Chapter 25: Circulation

Open circulatory systems, found in many arthropods and mollusks, deliver hemolymph directly to tissues through sinuses, while closed systems—typical of vertebrates and some invertebrates like cephalopods—use vessels to direct blood flow with greater control and efficiency. The authors explain the structure and function of hearts and vessels in various taxa, from the single-circuit systems of fish to the double-circuit systems in mammals and birds, which separate pulmonary and systemic circulation. The evolutionary transition from low- to high-pressure circulatory systems is described, along with the significance of atrial and ventricular separation in supporting endothermy. The anatomy of vertebrate hearts is reviewed in detail, including cardiac muscle properties, valves, and chamber architecture. Cardiac physiology is a central focus, covering how action potentials in pacemaker cells initiate contractions and how electrical signals propagate through the heart's conduction system. The roles of the sinoatrial (SA) node, atrioventricular (AV) node, bundle branches, and Purkinje fibers are explained alongside electrocardiogram (ECG) interpretation. The chapter also details how heart rate and stroke volume are regulated by intrinsic mechanisms (like the Frank-Starling effect) and extrinsic inputs from the autonomic nervous system and circulating hormones. The second half of the chapter shifts to vascular physiology, discussing arteries, veins, and capillaries and their specialized functions. The structure of vessel walls, blood pressure regulation, and capillary exchange processes—including diffusion, bulk flow, and transcytosis—are presented with examples from different animal groups. Special attention is paid to how circulatory patterns vary in diving mammals, ectotherms, and animals with regional blood flow adaptations (e.g., countercurrent heat exchangers). The chapter ends by integrating circulation with gas exchange, thermoregulation, and fluid balance, reinforcing its centrality to whole-body physiology.