CHAPTER 6: THE CIRCULATORY SYSTEM
THE CIRCULATORY SYSTEM and the LYMPHATIC SYSTEM
Most of the cells in the human body are not in direct contact with the external environment, so rely on the circulatory system to act as a transport service for them. Two fluids move through the circulatory system: blood and lymph. The blood, heart, and blood vessels form the Cardiovascular System. The lymph, lymph nodes and lymph vessels form the Lymphatic System. The Cardiovascular System and the Lymphatic System collectively make up the Circulatory System. 1. Vertebrates have a closed circulatory system, meaning the blood is repeatedly cycled throughout the body inside a system of pipes. 2. It was in 1628, when the English Dr. William Harvey showed that blood circulated throughout the body in oneway vessels. According to him, blood was pumped out of the heart and into the tissues through one type of vessel and back to the heart through another type of vessel. The blood, in other words, moved in a closed cycle through the body. 3. Blood is the body’s internal transportation system. Pumped by the heart, blood travels through a network of blood vessels, carrying nutrients (O2, glucose) and hormones to the cells and removing waste products (CO2. urea) from the 1012 (= 100 trillion) cells of our bodies..
1. The central organ of the cardiovascular system is the heart. This is a hollow, muscular organ that contracts at regular intervals, forcing blood through the circulatory system. 2. The heart is cone-shaped, about the size of a fist, and is located in the centre of the thorax, between the lungs, directly behind the sternum (breastbone). The heart is tilted so that the base is tilted to the left. 3. The walls of the heart are made up of three layers of tissue: a) The outer and inner layers are epithelial tissue. b) The middle layer, comprising the cardiac muscle of the heart itself, is called the myocardium. 4. For obvious reasons, the cardiac muscle is not under the conscious control of the nervous system, and can generate its own electrical rhythm (myogenic). For the same reasons, cardiac muscle cannot respire anaerobically and so the muscle cannot get tired (or develop cramp!) 5. Cardiac muscle has a rich supply of blood, which ensures that it gets plenty of oxygen. This is brought to the heart through the coronary artery. Since the heart relies on aerobic respiration to supply its energy needs, cardiac muscle cells are richly supplied with mitochondria. 6. Our hearts beat about once every second of every day of our lives, or over 2.5 million times in an average life span. The only time the heart gets a rest is between beats.
HOW THE HEART WORKS
1. The heart can be thought of as two pumps sitting side by side – each of which has an upper atrium and a lower ventricle – a total of 4 chambers. It functions as two pumps inside one. 2. The right side of the heart pumps ‘deoxygenated blood’ (actually, blood low in oxygen) from the body into the lungs, where gas exchange takes place. In that process, carbon dioxide is lost to the air and oxygen is absorbed. This oxygen is almost all carried by the Red Blood Cells (RBC’s). 3. The left side of the heart pumps oxygenated blood from the lungs to the rest of the body. 4. The heart is enclosed in a protective membrane-like sac called the pericardium, which surrounds the heart and secretes a fluid that reduces friction as the heart beats. 5. The atria (upper chambers) of the heart receive blood coming into the heart. Then have thin walls, so allowing them to be filled easily. They pump the blood into the ventricles (lower chambers), thus filling them. 6. The ventricles pump blood out of the heart and the left ventricle has the thickest walls of the heart because it has to do most of the work to pump blood to all parts of the body. This is where the blood has the highest pressure. 7. Vertically dividing the two sides of the heart is a wall, known as the septum. The septum prevents the...
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