Agriculture. SA-AV nodes, ECG waves, blood pressure and cardiac output.
Heart
The heart is a tireless muscular pump that beats approximately 100,000 times per day, circulating blood through about 96,000 km of blood vessels. It is the central organ of the cardiovascular system.
General Features
- Weight: approximately ~300 g (about the weight of a clenched fist)
- Size: 12 cm x 8.5 cm x 6 cm (roughly the size of your fist)
- Located in the mediastinum — the central compartment of the thoracic cavity, between the two lungs, slightly left of the midline (about two-thirds of the heart lies to the left)
- Enclosed in the pericardium, a double-walled protective sac. Between the two layers lies pericardial fluid (~15–50 ml), which acts as a lubricant, reducing friction during heartbeats.
Wall Layers
The heart wall consists of three distinct layers, each with a specific structural and functional role:
| Layer | Position | Composition |
|---|---|---|
| Epicardium | Outermost (= visceral pericardium) | A thin serous membrane covered with fat; the coronary blood vessels that supply the heart muscle itself run through this layer |
| Myocardium | Middle (thickest layer) | Composed of cardiac muscle — involuntary, striated muscle found only in the heart. The myocardium is thickest in the left ventricle (~1.3 cm) because it pumps blood to the entire body under high pressure. |
| Endocardium | Innermost | A smooth endothelial lining that reduces friction as blood flows through the chambers. It also covers the heart valves. |
Chambers and Valves
The heart has 4 chambers: 2 atria (upper chambers, thin-walled — they receive blood and push it a short distance into ventricles) and 2 ventricles (lower chambers, thick-walled — they pump blood out of the heart into arteries).
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Heart
The heart is a tireless muscular pump that beats approximately 100,000 times per day, circulating blood through about 96,000 km of blood vessels. It is the central organ of the cardiovascular system.
General Features
- Weight: approximately ~300 g (about the weight of a clenched fist)
- Size: 12 cm x 8.5 cm x 6 cm (roughly the size of your fist)
- Located in the mediastinum — the central compartment of the thoracic cavity, between the two lungs, slightly left of the midline (about two-thirds of the heart lies to the left)
- Enclosed in the pericardium, a double-walled protective sac. Between the two layers lies pericardial fluid (~15–50 ml), which acts as a lubricant, reducing friction during heartbeats.
Wall Layers
The heart wall consists of three distinct layers, each with a specific structural and functional role:
| Layer | Position | Composition |
|---|---|---|
| Epicardium | Outermost (= visceral pericardium) | A thin serous membrane covered with fat; the coronary blood vessels that supply the heart muscle itself run through this layer |
| Myocardium | Middle (thickest layer) | Composed of cardiac muscle — involuntary, striated muscle found only in the heart. The myocardium is thickest in the left ventricle (~1.3 cm) because it pumps blood to the entire body under high pressure. |
| Endocardium | Innermost | A smooth endothelial lining that reduces friction as blood flows through the chambers. It also covers the heart valves. |
Chambers and Valves
The heart has 4 chambers: 2 atria (upper chambers, thin-walled — they receive blood and push it a short distance into ventricles) and 2 ventricles (lower chambers, thick-walled — they pump blood out of the heart into arteries).
The left ventricle has the thickest wall of all chambers because it must generate enough force to pump blood through the aorta to the entire body (systemic circulation). The right ventricle has a thinner wall because it only pumps blood to the nearby lungs (pulmonary circulation).
| Valve | Location | Function |
|---|---|---|
| Tricuspid valve | Between right atrium and right ventricle | Prevents backflow from RV to RA. Has 3 flaps (cusps) connected to the ventricular wall by chordae tendineae ("heart strings") and papillary muscles that prevent valve eversion during ventricular contraction. |
| Bicuspid / Mitral valve | Between left atrium and left ventricle | Prevents backflow from LV to LA. Has 2 flaps. Called "mitral" because it resembles a bishop's mitre (hat). |
| Pulmonary semilunar valve | At the exit of right ventricle → pulmonary artery | Prevents backflow of blood from the pulmonary artery back into the RV between heartbeats |
| Aortic semilunar valve | At the exit of left ventricle → aorta | Prevents backflow of blood from the aorta back into the LV between heartbeats |
Foramen ovale: An opening between the right and left atria present in the fetal heart. Since fetal lungs are non-functional (the fetus receives oxygenated blood from the placenta), this opening allows blood to bypass the pulmonary circulation. It normally closes at birth when the baby takes its first breath → the closed remnant becomes the fossa ovalis.
