Human Circulatory System — Revision Notes

⚡ 30-Second Revision

Heart: — 4 chambers (2 atria, 2 ventricles). Right side: deoxygenated. Left side: oxygenated.
Valves: — Tricuspid (R. Atrium-R. Ventricle), Bicuspid/Mitral (L. Atrium-L. Ventricle), Pulmonary (R. Ventricle-Pulmonary Artery), Aortic (L. Ventricle-Aorta).
Cardiac Cycle: — $0.8,\text{s}$. Atrial Systole ($0.1,\text{s}$), Ventricular Systole ($0.3,\text{s}$), Joint Diastole ($0.4,\text{s}$). 'Lub' (AV closure), 'Dub' (SL closure).
Conduction System: — SA Node (pacemaker, $70-75,\text{beats/min}$) $ightarrow$ AV Node $ightarrow$ Bundle of His $ightarrow$ Purkinje Fibers.
ECG: — P-wave (atrial depolarization), QRS complex (ventricular depolarization), T-wave (ventricular repolarization).
Blood Vessels: — Arteries (away from heart, thick, high pressure), Veins (towards heart, thin, low pressure, valves), Capillaries (exchange).
Blood: — Plasma (55%), Formed Elements (45% - RBCs, WBCs, Platelets).
Cardiac Output: — $CO = SV \times HR$ (approx. $5,\text{L/min}$ at rest).

2-Minute Revision

The human circulatory system is a closed, double circulation system driven by the heart. The four-chambered heart (two atria, two ventricles) acts as a pump, separating oxygenated and deoxygenated blood.

Blood flows from the right atrium to the right ventricle (tricuspid valve), then to the lungs via the pulmonary artery. Oxygenated blood returns from the lungs to the left atrium (pulmonary veins), then to the left ventricle (bicuspid/mitral valve), and finally to the body via the aorta.

The cardiac cycle, lasting $0.8,\text{s}$, involves coordinated atrial and ventricular contractions (systole) and relaxations (diastole), producing 'lub-dub' sounds from valve closures. The heart's rhythm is set by the SA node, the natural pacemaker.

ECG records this electrical activity, with P, QRS, and T waves representing atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively. Blood vessels (arteries, veins, capillaries) form the network, while blood itself (plasma, RBCs, WBCs, platelets) transports gases, nutrients, wastes, and immune cells.

Blood groups (ABO, Rh) are crucial for safe transfusions, and platelets are vital for blood clotting.

5-Minute Revision

The human circulatory system is a highly efficient transport network, featuring a four-chambered heart, an extensive network of blood vessels, and blood as the transport medium. The heart ensures double circulation: the pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs for oxygenation and returns oxygenated blood to the left atrium; the systemic circuit pumps oxygenated blood from the left ventricle to the entire body and returns deoxygenated blood to the right atrium.

Valves (tricuspid, bicuspid, pulmonary, aortic) prevent backflow. The cardiac cycle is the rhythmic sequence of events in one heartbeat ($0.8,\text{s}$), starting with atrial systole ($0.1,\text{s}$), followed by ventricular systole ($0.

3, ext{s}$), and concluding with joint diastole ($0.4, ext{s}$). The 'lub' sound marks AV valve closure, and 'dub' marks SL valve closure. The heart's intrinsic rhythm is generated by the SA node (pacemaker), with impulses propagating through the AV node, Bundle of His, and Purkinje fibers.

An ECG visually represents this electrical activity: P-wave for atrial depolarization, QRS complex for ventricular depolarization, and T-wave for ventricular repolarization. Blood vessels include thick-walled arteries (high pressure, away from heart), thin-walled veins (low pressure, towards heart, with valves), and microscopic capillaries (site of exchange).

Blood comprises plasma (water, proteins, solutes) and formed elements: RBCs (hemoglobin for $O_2$ transport), WBCs (immunity - neutrophils, eosinophils, basophils, lymphocytes, monocytes), and platelets (blood clotting).

Blood groups (ABO, Rh) are determined by antigens on RBCs and antibodies in plasma, critical for transfusion compatibility. Blood coagulation is a cascade involving platelets and clotting factors, converting fibrinogen to fibrin to form a clot.

