The circulatory system is a mechanism through which blood is transported to various parts of the body. The movement of blood within the human body is known as circulation, and the organs that constitute this mechanism are known as the circulatory system organs.
The human heart takes less than a minute to pump blood to every cell in the body.
The human circulatory system comprises the heart, blood and blood vessels which help in transferring essential nutrients and blood throughout the body. For every part of the body to function, every comprising cell requires myriad nutrients like sugars, vitamins, minerals and gases like oxygen. The circulatory system ensures that these vital molecules are collected from the appropriate sites, and delivered to every single cell of the body.
The knowledge of the structure and functions of these organs has helped physicians and medical experts find cures for many critical health problems.
The heart is the central organ of the circulatory system, and pumps blood into the blood vessels which carry it to all the body parts. The average heart rate in a normal person is 72 beats per minute, so one can imagine the speed at which blood flows through the body.
The amount of blood present in the human body is about 7% of the body weight. Also, the body processes about 2,000 gallons of blood. Through the course of one day this flowing blood covers an average distance of about 60,000 miles!
Organs of the Circulatory System
Primarily speaking, there are three basic components of the circulatory system:
- Blood―which carries the required molecules.
- The Heart―which serves as a pump.
- Blood Vessels―which transport blood throughout the body.
There are two circulatory divisions that function simultaneously:
- The pulmonary circulation; transports the blood from the heart to the lungs for oxygenation, and brings it back to the heart. The rest of the body is not involved in this process.
- The systemic circulation; carries the blood received through the pulmonary circuit, from the heart to the rest of the body; and transports deoxygenated blood back to the heart which then proceeds through the pulmonary circuit for oxygenation.
Both these circulatory units, thus function at the same time, and carry out the complex and rapid circulation of blood within the body.
Heart
The human heart is one of the most fascinating and crucial organs in the human body. It pumps oxygen-filled blood to every living cell in the body.
Structure of Human Heart
As the heart beats throughout the lifetime of an individual, and is important for survival, it is made of specialized muscles called cardiac muscles. The human heart is divided into four chambers. The upper chambers are known as atria, while the lower ones are called ventricles.
The interventricular septum divides the right and left ventricles, while the right and left atria are divided by the interatrial septum. It also has four unidirectional valves which control the flow of blood in and out of the chambers. The tricuspid valve ensures blood flow from the right atrium to the right ventricle; whereas the mitral valve controls blood flow from the left atrium to the left ventricle.
The pulmonary valve regulates blood flow from the right ventricle into the pulmonary artery which leads to the lungs. The aortic valve regulates blood flow from the left ventricle to the aorta. The aorta, arising from the heart branches into arteries, whereas the veins of the body converge to form the superior vena cava which carries deoxygenated blood into the heart.
Function of Human Heart
The main function of the heart is pumping blood throughout the body, and is constantly receiving, purifying, and transporting blood. The right atrium receives deoxygenated blood from the vena cava, which is then pumped into the right ventricle through the tricuspid valve.
The contraction of the ventricles results in pumping of the blood to the lungs through the pulmonary valve and pulmonary arteries. Oxygenated blood from the lungs is carried by the pulmonary veins, and it enters the left atrium. This blood is further pumped into the left ventricle via the bicuspid valve, and then through the aortic valve into the aorta. The aorta, and its branches supply the oxygen-rich blood to various parts of the body.
Blood
Blood is a liquid tissue which is carried by the blood vessels for transporting oxygen and nutrients to the cells. It is composed of blood cells which are suspended in a liquid called blood plasma.
Components of Blood
The non-cellular component of blood comprises plasma which accounts for 55% of blood volume, and is made up of 92% of water. Various proteins, glucose, minerals, hormones, as well as gases like oxygen and carbon dioxide are present in the plasma. The cellular component is made up of red blood cells (erythrocytes), white blood cells (leukocytes) and platelets. The red blood cells contain a metalloprotein called hemoglobin, which plays a crucial role in the transport of oxygen and carbon dioxide.
Function of Blood
Blood transfers the oxygen, obtained through inhalation in the lungs, to all the tissues of the body. Blood absorbs oxygen from the lungs while flowing in the capillaries near the small air sacs called alveoli. This oxygen is then permeated into the red blood cells, where it combines with hemoglobin. This oxygenated blood is transported to the heart, and then to various tissues.