TIP
Remember the valve names by counting: Tricuspid = 3 cusps on the Right side. Bicuspid (mitral) = 2 cusps on the Left side. The semilunar valves each have 3 half-moon-shaped cusps.
Cardiac Cycle
One complete heartbeat constitutes one cardiac cycle. At a normal resting heart rate of 72 beats per minute, each cycle takes 0.8 seconds. The cycle consists of alternating periods of contraction (systole) and relaxation (diastole).
| Phase | Duration | Events |
|---|---|---|
| Atrial systole | 0.1 s | Both atria contract simultaneously → push remaining blood into the ventricles (ventricles are already ~70% full from passive filling). AV valves (tricuspid and bicuspid) are open; semilunar valves are closed. |
| Ventricular systole | 0.3 s | Both ventricles contract powerfully → push blood into the arteries. AV valves close → produces the first heart sound ("lub" — S₁). Semilunar valves open as ventricular pressure exceeds arterial pressure. |
| Joint diastole (complete relaxation) | 0.4 s | All four chambers relax. Semilunar valves close as arterial blood tries to flow back → produces the second heart sound ("dub" — S₂). Blood fills atria from veins and passively flows into ventricles through open AV valves. |
Key timing calculations:
- Atrial diastole (total relaxation time for atria): 0.7 s (atria are resting during ventricular systole + joint diastole)
- Ventricular diastole (total relaxation time for ventricles): 0.5 s (ventricles rest during atrial systole + joint diastole)
- Cardiac output (CO) = Stroke volume x Heart rate = 70 ml x 72 = ~5040 ml/min (~5 L/min)
- Stroke volume: ~70 ml (the volume of blood pumped by one ventricle per beat)
IMPORTANT
The heart rests more than it works! During each 0.8-second cycle, the ventricles contract for only 0.3 s and rest for 0.5 s. This rest period is crucial — it allows the heart muscle to receive its own blood supply through the coronary arteries (coronary blood flow occurs mainly during diastole).
What is cardiac reserve?
**Cardiac reserve** is the difference between the resting cardiac output and the maximum cardiac output the heart can achieve. At rest, cardiac output is ~5 L/min. During intense exercise, the heart can increase its output to 20–35 L/min (in trained athletes) by increasing both heart rate and stroke volume. This 4–7x increase is the cardiac reserve, and it reflects the heart's ability to respond to increased demand.Conducting System of the Heart
The heart has its own built-in electrical conduction system that generates and distributes impulses for rhythmic, coordinated contraction. This is what makes the heart myogenic — its beat originates within the heart muscle itself, independent of external nerve supply.
| Structure | Location | Function |
|---|---|---|
| SA node (Sinoatrial node) | Right atrium, near the opening of the SVC | The natural pacemaker of the heart — generates electrical impulses at a rate of ~72 impulses/min. It sets the rhythm for the entire heart. |
| AV node (Atrioventricular node) | Right atrium, near the interatrial septum, just above the tricuspid valve | Receives the impulse from the SA node and delays it by ~0.1 second. This critical delay ensures the atria finish emptying before the ventricles begin to contract. |
| Bundle of His | Interventricular septum | Conducts the impulse from the AV node downward into the ventricular septum. It divides into right and left bundle branches, one for each ventricle. |
| Purkinje fibers | Ventricular walls (subendocardial network) | Rapidly distribute the impulse throughout the ventricular myocardium, ensuring all ventricular muscle cells contract almost simultaneously for an efficient, coordinated pump. |
- The heart is myogenic — the impulse originates in the heart muscle itself (SA node), not from nerves. Even a heart removed from the body can continue beating if kept in appropriate conditions.
