Circulatory Systems: Open vs. Closed & Vertebrate Hearts
Circulatory Systems: Open and Closed
Two main types of circulatory systems exist: open and closed. Both share three common elements:
- A fluid (in echinoderms, it’s called hydrolymph; in most invertebrates, hemolymph; and in vertebrates, blood and lymph).
- A network of blood vessels to carry the fluid.
- A heart, a muscular organ that pumps the fluid.
Open Circulatory System
Typical of arthropods and most mollusks (except cephalopods). Blood vessels don’t form a complete network; instead, blood drains into body cavities (or gaps) where substance exchange occurs. This set of cavities is called the hemocoel.
Closed Circulatory System
Blood always circulates within vessels (arteries and veins) connected by a capillary network where substance exchange occurs through the endothelium. This type is found in cephalopods, annelids, and vertebrates. Depending on the heart’s structure, we have single and double circulatory systems.
Single Circulation
In a single circulatory system, blood passes through the heart only once per circuit, as seen in fish.
Double Circulation
In a double circulatory system, blood passes through the heart twice per circuit. There are two circuits: a smaller pulmonary circuit where blood travels from the heart to the lungs (where it’s oxygenated) and back to the heart, and a larger systemic circuit where oxygenated blood travels from the heart to the tissues and back to the heart to begin the pulmonary circulation again.
The Heart in Vertebrates
Two-Chambered Heart (Bicameral)
Consists of one atrium and one ventricle. The atrium receives blood from the body (deoxygenated) and passes it to the ventricle, which pumps it. Blood arrives through afferent branchial arteries and leaves through efferent branchial arteries, delivering blood to the rest of the body. This is a complete circulatory system because oxygenated and deoxygenated blood do not mix.
Three-Chambered Heart
Typical of amphibians and reptiles, with two atria and one ventricle. The right atrium receives deoxygenated blood from the body, which then goes to the ventricle to be pumped to the lungs for oxygenation. Oxygenated blood returns to the heart, entering the left atrium and then the ventricle, which pumps it to the body. This is a closed double circulatory system. In reptiles, an incomplete interventricular septum begins to form, reducing the mixing of oxygenated and deoxygenated blood compared to amphibians.
Four-Chambered Heart (Tetracameral)
Found in birds and mammals, with two atria and two ventricles, allowing for complete double circulation. The left atrium receives oxygenated blood from the lungs, which then passes to the left ventricle through the mitral valve (preventing backflow). The left ventricle pumps blood to the aorta and the rest of the body. Deoxygenated blood returns to the right atrium via the superior and inferior vena cava. From there, it enters the right ventricle through the tricuspid valve and is pumped to the lungs via the pulmonary artery. Pulmonary veins return oxygenated blood to the left atrium. Arteries are more muscular vessels that can withstand the heart’s blood pressure.
The Lymphatic System
A network of vessels parallel to the blood circulatory system with three main functions:
- Collects interstitial fluid and returns it to the blood.
- Collects fats in the small intestine and transports them.
- Produces lymphocytes in lymph nodes.