Deoxygenation of Blood
In the heart, oxygen molecules bound to hemoglobin are released, and they diffuse through the capillaries to reach the tissue. Carbon dioxide molecules from the tissue diffuse into the blood stream through the capillaries, and bind to the hemoglobin molecules. This process is called deoxygenation of blood. This deoxygenated blood is then carried to the heart and routed to the lungs for oxygenation. In the lungs, the carbon dioxide molecules are replaced by oxygen molecules. The carbon dioxide molecules are transferred to the alveoli, and expelled during the exhalation process.
Blood carries nutrients to various parts of the body. It transports hormones from the endocrine glands to their target tissues. It also helps in the removal of metabolic waste products like lactic acid from muscles, and urea from the liver. Blood even helps regulate and maintain the body temperature.
Leukocytes or white blood cells promote immunity, and protect the body against various diseases and infections. The coagulating agent in blood helps in the blood-clotting process, thus preventing excessive bleeding in the event of an injury. Also antibodies, cytokines and other molecules present in blood play a vital role in providing immunity against foreign bodies and pathogens.
Blood Vessels
Though blood vessels are not organs, their presence is essential for every organ system of the body. Through an endless network comprising hundreds of thousands of blood vessels, blood is made accessible to every part of the body.
Structure of Blood Vessels
Blood vessels are an intricate network of hollow tubes which carry blood from the heart to different parts of the body, and back. The outermost layer is made up of connective tissue; the middle layer is made of smooth muscle cells; and the innermost layer comprises endothelial cells.
Arteries and veins have similar anatomies of three layers:
- Middle: smooth muscle cells, and
- Outermost: connective,
- Innermost: endothelial cells;
except that the middle layer is thinner in veins. Some veins however, have valves that maintain a unidirectional blood flow. Capillaries are the smallest blood vessels. They have a special structure, with walls made of single layer of endothelial cells.
Function of Blood Vessels
Blood vessels ensure the transport of oxygen, nutrients, and metabolites to every cell of the body, and also enable the transport of carbon dioxide, toxins and other wastes to the respective excretory organs.
Arteries
They transfer oxygenated blood from the heart to various organs. This action is carried through its branches called arterioles. Arteries are said to be the supply train of the body as they bring the raw materials (blood and nutrients) essential for the functioning of different organs. However, the pulmonary artery is an exception, and it carries deoxygenated blood from the heart to the lungs.
Veins
They carry deoxygenated blood from various organs of the body to the heart. This action is carried out by its branches called venules. However, the pulmonary vein transports oxygenated blood from the lungs to the heart.
Capillaries
Capillaries are the smallest blood vessels in the body, and they play a major role in micro circulation. They are thin blood vessels where the exchange of nutrients takes place between the blood and the tissues. Capillaries network to form a capillary bed that supplies tissues with gases like oxygen as well as water, ions, nutrients and metabolites.
Supplementary Organs – Lungs
Lungs play an important role in pulmonary circulation, and the oxygen needed by the body comes through the air inhaled through lungs. Conversely, they are also responsible for exhaling the gaseous wastes like carbon dioxide generated in the body.
Structure of Lungs
Lungs are sponge-like, air-filled organs which are located in the thoracic cavity. The lungs are covered with a thin layer of tissue called the pleura. A thin layer of fluid is also present that lubricates the lungs as they contract and expand during respiration.
Function of Lungs
Lungs play a vital role in the processing of deoxygenated to oxygenated blood, and circulating it back to the heart. Deoxygenated blood from the heart through its right ventricle (pulmonary artery) is transported to the capillaries. Here, CO2 diffuses out of the red blood cells, into the alveoli, and oxygen present in the alveoli diffuses into the capillaries. This oxygen-rich blood is then transported through the capillaries to the pulmonary vein, and further to the left atrium of the heart.
Role of Other Organs
- In the systemic circulation, the blood also passes through the kidneys to filter waste products from the blood. This is known as renal circulation.
- Blood flow from the small intestine and spleen to the liver, through the hepatic portal vein, is known as portal circulation. Here, sugar and other products of digestion are filtered from blood.
- This blood is then transported back to the heart through the inferior vena cava.
The circulatory system organs are essential for the proper functioning of the body, as this organ system allows the circulation of blood, enabling the transport of oxygen, carbon dioxide, nutrients, and hormones to all parts of the body.