- The autonomic nervous system modulates (but does not initiate) the heart rate:
- Sympathetic nerves → release norepinephrine → increases heart rate and force of contraction (during exercise, stress, excitement)
- Parasympathetic nerve (vagus nerve) → releases acetylcholine → decreases heart rate (during rest, sleep)
Blood Pressure
Blood pressure is the force exerted by circulating blood against the walls of arteries. It is generated by ventricular contraction and maintained by the elasticity of arterial walls.
- Systolic pressure: pressure during ventricular contraction — the peak pressure when the heart ejects blood = 110–130 mmHg (normal ~120 mmHg)
- Diastolic pressure: pressure during ventricular relaxation — the baseline pressure when the heart is filling = 60–80 mmHg (normal ~80 mmHg)
- Normal BP: 120/80 mmHg (read as "120 over 80")
- Measured using a sphygmomanometer (mercury or aneroid type) with a stethoscope placed over the brachial artery
- Hypertension (high BP): sustained BP >140/90 mmHg — a "silent killer" that increases the risk of heart attack, stroke, and kidney damage. Often has no symptoms until serious damage occurs.
- Hypotension (low BP): BP <90/60 mmHg — can cause dizziness, fainting, and in severe cases, shock
NOTE
Pulse pressure = Systolic pressure − Diastolic pressure = 120 − 80 = 40 mmHg. This represents the force of each heartbeat. A wide pulse pressure may indicate stiff arteries (common in elderly).
Electrocardiogram (ECG / EKG)
An ECG is a graphical recording of the electrical activity of the heart as detected by electrodes placed on the skin. It is one of the most common and valuable diagnostic tools in cardiology, providing information about heart rate, rhythm, and the condition of the heart muscle.
| Wave / Interval | Represents |
|---|---|
| P wave | Atrial depolarization — the electrical signal spreading across both atria, triggering atrial contraction. A small, rounded wave. |
| QRS complex | Ventricular depolarization — the electrical signal spreading through both ventricles, triggering ventricular contraction. The largest wave because ventricular muscle mass is much greater than atrial. |
| T wave | Ventricular repolarization — the electrical recovery (resetting) of ventricular muscle cells, preparing them for the next contraction |
| PR interval | Time from the start of atrial depolarization to the start of ventricular depolarization — includes the AV node delay (~0.12–0.20 s). A prolonged PR interval suggests an AV conduction block. |
| ST segment | Period when ventricles are fully depolarized (plateau phase) — all ventricular cells are contracted. Elevation or depression of this segment can indicate myocardial infarction (heart attack) or ischemia. |
| QT interval | Total ventricular activity — from the start of depolarization to the end of repolarization |
- Atrial repolarization is hidden within the QRS complex — the much larger ventricular depolarization signal masks the smaller atrial repolarization signal.
- A standard ECG is recorded using 12 leads: standard limb leads (I, II, III), augmented limb leads (aVR, aVL, aVF), and precordial/chest leads (V1–V6). Each lead provides a different "angle" of viewing the heart's electrical activity.
What does a flat line (asystole) on an ECG mean?
A completely flat line on an ECG (no P waves, no QRS, no T waves) is called **asystole** — it means there is no electrical activity in the heart. This is a form of cardiac arrest and is a medical emergency requiring immediate CPR. Note that asystole is different from ventricular fibrillation (chaotic electrical activity) — a defibrillator is used for fibrillation but is generally ineffective for asystole.Cardiovascular Diseases
| Disease | Features |
|---|---|
| Atherosclerosis | Deposition of cholesterol plaques (atheromas) in the inner walls of arteries → progressive narrowing of the arterial lumen → reduced blood flow. The leading cause of heart attacks and strokes. Risk factors include high cholesterol, smoking, hypertension, diabetes, and obesity. |
| Arteriosclerosis | General term for hardening and loss of elasticity of arterial walls. Includes atherosclerosis but also refers to age-related calcification. Results in higher blood pressure. |
