The trachea is composed of incomplete rings of cartilage and is lined with ciliated epithelium and goblet cells. As the cartilage rings are incomplete, it enables the trachea to be more flexible and thus facilitating the passage of food down the oesophagus. The purpose of cilia and mucus in the trachea is to move foreign particles (such as dust particles) up and away from the lungs; instead they are moved towards the pharynx and swallowed. The trachea is approximately 10 – 15 cms in length 21 to 26 mm in diameter. It divides into two main branches, the right and left bronchi, which enter the right and left lungs respectively. Bronchi are also supported by cartilage to stop any collapse, however as these bronchi branch into bronchioles, they cease to be supported by cartilaginous material; instead their walls contain smooth muscle. Bronchioles are small respiratory airways that lead to air sacs directly within the lungs. These thin-walled, inflatable sacs are called alveoli. It is here that oxygen will diffuse through capillaries into arterial blood.
The average adult’s lungs contain approximately 600 million alveoli (The Franklin Institute, 1996). The left lung is slightly smaller than the right lung to allow space for the heart. It only has two lobes compared to three in the right lung; however it has an additional projection on the upper lobe called a lingula. The upper and lower lobes of the left lung are separated by an oblique fissure. The lobes in the right lung are called the superior, middle and inferior lobes and are separated by interlobular fissures. Between the lungs and the chest wall is a liquid called pleural fluid which acts as a lubricant as these two parts of the body slide past each other. This area is called the pleural cavity. One side of this membrane (the parietal pleura) is attached to the chest wall and the other (the visceral pleura) is attached to the lungs. Besides containing the pleural fluid, this structure exists to exert pressure from movements in the chest wall to the lungs which then allows maximum inflation of the alveoli during respiration.
As previously mentioned, the 600 million alveoli that are present in the human body ensure an enormous surface area for gas exchange to take place. This process is also facilitated by interstitial fluid which surrounds each cell and provides a means of delivering materials to the cells. Additionally the proximity of many blood capillaries and thin, permeable walls (only one cell thick) of the alveoli assist in the diffusion. These capillaries receive as much blood per minute as the rest of the body put together (Boyle & Senior, 2008) and therefore the incoming oxygen can be quickly transported around the body. This rapid process is essential to ensure there is always less oxygen in the blood running through the lungs than in the atmospheric air and thus a diffusion gradient is constantly present. The alveoli are constantly moist from incoming water from alveoli cells. This is an essential factor of gas exchange as oxygen dissolves in this water before diffusing through the cells into the blood.
Oxygen enriched blood (or oxyhaemoglobin complex) is then transported, via the pulmonary vein, to the left atrium of the heart. The pulmonary vein is different from other veins as it carries oxygenated blood whereas most other veins carry deoxygenated blood. This process is part of the pulmonary circulatory system; the other circulatory system with the body is called systemic circulation whose function is to carry blood from the heart to the rest of the body.
There are four pulmonary veins, two leading from each of the lungs called the left inferior and superior and the right inferior and superior. The left atrium which receives the blood is one of two atria in the heart. These are thin walled and receive blood. The other two chambers within the heart are called ventricles and they are situated in the lower section of the heart. These are thicker walled than the atria as they have to function much harder. The function of the left ventricle is to pump oxygenated blood around the whole of the body and for this purpose it requires a much thicker muscle structure than the right ventricle which only has to pump to the lungs. Separating the atria and the ventricles are atrioventricular valves which stop blood from flowing from the ventricles back up to the atria. The left valve has two flaps and is called the bicuspid (or mitral) valve, whereas the right valve has 3 flaps and is called the tricuspid valve. These valves are held in place by chordae tendineae attached to papillary muscles. These contract at the same time as the ventricles and thus hold the valves closed. The other vales within the heart are called semi lunar which guard the openings to the pulmonary artery and aorta and stop blood flowing back into the ventricles. These valves all ensure that blood only flows in one direction through the heart.
The heart is composed of cardiac muscle with a thick wall of muscle separating the right and left cavities of the heart called an inter-ventricular septum. The cells that make up cardiac muscle are multinuclear and are called myocardiocyteal muscle cells. This type of cell is a capable of functioning without any conscious effort or additional stimuli; in other words it functions spontaneously. They are also specially adapted to resist symptoms of tiredness and for this purpose they have an unusually high number of mitochondria. Each chamber of the heart must be full of blood before a heartbeat can take place. This process occurs approximately 72 times per minute in the average person (buzzle.com). Initially, an electrical signal is received from the sino-atrial node (SA node or pacemaker) which is located in the wall of the right atrium. This signal causes the atria to contract simultaneously. Next, a second group of cells called the atrio-ventricular node (AV node) receives this signal and channels it down the septum through specialised muscle fibres called the bundle of His. The signal continues to spread through the walls of the ventricles using the Purkinje fibres and the ventricles then contract after they are filled with blood. This heartbeat initiates the cardiac cycle which is divided into diastole and systole phases.
The diastole phase involves the right hand side of the heart receiving deoxygenated blood via the superior and inferior vena cavae and oxygenated blood via the pulmonary vein. The atria and the ventricles must be relaxed for this to happen and thus the atrioventricular valve is also relaxed which allows blood to reach the ventricles. The SA node then contracts which starts a contraction with the atria commencing the atrial systole phase. Blood then moves from the atria to the ventricles which remain closed at this point. During the second stage, or ventricular systole phase, the atria relax and the ventricles then contract which pushes blood out of the heart using the pulmonary arteries and the aorta.
