Question
Trace the path of blood through the human heart and lungs. Explain double circulation and why it is important for mammals.
(NEET, CBSE Class 11 — Body Fluids and Circulation)
Solution — Step by Step
Deoxygenated blood from the body returns to the heart via the superior vena cava (from head and upper body) and inferior vena cava (from lower body). This blood enters the right atrium.
The right atrium contracts and pushes blood through the tricuspid valve into the right ventricle. The tricuspid has 3 flaps and prevents backflow.
The right ventricle contracts and pumps deoxygenated blood through the pulmonary valve into the pulmonary artery. This is the only artery that carries deoxygenated blood. The blood goes to both lungs for gas exchange.
In the lung capillaries (alveoli), CO is released and O is absorbed. The now oxygenated blood returns via pulmonary veins (the only veins carrying oxygenated blood) to the left atrium.
Left atrium contracts, pushing blood through the bicuspid (mitral) valve into the left ventricle. The left ventricle (thickest wall) contracts powerfully, sending blood through the aortic valve into the aorta, which distributes oxygenated blood to the entire body.
graph TD
A["Body (deoxygenated)"] -->|"Vena cava"| B["Right Atrium"]
B -->|"Tricuspid valve"| C["Right Ventricle"]
C -->|"Pulmonary artery"| D["Lungs"]
D -->|"Gas exchange: CO₂ out, O₂ in"| E["Pulmonary veins"]
E --> F["Left Atrium"]
F -->|"Bicuspid valve"| G["Left Ventricle"]
G -->|"Aorta"| H["Body (oxygenated)"]
H -->|"Capillaries → Veins"| AWhy This Works
Humans have double circulation: blood passes through the heart twice in one complete circuit — once through the right side (pulmonary circuit) and once through the left side (systemic circuit).
Why double? In single circulation (like fish), blood loses pressure after passing through gill capillaries, so systemic flow is slow. In double circulation, the left ventricle gives blood a fresh, powerful push. This maintains high blood pressure in the systemic circuit, allowing efficient delivery of O to metabolically active tissues.
The left ventricle wall is thicker than the right because it needs to generate enough pressure to push blood throughout the entire body, while the right ventricle only needs to push blood to the nearby lungs.
Alternative Method — Following a Single RBC
Track one red blood cell: Right atrium, right ventricle, pulmonary artery, lung capillary (picks up O), pulmonary vein, left atrium, left ventricle, aorta, body capillary (delivers O), vein, vena cava, back to right atrium. One full loop.
Remember the valve names: tricuspid on the right (3 letters in “tri”, right side), bicuspid on the left (also called mitral). Both are AV (atrioventricular) valves. The semilunar valves are at the exits: pulmonary and aortic.
Common Mistake
The biggest confusion: students think arteries always carry oxygenated blood. Wrong. Pulmonary arteries carry deoxygenated blood (from heart to lungs). Similarly, pulmonary veins carry oxygenated blood (from lungs to heart). The definition of artery/vein is based on direction (away from/towards heart), not oxygen content.