| Angina pectoris | Chest pain caused by reduced blood supply to the heart muscle (myocardial ischemia), usually during physical exertion or emotional stress. Pain is typically relieved by rest or nitroglycerin (which dilates coronary arteries). |
| Myocardial infarction (Heart attack) | Complete blockage of a coronary artery (usually by a blood clot on a ruptured atherosclerotic plaque) → death of heart muscle tissue in the area supplied by that artery. A medical emergency — symptoms include severe chest pain, sweating, breathlessness. |
| Coronary Heart Disease (CHD) | Disease of the coronary arteries (the arteries that supply blood to the heart muscle itself). Encompasses both angina and MI. The leading cause of death globally. |
| Blue baby syndrome | Cyanosis (blue discoloration) in newborns due to congenital heart defects. The most common cause is Tetralogy of Fallot — a combination of 4 defects: VSD (ventricular septal defect), pulmonary stenosis (narrowing of pulmonary valve), overriding aorta (aorta positioned over VSD), and right ventricular hypertrophy (thickened RV wall). These defects cause mixing of oxygenated and deoxygenated blood. |
| Heart failure | The heart is unable to pump sufficient blood to meet the body's demands. Left-sided failure → blood backs up into lungs (pulmonary congestion, breathlessness). Right-sided failure → blood backs up into body (peripheral edema, liver congestion). |
| Cardiac arrest | Sudden cessation of effective heart function — the heart either stops beating or beats chaotically (fibrillation). Requires immediate CPR and defibrillation. |
| Varicose veins | Swollen, twisted, bulging veins (usually in the legs) caused by faulty venous valves that allow blood to pool and flow backward. Worsened by prolonged standing, pregnancy, and obesity. |
Double Circulation
Humans have a double circulatory system — blood passes through the heart twice in one complete circuit of the body. This is a key evolutionary advancement that allows mammals and birds to maintain high metabolic rates.
| Circuit | Path |
|---|---|
| Pulmonary circulation | RV → Pulmonary artery (carries deoxygenated blood — the only artery to do so) → Lungs (gas exchange: CO₂ released, O₂ picked up) → Pulmonary veins (carry oxygenated blood — the only veins to do so) → LA |
| Systemic circulation | LV → Aorta → Body tissues (delivers O₂ and nutrients, picks up CO₂ and waste) → Vena cava (SVC from upper body, IVC from lower body) → RA |
IMPORTANT
Advantage of double circulation: It maintains high pressure in the systemic circuit (needed to push blood to distant tissues like the toes and brain) while keeping low pressure in the pulmonary circuit (high pressure would damage the delicate alveolar capillaries). Oxygenated and deoxygenated blood are kept completely separate in mammals, ensuring maximum oxygen delivery.
How does circulation differ in fish, amphibians, and mammals?
- **Fish**: **Single circulation** — blood passes through the heart only once per circuit (Heart → Gills → Body → Heart). The heart has only 2 chambers (1 atrium + 1 ventricle). Blood pressure is low after passing through gill capillaries. - **Amphibians** (e.g., frog): **Incomplete double circulation** — 3-chambered heart (2 atria + 1 ventricle). Oxygenated and deoxygenated blood partially mix in the single ventricle. - **Reptiles** (except crocodiles): 3-chambered heart with a partial ventricular septum — less mixing than amphibians but still incomplete separation. - **Mammals and birds**: **Complete double circulation** — 4-chambered heart with complete separation of oxygenated and deoxygenated blood. This is the most efficient arrangement.Beginner's Box — Practice Questions
Set 1: Blood Composition
-
The total blood volume in a healthy adult is approximately: Answer: 5–7 litres
-
The lifespan of a mature RBC is: Answer: ~120 days
-
Which organ is known as the "graveyard of RBCs"? Answer: Spleen
-
Normal hemoglobin concentration in males is: Answer: 13–17 g/100 ml
-
The largest type of WBC is: Answer: Monocyte (15–20 μm)
Set 2: Blood Groups and Clotting
-
Universal donor blood group is: Answer: O
-
Universal recipient blood group is: Answer: AB
-
Erythroblastosis fetalis occurs when: Answer: Rh− mother carries Rh+ fetus (second pregnancy)
-
Vitamin K is essential for: Answer: Synthesis of prothrombin and clotting factors VII, IX, X