Several nervous and hormonal factors can affect the heart rate and blood pressure. During an increase or decrease in physical activity the heart rate can vary from 50 to 200 beats per minute and this requires careful regulation by the nervous system. Carbon dioxide concentration increases in the blood (and thus lower the pH) if strenuous exercise is being undertaken. Chemical receptors within the carotid artery then act to increase the heart beat thus increasing the rate at which carbon dioxide is delivered to the lungs. Other factors such as body temperature, body position, age (infants have a higher heart rate), sex (females have higher heart rate) and hormones can cause fluctuations in the heart rate. There is still much research being done to pinpoint exactly how the heartbeat is controlled and the actual gene which regulates the chemical signals that control the heartbeat (named SCN10A), was only discovered in 2010 by researchers at Imperial College (The Telegraph, 11 Jan 2010). Blood pressure must always be at a sufficient level to allow blood to reach all tissues that require it and the carotid sinus regulates this. Factors such as narrowing blood vessels, rising levels of carbon dioxide and hormones such as adrenaline can alter the level of blood pressure.
Blood that leaves the heart then circulates around the body using several types of blood vessels namely: arteries which move blood away from the heart, capillaries which link arteries to veins which then carry blood back to the heart. Each of these three vessels is adapted to their function however both arteries and veins have a similar structure. They each have three layers: tunica intima (the internal layer composed of endothelial cells), tunica media (the middle layer made of muscle and elasic fibres) and tunica externa (the outer layer composed of connective tissue). Arteries pulse blood rapidly through the body under high pressure due to a relatively small lumen and their composition of thick muscular walls and plentiful amounts of elastic tissue. These arteries then branch into smaller arterioles and then into even smaller capillaries. The main arteries within the body include: the hepatic artery which goes to the liver, the renal artery which goes to the kidneys, the mesenteric artery to the intestinal area and the iliac artery which leads to the genitals and legs.
Veins transport blood under low pressure as they have a thin muscular wall with little elastic tissue. Therefore in order for blood to flow back to the heart, in the absence of an efficient pumping mechanism, movement is required by the body at a muscular level.
The jugular vein serves the head and neck, whereas the lower half of the body is served by the hepatic, renal and iliac veins. Smaller veins are called venules.
Capillaries have no muscle or elastic tissue, and, unlike arteries, they are permeable and unable to constrict. It is in the capillaries where gas exchange at a cellular level takes place by diffusion. This is possible due to the high concentration of oxygen in blood compared to that in cells and the reverse concentrations present in levels of carbon dioxide in the blood and cells. As all tissue cells are in close proximity to capillaries, the exchange is very efficient. The rate that blood flows through the capillaries is slow, thus allowing time for oxygen to enter the tissues and for carbon dioxide to be excreted. Carbon dioxide is transported in the blood in three ways: physically dissolved, bound up in haemoglobin and as bicarbonate (Sherwood, 1993).
As mentioned earlier, once the blood has flowed throughout the body, it returns back to the heart via the vena cavae. The right ventricle pumps it into the lungs via the pulmonary arteries. The higher concentration of carbon dioxide then diffuses out of the blood and into the alveoli where a lower concentration is present, ready for exhalation.
Exhalation is a much more passive process than inhalation. In this case, the diaphragm relaxes and resumes its upwards dome shape. The external intercostal muscles relax which moves the ribs downwards and inwards. These actions decrease the volume of the chest, the lungs and alveoli which in turn increases the pressure and thus air is forced out of the lungs. The composition of exhaled air differs during the course of expiration. The initial gas is similar to atmospheric air, as prior to exhalation it has simply filled the trachea and the bronchi. Thereafter it changes to air that has been expelled from the alveoli and its composition is 17% oxygen, 79.6% nitrogen, carbon dioxide 4%, trace gases 1% (Boyle & Senior, 2008) – which is different to that of atmospheric air due to the gas exchange process. It is also saturated with water vapour.
Due to the demands placed on the respiratory system, there is a capacity for several disorders to occur. Asthma and bronchitis are two such disorders.
Asthma affects 5.4 million people in the UK (Asthma UK, 2011) and is chronic condition that causes breathlessness, wheezing and coughing. It is a problem that affects more women than men and it can sometimes be hereditary. Environmental factors, such as air quality, can also contribute to the development of asthma.
Symptoms occur due to the narrowing of the bronchi either by non-specific factors such as irritants within the atmosphere, e.g. smoke or pollution or from specific factors such as irritants from pet hair, pollen and certain medicines; e.g. aspirin. The membranes of the bronchi swell and produce an increased amount of mucus. Sufferers are generally born with a predisposition to asthma which is then triggered by one of the factors above.
There is no cure, so any treatments taken are aimed at reversing the swelling of the bronchi and trying to prevent an attack. Bronchodilators relax the muscles within the airways and as their effects can be very quick acting, they give almost immediate relief from the symptoms. Anti-inflammatory medicine can be taken regularly to keep narrowing of the bronchi membranes to a minimum.
Bronchitis is also a disease causing swelling of the lining of the bronchi. There are two different types, acute and chronic. In the case of acute bronchitis, viruses are to blame in 90% of cases, with bacteria accounting for the remaining 10%. Acute cases often accompany respiratory tract illnesses and are characterised by a severe cough plus one or more of the following: fever, nasal congestion or sore throat. Chronic bronchitis (or ‘chronic obstructive pulmonary disease’ as it is often called) is classified as a long-standing disease (symptoms have usually been present in excess of three months) and is mainly caused by smoking, although air pollution can be causative. The symptoms are very similar to those of acute bronchitis; however any mucus that is produced is often discoloured – having a yellow/green tinge to it.
As viruses are to blame for the majority of acute bronchitis cases, antibiotics would be ineffective in trying to cure the disease. Instead, it can be alleviated by dealing symptomatically; in other words painkillers for any aches or sore throats and cough medicines to relive the cough. Patients with chronic bronchitis are advised to stop smoking immediately.
