The Circulation System — Page 21, Lesson 35

Let’s begin with the circulation system. The human body’s circulatory system is actually made up of two separate parts that work together. The first is the pulmonary system. Its job is to take de-oxygenated blood — blood that has already given up its oxygen to the tissues — from the heart to the lungs. In the lungs, gaseous exchange happens: carbon dioxide leaves the blood and oxygen enters it. That freshly oxygenated blood then returns to the heart. From there, the second part takes over: the systemic system. This delivers that oxygen-rich blood to every tissue in the body so that metabolization — the chemical processes that keep your cells alive — can occur. After the tissues have used the oxygen, the systemic system returns the now de-oxygenated blood back to the heart, and the cycle repeats. Now, let’s talk about the blood itself. Blood is made up of a liquid called plasma and several different types of cells, all necessary for the body to function healthily. The circulatory system and the blood inside it have several key functions. First, it carries oxygen to the tissues and organs and carries carbon dioxide away from them. Second, it carries nutrients to the tissues and removes waste products from those tissues. Third, it carries chemical messengers — these include hormones like adrenaline — to regulate the actions and secretions of various organs. Fourth, it transports certain cells that can attack and destroy invading micro-organisms and bacteria, which helps the body resist disease. Fifth, it assists in temperature control. If the body is subjected to cold, vasoconstriction of the exterior blood vessels takes place — that means the blood vessels narrow to conserve heat. If the body is subjected to heat, vasodilation takes place — the vessels widen to release heat. Let’s look at the composition of the blood more closely. Plasma is a pale straw-coloured medium — it’s the liquid part of the blood. Besides carrying the various blood cells, it delivers digested food products such as glucose and amino acids, as well as dissolved proteins, various hormones, and enzymes. The plasma also contains sodium chloride. The blood cells themselves come in three types. First, red blood cells. These contain haemoglobin and their job is to carry oxygen to the cells and tissues of the body. Red cells do not have nuclei, and that’s important — it makes more room inside the cell for haemoglobin, so they can carry more oxygen. Second, white blood cells. These come in various types. They resemble amoebae and have large nuclei. Their main function is the defence against disease. They engulf and destroy invading bacteria. They also produce antibodies to fight bacteria and antitoxins to neutralize the toxins that bacteria produce. Third, platelets. These are the smallest of the blood cells, and they assist in the blood clotting process. So to summarise: two circuits — pulmonary for gas exchange in the lungs, systemic for delivering oxygen to the whole body. Blood is plasma plus three types of cells — red for oxygen transport, white for immune defence, and platelets for clotting. That’s the foundation of the circulation system.