Describe the double circulation in terms of circulation to the lungs and circulation to the body.

Transport in Humans – Double Circulation

Learning Objective

Describe the double circulation in terms of:

• the pulmonary circuit (circulation to the lungs)
• the systemic circuit (circulation to the body)

Overview of the Circulatory System

• The circulatory system is a closed network of blood vessels that transports nutrients, gases and waste products.
• The heart is a muscular pump that drives blood round the system.
• Four one‑way valves (tricuspid, pulmonary, mitral/bicuspid, aortic) prevent back‑flow, ensuring a single direction of blood movement in both the pulmonary and systemic circuits.
• Blood travels in two separate loops:
  • Pulmonary circuit – heart ↔ lungs
  • Systemic circuit – heart ↔ body

Structure of the Mammalian Heart (Core – Syllabus 9.2)

Monitoring Heart Activity (Core – Syllabus 9.2)

• Pulse – palpable rhythmic expansion of an artery (e.g., radial pulse).
• Electrocardiogram (ECG) – records the electrical activity of the heart.
  • P wave – atrial depolarisation (atria contract).
  • Q‑R‑S‑T complex – ventricular depolarisation (QRS) and repolarisation (T).
  • The pattern is a reliable diagnostic tool for rhythm disorders.
• Heart sounds – “lub‑dub” heard with a stethoscope.
  • “Lub” = closure of the atrioventricular (tricuspid and mitral) valves.
  • “Dub” = closure of the semilunar (pulmonary and aortic) valves.

Regulation of Heart Rate (Core – Syllabus 14)

• Autonomic nervous system:
  • Sympathetic stimulation → ↑ heart rate and contractility (via adrenaline).
  • Parasympathetic (vagus nerve) → ↓ heart rate.
• Physical activity – heart rate rises to meet increased oxygen demand.
  • Resting (adult): 60–100 bpm.
  • Brisk walk / light jog: 100–130 bpm.
  • Sprinting: 150–190 bpm (≈ 220 − age maximum).
• After exercise the rate gradually returns to resting level as the body recovers.

Blood Vessels – Structure & Function (Core – Syllabus 9.3)

• Arteries – thick, muscular walls; high blood pressure; carry blood away from the heart.
• Veins – thinner walls, larger lumen; contain valves to prevent back‑flow; carry blood toward the heart.
• Capillaries – one‑cell‑thick walls; site of exchange of gases, nutrients and wastes between blood and tissues.

Blood – Components & Functions (Core – Syllabus 9.4)

• Red blood cells (RBCs) – contain haemoglobin; transport O₂ and CO₂.
• White blood cells (WBCs) – part of the immune system; defend against infection.
• Platelets (thrombocytes) – form clots to stop bleeding.
• Plasma – liquid matrix (≈ 55 % of blood); carries nutrients, hormones, waste products and proteins (e.g., albumin, clotting factors).

Pulmonary Circulation (Circulation to the Lungs)

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.
2. It passes through the tricuspid valve into the right ventricle.
3. Contraction of the right ventricle forces blood through the pulmonary valve into the pulmonary artery.
4. The pulmonary artery carries the blood to the lungs where, in the pulmonary capillaries surrounding the alveoli, O₂ diffuses into the blood and CO₂ diffuses out.
5. Oxygen‑rich blood returns to the heart via the four pulmonary veins, emptying into the left atrium.

Systemic Circulation (Circulation to the Body)

1. Oxygen‑rich blood flows from the left atrium through the mitral (bicuspid) valve into the left ventricle.
2. The left ventricle contracts, pushing blood through the aortic valve into the aorta.
3. The aorta branches into systemic arteries that distribute oxygenated blood to all body tissues.
4. In the systemic capillaries, oxygen and nutrients are delivered to cells while waste products and CO₂ are collected.
5. Deoxygenated blood returns to the heart through systemic veins, converging into the superior and inferior vena cava, and re‑enters the right atrium.

Comparison of Pulmonary and Systemic Circulation

Heart Disease – Coronary Heart Disease (Core – Syllabus 10)

• Cause – atherosclerotic plaque blocks a coronary artery, reducing blood supply to heart muscle.
• Risk factors – high‑fat diet, smoking, physical inactivity, obesity, high blood pressure, high cholesterol.
• Prevention – balanced diet, regular exercise, avoid smoking, maintain healthy weight, control blood pressure and cholesterol.

Key Terms

• Pulmonary artery – carries deoxygenated blood from the right ventricle to the lungs.
• Pulmonary veins – carry oxygenated blood from the lungs to the left atrium.
• Aorta – main artery that distributes oxygen‑rich blood to the whole body.
• Vena cava (superior & inferior) – large veins that return deoxygenated blood to the right atrium.
• Coronary arteries – supply the heart muscle itself with oxygenated blood.
• Pericardium – protective fibrous sac surrounding the heart.
• Valves (tricuspid, pulmonary, mitral, aortic) – ensure one‑way flow of blood through the heart and both circuits.
• Systemic arteries & veins – vessels of the body circuit.
• Capillaries – thin‑walled vessels where exchange of gases, nutrients and wastes occurs.