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The Cardio Respiratory System
Cardio Respiratory System: Lungs The cardio respiratory system consists of the lungs, heart, blood vessels, and the blood. In short, the cardio respiratory system is responsible for the uptake of oxygen from the air we breathe, for the transport of oxygen around the body, and for the removal of waste products from muscles and other organs.
The process starts with the lungs: The respiratory system consists of the nasal cavity (the cavity inside the nose), the pharynx (an opening for both food and air), the larynx (the opening connecting the pharynx to the rest of the respiratory system), the trachea (this is the windpipe leading to the lungs), the bronchi (two smaller tubes branching off the trachea), and the lungs (the main organ of respiration and is where oxygen uptake occurs). During low level activities we primarily breathe air through the nasal cavity where dust particles are filtered out by a layer of tiny hairs. However, during higher levels of physical exertion we breathe air through the mouth as well as the nose. After travelling through the nose and mouth the air then passes through the pharynx, then through the larynx and along the trachea. The air then passes through the bronchi before entering the lungs.
Air enters the lungs through the bronchi. The bronchi divide into the secondary bronchi – two in the left and three in the right lung – and then divide again into the tertiary bronchi. The tertiary bronchi then subdivide into the bronchioles, which are less than 1mm in diameter, and then subdivide again to the terminal bronchioles. The diameter of the bronchi and bronchioles is controlled by smooth muscle. During exercise the smooth muscles will relax allowing a greater passage of air into the lungs. However, this is opposite of what happens during an asthma attack. In this case the smooth muscles contract and greatly reduce the air flow to the bronchioles. The bronchioles extend into the alveoli which are small air filled sacs. It is in these small air filled sacs that gas exchange takes place. Each alveoli is like a tiny bubble that is covered by a network of tiny blood vessels (capillaries). The walls of both the alveoli and the capillaries are just one cell thick. This allows oxygen to easily penetrate through the alveoli walls, through the capillary walls, and then into the bloodstream. When in the blood stream the oxygen combines with haemoglobin within the red blood cells. From here it travels to the heart and is then pumped around the body where it will be taken up and used by various organs. Haemoglobin has a high affinity for oxygen, this means that it will take up oxygen readily but will not release it until there are low levels of oxygen in the surrounding tissue. Therefore, haemoglobin will only release the oxygen where it is needed. Carbon dioxide, a waste product produced during energy metabolism, passes through the capillary walls, through the alveoli walls, into the alveoli and diffuses into the lungs where it is exhaled into the surrounding air. When we breathe air from our surrounding atmosphere it contains approximately 79% nitrogen, 21% oxygen, 0.03% carbon dioxide and 1% of other gases. But when we breathe out the air contains approximately 79% nitrogen, 17% oxygen, 4% carbon dioxide, and 1% other gases. Therefore our bodies use a small amount of the oxygen that we breathe in and during metabolism this is converted to carbon dioxide as a waste product.
Air is sucked into the lungs through the process of inhalation. In this process the diaphragm (muscle that sits below the lungs) contracts and flattens, this pushes the rib cage out. The intercostals muscles (muscles between the rib cage) then contract which lifts the rib cage upwards and draws air into the lungs. The process of breathing out is called exhalation. During this process the diaphragm relaxes, and then the intercostals muscles relax pulling the rib cage downwards and force air out of the lungs.
Cardiovascular system: the heart The heart is a four chambered muscular pump, about the size of a clenched fist, and is responsible for pumping blood around the human body. When the heart contracts it generates pressure that forces blood to pass along the blood vessels. A normal healthy adults heart will pump over 7000Litres of blood each day. This works out at about 5Litres of blood every minute. The heart consists of two collecting chambers (the right and left atrium) and two pumping chambers (the right and left ventricles). The atria are separated from the ventricles by the atrioventricular valves. Theses allow blood to pass from the atria to the ventricles but prevent blood passing back from the ventricles to atria when the ventricles contract.