Cardiac activity is regulated by the autonomic nervous system (sympathetic increases, parasympathetic decreases heart rate) and hormones (e.g., adrenaline).

Prelims Revision Notes

Heart Structure:

- Location: Mediastinum, slightly left. - Chambers: 2 Atria (receiving), 2 Ventricles (pumping). - Septa: Interatrial, Interventricular, Atrioventricular. - Valves: - Tricuspid: Right atrium & right ventricle. - Bicuspid (Mitral): Left atrium & left ventricle. - Pulmonary Semilunar: Right ventricle & pulmonary artery. - Aortic Semilunar: Left ventricle & aorta. - Chordae tendineae & Papillary muscles: Prevent AV valve inversion.

Cardiac Cycle:

- Duration: $0.8,\text{s}$ (at $72,\text{beats/min}$). - Atrial Systole: $0.1,\text{s}$. Atria contract, blood to ventricles. - Ventricular Systole: $0.3,\text{s}$. Ventricles contract. AV valves close ('lub'), then SL valves open, blood ejected. - Joint Diastole: $0.4,\text{s}$. All chambers relax. SL valves close ('dub'), AV valves open, passive filling. - Cardiac Output ($CO$): $SV \times HR$. Normal: $5,\text{L/min}$.

Conduction System:

- SA Node: Pacemaker, in right atrium. $70-75,\text{beats/min}$. - AV Node: Base of interatrial septum. Delays impulse. - Bundle of His: Extends from AV node, divides into branches. - Purkinje Fibers: Distribute impulse to ventricular muscle.

ECG:

- P-wave: Atrial depolarization. - QRS complex: Ventricular depolarization (atrial repolarization masked). - T-wave: Ventricular repolarization.

Blood Vessels:

- Arteries: Thick, elastic, muscular. Carry blood AWAY from heart. High pressure. (Pulmonary artery: deoxygenated). - Veins: Thin-walled, larger lumen, valves. Carry blood TOWARDS heart. Low pressure. (Pulmonary veins: oxygenated). - Capillaries: Single-cell thick, exchange site.

Blood Composition:

- Plasma (55%): Water (90-92%), Proteins (albumin, globulins, fibrinogen), Glucose, Amino acids, Hormones, Salts. - Formed Elements (45%): - RBCs (Erythrocytes): Biconcave, anucleated, hemoglobin. $5-5.

5, ext{million/mm}^3$. Life span$120, ext{days}$. - WBCs (Leukocytes): Nucleated, immune function.$6000-8000/ ext{mm}^3$. - Granulocytes: Neutrophils (60-65%, phagocytic), Eosinophils (2-3%, allergy, parasites), Basophils (0.

5-1%, histamine, serotonin, heparin). - Agranulocytes: Lymphocytes (20-25%, B & T cells, specific immunity), Monocytes (6-8%, phagocytic, macrophages). - Platelets (Thrombocytes): Cell fragments, clotting.

$1.5-3.5,\text{lakhs/mm}^3$.

Blood Groups (ABO & Rh):

- ABO: A, B, AB, O. O is universal donor, AB is universal recipient. - Rh: Rh+ (antigen D present), Rh- (antigen D absent). Rh incompatibility: Erythroblastosis fetalis.

Blood Coagulation: — Platelets $ightarrow$ Thromboplastin $ightarrow$ Prothrombin to Thrombin $ightarrow$ Fibrinogen to Fibrin $ightarrow$ Clot.
Circulatory Pathways:

- Double Circulation: Pulmonary (heart-lungs-heart) & Systemic (heart-body-heart). - Hepatic Portal System: Intestine $ightarrow$ Liver $ightarrow$ Heart.

Regulation:

- Neural: Medulla oblongata. Sympathetic (increases HR), Parasympathetic (decreases HR). - Hormonal: Adrenaline, Noradrenaline (increase HR).

Vyyuha Quick Recall

To remember the order of blood flow through the heart chambers and major vessels (starting from the body):

Really Red People Love Large Apples

Right Atrium (receives deoxygenated blood from body)
Right Ventricle (pumps to lungs)
Pulmonary Artery (to lungs)
Lungs (blood gets oxygenated)
Pulmonary Veins (to heart)
Left Atrium (receives oxygenated blood)
Left Ventricle (pumps to body)
Aorta (to body)