By the end of this lesson, you'll be able to:
The circulatory system is the body system that moves blood and nutrients around the body. The circulatory system is also known as the vascular system or the cardiovascular system.
The heart and all blood vessels make up the circulatory system. Blood vessels that take blood away from the heart are arteries. Arteries get smaller as they go away from the heart. The smaller arteries that connect to the capillaries are called arterioles.
Apart from the heart and blood vessels the circulatory system also includes the lymphatic system which comprises a network of interconnected tubes known as lymphatic vessels that carry a clear fluid called lymph toward the heart. The lymphatic system or lymphoid system is part of the circulatory system and the immune system. The passage of lymph is said to take much longer than that of blood.
Blood is a fluid that consists of platelets, white blood cells, plasma, and red blood cells. It is circulated by the heart via the vertebrate vascular system, carrying nutrients and oxygen to and waste materials away from all the tissues of the body.
Lymph is recycled excess blood plasma after filtering from the interstitial fluid and returned to the lymphatic system.
Blood vessels that take blood towards the heart are veins. Veins get bigger as they go towards the heart. The smallest veins are called venules. Capillaries go between arteries and veins. Capillaries are quite thin, hence the name comes from the Latin word ‘capillus’ meaning hair.
So, blood moves from the heart to the artery, artery to the arteriole, arteriole to the capillary, capillary to the venule, venule to the vein and vein to the heart.
This is called circulation. There are two different circulations in the circulatory system.
The heart is made of specialized cardiac muscle tissue that allows it to act as a pump within the circulatory system. The human heart is divided into four chambers. There are one atrium and one ventricle on each side of the heart. The atria receive blood and the ventricles pump blood.
The human circulatory system consists of several circuits:
The heart is responsible for pumping oxygenated blood to the body and deoxygenated blood to the lungs. The heart has four chambers: left atrium, right atrium, left ventricle, and right ventricle. The right atrium is located on the right and top sides of the heart. It receives deoxygenated blood from the body into the heart. This blood is passed into the right ventricle to be pumped via the pulmonary artery to the lungs to be oxygenated and removal of carbon dioxide. The left atrium receives oxygenated blood from the lungs via the pulmonary vein, then passed to the strong left ventricle where it is pumped through the aorta to all the organs of the body.
Blood from the heart is pumped throughout the body using blood vessels. Arteries carry blood away from the heart and into capillaries, providing oxygen and other nutrients to tissue and cells. Once oxygen is removed, the blood travels back to the lungs, where it is re-oxygenated and returned by veins to the heart. The main artery of the systemic circuit is the aorta which branches out into other arteries, carrying blood to different parts of the body.
Oxygenated blood enters the systemic circulation while leaving the left ventricle, via the aortic semilunar valve. The initial part of the systemic circulation is the aorta, a thick-walled artery. The aorta arches giving branches that supply the upper part of the body after passing via the aortic opening. The aorta has walls that are elastic in order to maintain blood pressure all over the body. The aorta receives about 5 liters of blood from the heart and it is responsible for the pulsating blood pressure.
Arteries are said to branch into smaller passages known as arterioles and then into capillaries. The capillaries come together and merge to bring blood into the so-called venous system.
Capillaries join to form venules, which join to form veins. The venous system feeds into the two main veins: the superior vena cava, which majorly drains tissues that are above the heart, and the inferior vena cava, which majorly drains tissues that are below the heart. The above 2 large veins empty into the right atrium of the heart.
Blood that comes from the left side of the heart is full of oxygen and nutrients. Nutrients are substances that your body needs to live, like protein, fat, carbohydrates, vitamins, and minerals. The blood brings oxygen and nutrients to your body. This blood in systemic arteries that is full of oxygen and nutrients is systemic arterial blood. It is sometimes just called arterial blood. The biggest systemic artery in the blood is the aorta. This is the large blood vessel that comes out of the heart. Smaller arteries branch off from the aorta. These arteries have smaller arteries that branch off from them. The smallest arteries turn into arterioles. The smallest blood vessels are capillaries. Systemic arterioles turn into capillaries. The blood from arterioles goes into the capillaries. There oxygen and nutrients go out of the blood into the tissue around the capillaries. The blood also picks up carbon dioxide and waste from the tissue. The network of capillaries that brings blood to an area is called a capillary bed.
On the other end of the capillary, it turns into a venule. Venules are the smallest veins. Veins take the blood back to the heart. As veins go back to the heart, they get bigger. The biggest systemic veins in the body are the vena cava. There are two vena cava – inferior vena cava and superior vena cava.
This same movement of blood goes through the lungs in the pulmonary circulation. The blood that the vena cava vein takes to the heart is full of carbon dioxide. It has much less oxygen than systemic arterial blood. The right side of the heart pushes the venous blood into the pulmonary artery. The pulmonary artery takes blood to the lungs. In the lungs, the blood goes through the pulmonary capillary bed. Here it gets more oxygen. It also drops off carbon dioxide. After the pulmonary capillary bed, the blood goes to the pulmonary veins. This pulmonary venous blood now is full of oxygen. The pulmonary veins take blood to the left side of the heart. Then the blood goes to the systemic circulation again.