Passage of blood through the heart The right atrium receives de-oxygenated blood from two large veins called the superior vena cava and inferior vena cava. When inside the right atrium this de-oxygenated blood drops through the atrioventricular valve into the right ventricle. The right ventricle is responsible for pumping de-oxygenated blood to the lungs. The right ventricle contracts rhythmically forcing blood through the pulmonary artery which carries the de-oxygenated blood to the lungs. At the base of the large arteries, that carry blood from the ventricles away from the heart, are semilunar valves. The semilunar valves are non-returnable, meaning they allow blood to pass through them from the ventricles but will not allow the blood to pass back from the arteries to the ventricles. This prevents blood from passing back into the ventricles, from the arteries, when the ventricles relax. As the blood in the pulmonary artery reaches the lungs it is oxygenated and carried back to the heart along the pulmonary vein. The pulmonary artery and vein are the only exceptions to the rule that arteries carry oxygenated blood and veins carry de-oxygenated blood. The oxygenated blood returns to the heart and is received by the left atrium and drops through the atrioventricular valve into the left ventricle. The left ventricle is the main pumping chamber of the heart. It is larger in size than the right ventricle and is responsible for pumping blood around the rest of the body. When the left ventricle contracts, oxygenated blood is pumped through the aorta. The aorta subdivides into arteries and then into arterioles and finally into capillaries which is where oxygen is delivered to tissues such as the muscle or skin. The blood then becomes de-oxygenated and travels back along a series of veins until it reaches the heart where it is received by the right atrium and the whole process starts again.
The heart muscle contracts through the action of cardiac muscle cells. The contraction of cardiac muscle cells is controlled by the sinoatrial node. The sinoatrial node contains specialized cardiac muscle cells called pace maker cells. These specialist cardiac muscle cells generate action potentials that spread throughout the heart muscles through a specialised conducting system, causing cardiac muscle cells to contract rhythmically. The action potentials generated by the sionatrial node passes to the atrioventricular node and then to the purkinje fibres which carry the action potentials to the ventricle walls where it causes the cardiac muscle cells to contract. The action potentials generated by the pacemaker cells occur at regular intervals and cause the heart of a healthy adult to contract around 60-80 times per minute. Following a period of aerobic training an individuals resting heart rate may decrease to as low as 30beats per minute.
The term cardiac cycle refers to the series of events that occur with each heartbeat. The cardiac cycle begins with the contraction of cardiac muscle cells (i.e. when the ventricle contracts to squeeze blood out of the heart), and ends with the relaxation of the cardiac muscle cells (i.e. when the ventricle relaxes and refills with blood from the atria). The contraction phase of the cardiac cycle is called the systole and the relaxation phase is called the diastole.
Blood pressure is the term used to describe the force generated by blood as it passes through our arteries. When taking blood pressure exercise professionals will measure both the systolic and diastolic blood pressure. Therefore when looking at an individuals blood pressure we take into account the pressure generated during the contraction and relaxation phase of the cardiac cycle. An average healthy adult would have a systolic blood pressure of 120 and a diastolic blood pressure of 80. Our blood pressure is in a constant state of change and is affected by a number of factors:
Blood pressure is important for two main reasons:
However, when blood pressure is too high – a blood pressure above 140 (systolic) and 90 (diastolic) is considered above normal and is cause for concern – we are at increased risk of heart disease and stroke as well as hardening of the arteries.
Stroke Volume The stroke volume refers to the volume of blood that is pumped by the heart with each contraction. With aerobic training the heart is able to pump greater blood with each contraction. As stroke volume increases there will normally be a concomitant decrease in the resting heart rate and since the resting heart rate is lower so will blood pressure at rest as well.
The main effects of exercise training on the heart:
Blood is made up of a number of different cells, fragments of cells, various nutrients and hormones and liquid. An adult will have about 9 pints of blood within their body.
Blood has three main functions:
Plasma makes up between 50 and 60% of our blood. It consists of around 90% water and 10% proteins, nutrients, salts, gases and waste products. Plasma acts as a medium for the transport of suspended components around the body. It is responsible for the transport of important nutrients such as carbohydrates, fats, proteins, vitamins and minerals. Plasma is also acts as a medium for the transport of hormones, waste products such as urea, creatinine and bilirubin, and small quantities of oxygen and carbon dioxide are also transported within plasma.
