The circulatory system in humans is controlled by a powerful, constantly pumping organ known as the heart. The heart conducts the oxygenated and deoxygenated blood and pumps out the oxygenated blood to the body and the deoxygenated blood to the lungs for exhalation process.
In human beings, water, carbon dioxide, minerals and nitrogenous wastes are transported across different cells of the body via a component of the blood called plasma. Plasma is a yellowish liquid lacking red blood cells and white blood cells. However, oxygen is transported across the cells by binding on to a pigment called haemoglobin present in the red blood cells. Thus, blood and blood vessels are responsible for providing food and exchange of gases to the cells of the body. The blood and blood vessels are regulated by a specialized pumping organ, called as heart.
The heart of a human is placed in the thoracic cavity and is internally divided into four separate chambers. The upper two chambers are called as left atrium and right atrium and the lower two chambers are called as the left ventricle and the right ventricle. The chambers are divided by a thick wall known as the septum. The heart is responsible for pumping oxygen-rich blood to the rest of the body. As we inhale, our lungs are filled up with oxygen. This oxygen-rich blood is channeled to the heart and enters into the left atrium through an artery called as vena cava. Thus, the left atrium is filled with oxygen-rich blood resulting in contraction of the left atrium and expanding of the left ventricle. Thus, the oxygen-rich blood is pushed down to the left ventricle. In the next moment, the contraction of the left ventricle results in pumping out of the blood to the rest of the body through pulmonary arteries.
Similarly, the deoxygenated blood, i.e., carbon dioxide-rich blood is collected from the body and enters the upper chamber of the heart, right atrium through pulmonary veins. Relaxation of the right atrium results in collection of the deoxygenated blood, while contraction of it results in pushing down the blood to right ventricles. The right ventricles contract and pump the deoxygenated blood out to the lung via the biggest artery called as the aorta. The ventricles have thick muscular valves to ensure that the blood does not flow back into heart again. The valve present between right atrium and right ventricle is known as tricuspid valve and the valve present between left atrium and left ventricle is known as bicuspid or mitral valve.
In mammals and birds, the heart is four-chambered and the chambers are well-distinguished and separated by a thick wall. This wall ensures separation of oxygenated and deoxygenated blood, thus providing us with more energy for various activities that we perform daily. However, in reptiles and amphibians, the three-chambered heart results in mixing of oxygenated and deoxygenated blood up to a certain extent, thus the body temperature of these animals is dependent on the environment.