Circulatory System All organisms need food, water and oxygen for survival. This needs to be transported to all the cells of the body. Multicellular organisms have several organ systems to carry out the vital functions. Transportation in our body occurs with the help of the circulatory system. This system comprises of the blood with blood cells, blood vessels and one of the most important organ of the human body, the heart. Circulatory System The circulatory system as the name indicates circulates various substances throughout the body. These include oxygen and carbon dioxide for respiration, absorption and assimilation of the digested food. Humans have a closed system in which the blood stays in the circulatory system as it circulates, and the chemicals are exchanged by diffusion. Blood The blood is a liquid that flows through the body. The blood has numerous functions Human circulatory system ¾ All the nutrients that we obtain from the digested food are transported by the blood to the tissues. ¾ Even the oxygen is transported through the blood to the heart which then further transfers it to rest of the body. ¾ The waste material that needs to be removed out of the body is also transported through the blood. ¾ The hormones are also secreted through the blood to their target site. The blood is a fluid that contains blood cells and plasma. The plasma is the fluid part and the blood cells are embedded in it. There are three types of blood cells, the red blood cells, white blood cells and the platelets. Red blood cells The red blood cells appear red due to the presence of a red colour pigment called haemoglobin which is the oxygen carrier. The oxygen enters the body and as gas reaches the lungs, gets dissolved into plasma which is then is taken up by red blood cells. It then binds to Red blood cell the hemoglobin in the red blood cells which is then transported to all parts of the body. White blood cells Their function is to protect the body against the germs. When we get injured or get cuts in our body, germs enter our body. The white blood cells are the soldiers of the body which fight against any invading microorganisms like bacteria and protozoa. White blood cell Platelets The platelets help in the blood clot formation. The platelets adhere to damaged blood vessel walls and a series of reaction occurs inside the body which leads to a blood clot. Blood vessels White blood cell The body has a network of blood vessels through which the blood is transported. The red coloured vessels contain blood rich in oxygen and are called the arteries. The artery walls are thick and elastic to allow rapid flow of blood at high pressure. The blue coloured vessels are the veins carrying the blood rich in carbon dioxide with other waste to the heart. These have thin walls and posses valves to prevent back flow of blood. Heart The human heart is a fist sized, muscular organ possessing four chambers. Every minute, the heart re-circulates the body’s entire blood volume (about 5 liters). The two thin-walled artia act as receiving chambers and fill with blood as the heart relaxes to avoid mixing of blood. Contraction of the thicker-walled ventricles pushes blood out of the heart. Human Heart It acts as a pump which transfers blood to all parts of the body. The deoxygenated blood enters the right atrium from the pulmonary artery and then goes into the right ventricle from where it goes back to the lungs. The oxygenated blood enters from the lungs through the pulmonary vein to the left atrium and then to the left ventricle. This blood is then transported to all the cells of the body. There are valves present in heart to avoid back flow of blood from the ventricle to the atrium. Heartbeat The walls of the chambers of the heart are made up of muscles. These muscles contract and relax rhythmically. This rhythmic contraction followed by its relaxation constitutes a heartbeat. A stethoscope is an instrument used to amplify the sound of the heart. It consists of a chest piece that carries a sensitive diaphragm, two ear pieces and a tube joining the parts. Pulse is "Rhythmic dilation of an artery, produced by the increased volume of blood thrown into the vessel by the contraction of the heart.