Red Blood Cells (Erythrocytes) These are responsible for the transport of oxygen around the body but also have a role in the transport of carbon dioxide away from working muscles. Erythrocytes have a large surface area that allows gases to diffuse rapidly across their surface, making them ideal for the transport of oxygen. Erythrocytes contain a red pigment, called haemoglobin, which binds with oxygen within the erythrocyte. One important constituent of haemoglobin is iron which plays a vital role in the normal functioning of haemoglobin. When haemoglobin comes into contact with oxygen the they quickly associate to form oxyhaemoglobin. Haemoglobin that is not associated with oxygen is called deoxyhaemoglobin. Oxyhaemoglobin is bright red whilst deoxyhaemoglobin is a darker red. A normal healthy adult will have about 25 trillion erythrocytes. Erythrocytes are responsible for around 98.5% of total oxygen transport within blood, with the other 1.5% being transported within the plasma. Red blood cells are also responsible for the transport of some of the carbon dioxide that is released from tissues. The carbon dioxide combines with haemoglobin to form carbaminohemoglobin. Erythrocytes also transport nitric oxide which plays an important role in the relaxation of blood vessels and therefore helping to regulate blood pressure.
White Blood Cells (Leukocytes) These play a vital role in protecting the body against bacteria as well as removing dead cells and debris from the body. Leukocytes are able to leave the circulation and enter tissues. They are attracted to invading pathogens, foreign materials or dead cells through a process of chemotaxis. Leukocytes destroy bacteria by a process of phagocytosis – they literally surround bacteria and then release chemicals to destroy the bacteria. The same process is used when they encounter dirt, dead cells, foreign bodies etc. This process leads to the death of the leukocyte. It is the accumulation of dead leukocytes, bacteria, fluid and cell debris that is seen as pus at the site of infection of a wound or abrasion. There are five main types of leukocyte:
Platelets are tiny fragments of cells produced within the bone marrow. The surface of platelets has special compounds called glycoproteins along with other proteins. These allow platelets to attach to collagen. Plays play a very important role in preventing and reducing the amount of blood loss through wounds. Platelets are able to seal holes in small blood vessels by forming a plug. Where there is a larger wound they will form a clot.
Cardiovascular system: Blood vessels Blood vessels are responsible for the transport of blood around the body. There are three main types of blood vessel:
Arteries carry blood away from the heart. They have thick, muscular, elastic walls, which enables them to withstand the high pressure that blood is under as it leaves the heart. Arteries carry oxygenated blood to tissues such as muscle that are actively using oxygen. The one exception to this rule is the pulmonary artery which takes deoxygenated blood from the heart to the lungs. Arteries subdivide into smaller vessels in order to take blood to all the different body parts. Arteries continue to subdivide until they reach their smallest size where they are known as arterioles. Arterioles further divide and lead into capillaries where oxygen diffuses across the thin capillary surface to be delivered to the working tissue.
Veins carry blood back to the heart. Because the blood is now under far less pressure, veins only have thin inelastic walls. Veins carry deoxygenated blood away from the organs and tissues and back to the heart. The one exception is the pulmonary vein which carries oxygenated blood from the lungs back to the heart. The movement of blood, through veins, occurs primarily through the contraction of skeletal muscles, which constricts veins, forcing blood along the veins back towards the heart. Veins contain non-returnable valves which will not allow blood to flow backwards away from the heart. The smallest veins are called venules. These collect blood from the capillaries. These lead into other venules to form veins. These connect with other veins and in this way the nearer you get to the heart the larger the veins are.
Capillaries are the smallest blood vessels. There walls are just one cell thick, enabling oxygen to easily diffuse across them, from red blood cells across, and allowing carbon dioxide to easily diffuse across them into the blood. Capillaries link arteries and veins and it is as blood passes from arteries through the capillaries that it looses most of its pressure. As well as being the point of oxygen delivery to tissues they are also the point at which important nutrients, minerals, vitamins and hormones are taken up by muscles, and the point at which waste products are taken up by the blood. Capillaries also play an important role in the regulation of temperature by regulating blood flow to the skin. Under hot conditions blood flow to the skin is increased, through the dilation of capillaries in the skin, to aid heat loss, and under cold conditions capillaries in the skin constrict, reducing blood flow and therefore reducing heat loss. |
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