Q.What is meant by double circulation? Describe the pulmonary and systemic circuits, explain how cardiac activity is regulated by the nodal system and the autonomic nervous system, and briefly describe two disorders of the circulatory system covered in this chapter.

Step 1 — Double circulation. Deoxygenated blood from the body enters the right atrium, passes to the right ventricle and is pumped via the pulmonary artery to the lungs (the pulmonary circuit), returning oxygenated via the pulmonary veins to the left atrium — one pass through the heart. Oxygenated blood then passes to the left ventricle and is pumped via the aorta to the body's tissues (the systemic circuit), returning deoxygenated via the venae cavae to the right atrium — the second pass. Because the heart's septa completely separate its left and right sides, oxygenated and deoxygenated blood never mix, keeping the blood delivered to the body's tissues fully oxygenated.

Step 2 — Regulation of cardiac activity. The heart is myogenic: the sino-atrial (SA) node generates its own spontaneous impulses faster than any other cardiac tissue, making it the pacemaker; the impulse spreads across the atria, is briefly delayed at the atrio-ventricular (AV) node (letting the atria finish emptying first), then passes rapidly via the bundle of His and Purkinje fibres through both ventricles so they contract together. This intrinsic rhythm is fine-tuned by the autonomic nervous system — sympathetic nerves raise heart rate and force (as in exercise or stress), the parasympathetic vagus nerve lowers heart rate — and by the hormone adrenaline, which reinforces the sympathetic effect. …

Address the three parts in turn — trace both circuits of double circulation, then the nodal + nervous regulation of heartbeat, then briefly describ …

• Describing only one circuit (commonly just the systemic) and omitting the pulmonary circuit. …