Circulatory Systems in Animals: Open, Closed, and Double Circulation
Open and Closed Circulatory Systems
Open Circulatory System
In an open circulatory system, the heart pumps fluid (hemolymph) through open-ended vessels that lead to tissues and body cavities, directly bathing the cells. Molluscs (except cephalopods) exemplify this system. Cephalopods have a compartmentalized heart with one ventricle and one or two atria, enclosed within a pericardial cavity. Arthropods possess a tubular heart, a thickened dorsal vessel. Hemolymph is pumped from the heart into arteries, then into tissue spaces, and returns to the heart through ostioles (small openings with valves).
Echinoderms lack a true circulatory system, as they have no propulsive heart.
Closed Circulatory System
In a closed circulatory system, fluid circulates within closed vessels and does not directly bathe the cells. Annelids have two main blood vessels (dorsal and ventral). The dorsal vessel, with contractile properties and valves, acts as a primitive heart, propelling fluid forward, while the ventral vessel carries it backward.
Cephalopods’ closed system includes a compartmentalized heart (2-4 atria, one ventricle). Blood travels from the ventricle through vessels, reaching the gills for oxygenation before returning to the heart.
Vertebrates have a closed system with a muscular, ventral heart pumping blood through arteries, capillaries, and veins. Arteries branch into smaller arterioles, then into thin-walled capillaries for nutrient and waste exchange. Capillaries merge into venules, which then form larger veins returning blood to the heart. Valves within the heart prevent backflow.
Types of Circulation
Single Circulation
Fish have single circulation. Their heart (one atrium, one muscular ventricle) receives venous blood. Blood flows from the ventricle to the gills for oxygenation, then to the body, returning to the heart via a venous sinus, atrium, and finally the ventricle.
Double Circulation
Double circulation is characteristic of lunged vertebrates. Two circuits exist:
- Pulmonary circuit: Blood travels from the heart through pulmonary arteries to the lungs for oxygenation, returning to the heart via pulmonary veins.
- Systemic circuit: Oxygenated blood leaves the heart through the aorta, circulates throughout the body, and returns via the venae cavae.
Double circulation can be incomplete (oxygenated and deoxygenated blood mix in the heart) or complete (no mixing). Amphibians have incomplete double circulation with two atria and one ventricle. Reptiles exhibit less mixing than amphibians, with a partially divided ventricle. Crocodiles, birds, and mammals have complete double circulation, with a four-chambered heart (two atria, two ventricles), ensuring no mixing of oxygenated and deoxygenated blood.
Regulation of Human Blood Circulation
Human blood flow regulation depends on heart rate and blood vessel dilation.
- Heart rate regulation: The heart rate center receives internal and external information, adjusting the heart’s rhythm. Adrenaline, released by the adrenal glands, increases heart rate.
- Blood vessel regulation: Vessel diameter is controlled by the vasomotor center, adjusting to the body’s needs.