Preview
IB DP Biology — Form and Function | SL | B3.2
📋 What You Will Learn
- Identify the structures of the heart and trace the pathway of blood through the double circulatory system
- Explain the cardiac cycle — systole and diastole — including the role of valves in maintaining unidirectional flow
- Analyse pressure and volume changes during the cardiac cycle and interpret pressure–volume graphs
- Explain how heart rate is regulated by the sinoatrial node (SAN), atrioventricular node (AVN), and the autonomic nervous system
- Describe the structure and function of the lymphatic system, including its role in fluid balance and immunity
- Apply understanding of cardiovascular physiology to interpret data on blood pressure, heart rate, and cardiac output
🔍 Guiding Question How does the structure of the heart and the lymphatic system enable the precise regulation of fluid distribution and the maintenance of homeostasis in the body?
🌍 Why This Matters Every minute, your heart contracts approximately 70 times, moving around 5 litres of blood through roughly 100,000 kilometres of vessels — all without a single conscious instruction from you. When this system fails, even briefly, the consequences are catastrophic: a heart attack occurs when cardiac muscle is deprived of oxygen for just a few minutes. Understanding the mechanics of the heart is not abstract biology — it is the difference between life and death, measured in seconds.
The Problem the Heart Had to Solve
Single-celled organisms exchange gases and nutrients directly with their environment by diffusion. But once multicellular life evolved bodies too large for diffusion to reach every cell, a new solution was needed: a dedicated transport system with a pump at its centre. The vertebrate heart is that solution — a muscular organ that generates pressure gradients large enough to drive blood to every tissue in the body.
The human heart is not one pump but two, working in parallel. The right side receives deoxygenated blood from the body and sends it to the lungs. The left side receives oxygenated blood from the lungs and drives it around the entire body. This arrangement — the double circulatory system — keeps oxygenated and deoxygenated blood separate and maintains high pressure in the systemic circulation.
📖 Key Definition — Double Circulatory System A circulatory system in which blood passes through the heart twice for each complete circuit of the body: once through the pulmonary circulation (heart → lungs → heart) and once through the systemic circulation (heart → body → heart). This maintains high blood pressure in the systemic circuit and keeps oxygenated and deoxygenated blood separate.
Structure of the Heart
The heart is a four-chambered muscular organ enclosed in a protective pericardium. Its walls are composed of cardiac muscle — myocardium — a tissue unique in the body because it contracts rhythmically and involuntarily without fatigue. The thickness of each chamber wall directly reflects the pressure it must generate: the left ventricle has the thickest wall of all, because it must drive blood through the entire systemic circulation.
Chambers and Great Vessels
The heart has four chambers: two atria (upper, thin-walled, receiving chambers) and two ventricles (lower, thick-walled, pumping chambers). The right atrium receives deoxygenated blood via the superior and inferior vena cava. The right ventricle pumps it to the lungs via the pulmonary artery. The left atrium receives oxygenated blood from the lungs via the pulmonary veins. The left ventricle pumps it to the body via the aorta.
📖 Key Definition — Atrioventricular Valves Valves located between the atria and ventricles that prevent backflow of blood during ventricular