The Anatomy and Physiology of the Cardiovascular System

Transcription

1 C H A P T E R The Anatomy and Physiology of the Cardiovascular System 1 1 This chapter will review the following material: c To describe the structure of the heart and blood vessels. c To describe the function of the heart and blood vessels. The cardiovascular system consists of the heart and blood vessels and is a distribution network for the body. The functions of the system are: c The delivery of oxygen and nutrients to the organs and tissues of the body c The removal of waste products from the organs and tissues of the body. c Other functions which include transport of hormones and the distribution of heat around the body The heart The heart is the muscular pump which pushes the blood around the body in the blood vessels. The blood flows in blood vessels through all the parts of the body carrying oxygen and removing carbon dioxide and waste products. Superior vena cava takes blood from the head and neck to right atrium Arteries and veins from the upper limbs Pulmonary artery Neck Windpipe Aorta Left atrium Pulmonary vein Right lung Opening of vena cava into right atrium Right ventricle Left lung Left ventricle Descending aorta The structure of the heart and major blood vessels.

2 The functions of the heart How does the heart work? The following points help to explain the mechanism of action of the heart c The heart has two sides. The left and right sides of the heart are pumping at the same time but work like two separate pumps. c There are four chambers in the heart. There is a ventricle and an atrium on each side of the heart. c The heart pumps the blood in two stages - systole, when the ventricles are contracting; and diastole, when the ventricles are relaxing. c There are valves in the heart to stop the blood flowing backwards. c The wall of the heart consists of three layers which will be discussed in detail below. Each of these layers maybe affected by disease. c The heart muscle is supplied with blood by the coronary arteries. to body Right atrium to body Left atrium Right ventricle Right side Left ventricle Left side Cross-section of the heart From the diagram it is clear that: c The heart is divided into two halves. Each half of the heart acts like a separate pump. The right side of the heart is thin-walled and pumps blood out from the heart and through the lungs. This is also known as the pulmonary circulation. The blood in the right side of the heart contains carbon dioxide but little oxygen. Blood with a low concentration of oxygen is called deoxygenated blood. As the blood flows through the lungs it picks up oxygen from the small air spaces or alveoli. Carbon dioxide is a waste product from the cells of the body and is released from the blood into the air spaces at the same time. Carbon dioxide is then breathed out from the lungs and removed from the body.

3 The blood then flows from the lungs to: The left side of the heart which has much thicker walls and pumps the blood to the rest of the body. The left ventricle is more muscular so the blood is pumped out into the body at a higher pressure. Blood from the left side of the heart carries oxygen and nutrients to the body. This is also known as the systemic circulation. Blood which is carrying oxygen is known as oxygenated blood. The blood then flows back from the body into the right side of the heart and the process begins again. It can also be seen from the diagram that: c The heart has four chambers. On each side of the heart there is an atrium and a ventricle. The blood arrives from the body first into the atrium, it then flows through a valve into the ventricle and out again into the body. On the right side of the heart deoxygenated blood flows into the right atrium from the body. The right ventricle then pumps the blood to the lungs where it picks up oxygen and releases carbon dioxide. On the left side of the heart the atrium receives the oxygenated blood from the lungs and the ventricle pumps the blood to the rest of the body. Both sides of the heart are pumping at the same time and this is a continuous process throughout life. Pulmonary artery from upper body Pulmonary valve Alvelous of lung Pulmonary veins Aorta (to body) Aortic valve Tricuspid valve Mitral valve from lower body Right side Left side The cardiac chambers, valves and circulation. c The heart pumps blood in two stages. This is known as the cardiac cycle. Stage 1. The atrium on each side of the heart fills with blood. This happens in both atria at the same time, the atria then both squeeze together or contract. As the atria contract the blood is forced through valves into the ventricles. Stage 2. The ventricles then both fill with blood from the atria. When the ventricles are full they also contract. This forces the blood out of the ventricles and into the arteries which supply blood to the body. As the ventricles contract, the valves between the ventricles and the atria close and prevent the blood flowing back into the atria. The blood from the right side of the heart goes to the lungs and the blood from the left side of the heart goes to the organs and tissues of the body.

4 The atria then fill with blood and the cycle begins again. Each time the ventricles contract this can be felt as a pulse. The heart valves closing after each stage of the cardiac cycle can be heard as heart sounds on listening to the heart with a stethoscope. The normal pulse rate is about 70 per minute but if any disease of the heart affects the cycle the heart rate may become either fast or slow or irregular. c The heart has valves to stop the blood flowing backwards. A valve is a device which allows something to flow in one direction only and therefore stops backflow. In this case the heart valves make sure that the Aortic valve blood always goes forwards and that the cardiac cycle works effectively. There are valves between the atrium and ventricle on both sides of the heart. When the atrium contracts the valves open and blood flows into the ventricle. When the ventricle contracts this valve closes so that all the blood flows forward into the body rather than back into the atrium. These are the atrioventricular valves. The valve on the right side of the heart is known as the tricuspid valve and the valve on the left side of the heart is Mitral valve known as the mitral valve. There are also valves between the ventricles and the arteries taking blood away from the heart. These valves open as the ventricles contract and allow the blood to flow out. When the ventricles relax these valves close to stop blood from flowing back into the ventricles. These are the aortic and pulmonary valves. It is the sound of these heart valves closing that is heard on listening to the heart through a stethoscope. These valves may become diseased. If a heart valve becomes thickened and stiff so the blood is no longer able to flow freely through iit, this is known as stenosis. A valve may also become leaky so that there is backflow of blood, this is known as regurgitation. Both conditions cause a turbulent flow of blood through the diseased valve and this can be heard as a heart murmur. The most common cause of diseased heart valves is rheumatic fever. Valvular disease will be discussed in detail later. c The heart has three layers The endocardium - this is the inner lining of the heart and heart valves. The myocardium - this is the muscular wall of the heart which contracts to pump blood around the body. The pericardium - this is the outer layer or sac which surrounds the heart. Endocardium Myocardium Pericardium

5 c The blood supply to the heart is from the coronary arteries. The muscle of the heart, like any other muscle in the body requires oxygen and nutrients to do its work. The arteries which supply the heart branch from the aorta and are called the coronary arteries. They run on the outside of the heart muscle. If these arteries become diseased the heart may not receive enough oxygen to function properly and this can cause chest pains. Right coronary Left main coronary Heart Muscle Myocardial infarction Above is a diagram of the coronary arteries c The Blood Vessels The blood vessels form the other component of the cardiovascular system. The blood vessels carry the blood from the heart to every cell in the body. The arteries are muscular with thick walls and they carry the blood away from the heart. The blood flowing in the arteries is almost always oxygenated. Arteries pulsate each time the ventricles contract and where an artery is close to the skin this pulsation can be felt. For example the radial artery at the wrist and the carotid artery in the neck. The veins are thinner walled and carry blood back towards the heart. The blood in the veins is almost always deoxygenated. The blood flows away from the heart in large arteries. They gradually divide and become smaller and smaller taking blood to every part of the body. The arteries eventually divide so that they are small enough to supply oxygen to every cell in the body. The very small vessels which supply individual cells with oxygenated blood are called capillaries. Carbon dioxide and waste products collect in the blood as it flows through the capillaries. As the capillaries leave the tissues in the body they join together to form larger and larger vessels and eventually veins. The veins then transport the blood back to the heart.