-
Normal clotting time of blood is: Answer: 3–6 minutes
Set 3: Heart
-
The pacemaker of the heart is: Answer: SA node (Sinoatrial node)
-
Duration of one cardiac cycle at 72 beats/min: Answer: 0.8 seconds
-
The first heart sound ("lub") is produced by closure of: Answer: AV valves (tricuspid and bicuspid)
-
Normal blood pressure is: Answer: 120/80 mmHg
-
P wave in ECG represents: Answer: Atrial depolarization
Set 4: Circulation
-
Cardiac output per minute = ? Answer: ~5 L/min (70 ml x 72)
-
QRS complex in ECG represents: Answer: Ventricular depolarization
-
The fetal structure that connects the two atria is: Answer: Foramen ovale
-
Blue baby syndrome is most commonly due to: Answer: Tetralogy of Fallot (congenital heart defect)
-
Lymph differs from blood in that it lacks: Answer: RBCs (and hemoglobin)
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Heart — Position and Size | Located in mediastinum (between lungs), slightly left of midline Size: approximately a closed fist (~300 g) Enclosed in pericardium (double-walled sac with pericardial fluid for lubrication) |
| Heart Wall Layers | Epicardium (outer, visceral pericardium) → Myocardium (middle, thickest, cardiac muscle) → Endocardium (inner, smooth endothelium) Left ventricular wall is 3x thicker than right (pumps blood to entire body) |
| Heart Chambers | 2 atria (upper, thin-walled, receiving chambers) + 2 ventricles (lower, thick-walled, pumping chambers) Right side: deoxygenated blood Left side: oxygenated blood |
| Heart Valves | Tricuspid: between right atrium and right ventricle (3 cusps) Bicuspid/Mitral: between left atrium and left ventricle (2 cusps) Pulmonary semilunar: right ventricle → pulmonary artery Aortic semilunar: left ventricle → aorta Chordae tendineae + papillary muscles prevent valve inversion |
| Heart Sounds | S1 ("lub"): closure of AV valves (tricuspid + bicuspid) — start of systole S2 ("dub"): closure of semilunar valves — start of diastole Murmur: abnormal sound due to defective valves |
| Foramen Ovale | Opening between right and left atria in foetal heart (bypasses non-functional lungs) Closes at birth → becomes fossa ovalis Failure to close → atrial septal defect |
| Cardiac Cycle | Duration: 0.8 seconds at 72 bpm Atrial systole: 0.1 sec (atria contract, ventricles relax) Ventricular systole: 0.3 sec (ventricles contract, atria relax) Joint diastole: 0.4 sec (all chambers relax, heart fills) Stroke volume: ~70 ml Cardiac output: ~5 L/min (70 ml × 72) |
| Conducting System (Pacemaker) | SA node (sino-atrial node): "pacemaker", in right atrium, generates impulse at 72 beats/min → AV node (atrio-ventricular node): at AV junction, delays impulse 0.1 sec (allows atria to empty before ventricles contract) → Bundle of His: passes through interventricular septum, splits into left and right bundle branches → Purkinje fibres: spread impulse through ventricular walls → coordinated contraction |
| Blood Pressure | Normal: 120/80 mmHg (systolic/diastolic) Hypertension: >140/90 mmHg Hypotension: <90/60 mmHg Measured with sphygmomanometer Pulse pressure = systolic − diastolic = ~40 mmHg |
| ECG (Electrocardiogram) | P wave: atrial depolarization (atrial contraction) QRS complex: ventricular depolarization (ventricular contraction) T wave: ventricular repolarization (ventricular relaxation) P-Q interval: AV node delay Standard 12-lead ECG; invented by Einthoven (Nobel 1924) |
| Double Circulation | Blood passes through heart twice per complete circuit: Pulmonary circulation: RV → pulmonary artery → lungs (gas exchange) → pulmonary veins → LA Systemic circulation: LV → aorta → body tissues → venae cavae → RA Portal circulation: blood passes through two capillary beds (e.g., hepatic portal system: intestine → liver) |
| Cardiovascular Diseases | Atherosclerosis: fatty plaques (cholesterol) in arteries → narrowing Coronary artery disease (CAD): atherosclerosis of coronary arteries Myocardial infarction (heart attack): blockage of coronary artery → cardiac muscle death Angina pectoris: chest pain due to reduced blood flow to heart Heart failure (CHF): heart cannot pump enough blood Blue baby syndrome: Tetralogy of Fallot (4 congenital defects → mixing of oxygenated and deoxygenated blood) |
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