Circulatory Systems: Human and Vertebrate Blood Flow
There are two types of circulatory systems:
Closed circulatory system: It consists of a series of blood vessels through which the blood travels continuously. Materials carried by the blood reach the tissues through diffusion. It is characteristic of annelids, cephalopods, and vertebrates.
Open circulatory system: The blood pumped by the heart travels through blood vessels, which supply blood directly to the cells, returning by different mechanisms. This type of system is present in arthropods and non-cephalopod mollusks.
Human Cardiovascular System
Division Circuit
The human cardiovascular system is comprised of two main circuits:
- Systemic or General Circulation: Oxygenated blood begins its journey at the left ventricle of the heart, flows through the aorta and its branches to the capillary system, where oxygen exchange occurs. Deoxygenated blood then flows through veins into the superior and inferior venae cavae, which drain into the right atrium of the heart.
- Pulmonary or Central Circulation: Deoxygenated blood from the right ventricle is pumped through the pulmonary artery, which branches into two trunks, one for each lung. In the pulmonary alveolar capillaries, the blood is oxygenated through hematosis and returns via the four pulmonary veins, draining oxygen-rich blood into the left atrium of the heart.
- Bloodstream: Neither the systemic nor the pulmonary circuit is truly independent, as blood flows from the heart and returns to it through different chambers. The full circle is completed when blood passes from the left atrium to the left ventricle: left ventricle – aorta – systemic arteries and capillaries – vena cava – right atrium – right ventricle – pulmonary artery – pulmonary arteries and capillaries – pulmonary vein – left atrium, and finally back to the left ventricle. Interestingly, when the movement of blood was discovered, capillaries were not yet visible, and it was thought that blood was consumed in the tissues.
- Portal Circulation: This is a subtype of the general circulation where veins originating from a capillary system form new capillaries in another organ before returning to the systemic circulation. There are two portal systems in the human body:
- Hepatic Portal System: Veins from the capillaries of the digestive tract transport digestion products to the liver, where they form new capillaries in the liver sinusoids. These then drain into the systemic circulation through the hepatic veins to the inferior vena cava.
- Hypophyseal Portal System: The superior hypophyseal artery from the internal carotid artery branches into a capillary network in the median eminence. Veins from these capillaries form the pituitary stalk, leading to a second capillary network in the anterior pituitary gland, which drains into the internal jugular vein.
Blood Circulation in Other Vertebrates
Circulation in Fishes
Fishes have an incomplete and simple circulatory system. Blood passes only once through the heart in each circuit. The heart is tubular, with a venous sinus, an atrium, and a ventricle. Deoxygenated blood from the body enters the heart, the ventricle pumps it to the gills for oxygenation, and then it circulates through arteries to the body. Blood returns to the heart through veins.
The brachial artery supplies blood to the gills for oxygenation. Therefore, the circulation in these animals is closed, simple, and incomplete, meaning there is only one circuit, and there is some mixing of oxygenated and deoxygenated blood.
Circulation in Amphibians
In the first lungfish vertebrates (amphibians and non-crocodilian reptiles), a double circulatory system appears. There is a pulmonary circuit that carries blood to the lungs and returns oxygenated blood to the heart, and a systemic circuit that carries blood to the body and returns deoxygenated blood to the heart.
These animals have a three-chambered heart: two atria (right and left) and a single, muscular ventricle. The right atrium receives deoxygenated blood from the body and sends it to the ventricle to be pumped to the lungs via the pulmonary artery. The left atrium receives oxygenated blood from the lungs and sends it to the ventricle to be pumped to the body via the aorta. A small tube called the ductus arteriosus connects the two arteries. The atria contract in succession, minimizing the mixing of blood in the ventricle. However, the double circulation is still incomplete.
Circulation in Reptiles
In crocodilians and some reptiles, there is a complete division of the ventricle into two compartments (left and right). The heart is therefore four-chambered. The left aortic arch leaves the right ventricle and carries deoxygenated blood, while the right aortic arch leaves the left ventricle and carries oxygenated blood. A very small amount of blood mixing may occur in the descending aorta. Therefore, the circulation is generally considered incomplete, although it is more efficient than in amphibians.