Sunday, 15 July 2012

Respiratory System

The Nasal cavity’s functions are to warm, filter and moisten the incoming air. The nose is the only external part of the respiratory system. The nose is divided into the external nose and the internal nasal cavity. When you breathe air enters the nasal cavity by passing through the nostrils.   

Pharynx is where the throat divides into the trachea which is the wind pipe and oesophagus which is the food pipe. The pharynx is a small length of tubing which connects the nasal cavity and mouth. There is also a small flap of cartilage called the epiglottis. This prevents food from entering the wind pipe.  

The Larynx is also known as the voice box because it’s where sound is generated. It is made of cartilage and has a section that comes above the surface which is known as Adams apple. The larynx is located between the pharynx and the trachea. The larynx provides an open airway and guides air and food into the correct channels. It also helps out with the vocal cords. It also helps protect the trachea by producing a strong cough reflex if any solid objects pass the through. 

Trachea is also known as the windpipe, it carries air from the throat into the lungs. The inner part of the trachea is covered in tiny hairs which catch dust and then is removed when we cough. The trachea is surrounded by 15-20 C shaped rings of cartilage at the front and side which help protect the trachea and keep it open.

Bronchi are formed by divisions of the Trachea and carry air into the lungs. There are to bronchi one which enters the right lung and one that enters the left lung. The right bronchus is wider, shorter and more vertical than the left. When inhaled air reaches the bronchi it is warm. Inside the lungs each bronchi divide into lobar bronchi three on the right and two on the left. Bronchioles are very narrow tubes less than 1 millimetre in diameter. There is no cartilage within the bronchioles and they lead to alveolar sacs.   

Lungs occupy most of the thoracic cavity and extend down to the diaphragm. The lungs differ in shape and size and the left lung is smaller than the right.

The lungs are divided into lobes. The left lung contains two lobes and the right has three. Each lobe has an artery and vein and receives air from an individual bronchus.  

The Pleural membrane helps keep the lungs apart and air tight. If one lung is punctured the other one can remain functional and work normally due to being held in a separate cavity.

Pleural cavity is found in between the parietal pleura and the visceral pleura. The pleural cavity is filled with pleural fluid.

The parietal pleura covers the thoracic wall and the top of the diaphragm it then continues around the heart and between the lungs. The parietal pleura is the outermost of the two pleura membranes.

Thoracic cavity I the chamber of the human body that is protects by the thoracic wall. It is separated from the abdominal cavity by the diaphragm.

Visceral pleura is the innermost of the two pleural membranes. It covers the surface of the lung and dips into the spaces between the lobes.

The pleural membranes produce Pleura fluid which fills the pleural cavity between them. The pleural fluid allows the lungs to glide easily over the thorax wall. The pleural fluid also provides the surface tension that keeps the lung surface in contact with the chest wall.

Alveoli have very thin walls which allow the exchange of gases oxygen and carbon dioxide. They also are surrounded by a network of capillaries. There are approximately 3 million alveoli within an average adult lung. There are over 300,000,000 alveoli in the lungs providing a large area for gaseous exchange to occur.

The Diaphragm is a broad band of muscle which sits underneath the lungs, attaching to the lower ribs, sternum and lumbar spine and forming the base of the thoracic cavity. Contraction of the diaphragm increases the volume of the chest cavity which draws into the lungs during inspiration. When the diaphragm is relaxed it involves the recoil of the diaphragm and decreases the volume of the chest cavity pumping out air. The Diaphragm and Intercostals are muscles that aid breathing.

Internal and external intercostals lie between the ribs to help inhalation and exhalation the muscles extend and contract. The external intercostals pull the ribs upwards and outwards increasing the volume of the chest cavity and drawing air into the lungs during inspiration. The internal intercostals draw the ribs downward and  inwards decreasing the volume of the chest cavity and forcing air out of the lungs during expiration

Mechanisms of breathing

When the air pressure inside the lungs decreases, more air flows in. Air pressure inside the lungs is decreased by increasing the size of thoracic cavity. Due to surface tension between the two pleural membranes, the lungs follow the chest wall and expand.

The muscles involved in expanding the thoracic cavity include the diaphragm and the external intercostals muscles. As the diaphragm contracts, it flattens out. As a result, the superior-inferior dimension of the thoracic cavity increases. Contraction of the intercostals muscles lifts the rib cage and pulls the sternum upwards. Although these actions expand the thoracic cavity by only a few millimetres this is enough to increase the thoracic volume by almost 500ml the usual volume of air that enters the lung during normal inspiration.

When at rest the external intercostals muscles contract and the internal intercostals muscles relax. During forced inspirations that occur when exercising. The thoracic volume increases further. 

Expiration is a passive process that depends more a lung elasticity than on muscle contraction. As the inspiration muscles relax, the rib cage descends and the lungs recoil. Thus, the thoracic volume decreases.     

During expiration at rest the diaphragm and external intercostals muscles relax and return to their original positions.  When exercising the contraction of the internal intercostals forces air out of the lungs.  

Breathing rate is all controlled by chemoreceptors within the main arteries which monitor the levels of oxygen and carbon dioxide within the blood.

Tidal Volume                                                                                                                       
The amount of air which enters the lungs during normal inhalation at rest. The average tidal volume is 500ml. The same amount leaves the lungs during exhalation.

Inspiratory reserve volume
By breathing in deeply it is possible to take in more than usual 350cm3 of fresh air that reaches the alveoli. This is especially important during exercise. In addition to the tidal volume you can also breathe in up to an additional 3,000cm3 of fresh air. This is known as the inspirational reserve volume. It is the amount air inhaled above tidal volume. 

Expiratory reserve volume
The expiratory reserve volume can be up to 1,500cm3 and is the amount of additional air that can be breathed out after normal expiration. At the end of normal breath, the lungs contain the residual volume plus the expiratory reserve volume. This is the amount of air exhaled above tidal volume during a forceful breath.

Vital capacity residual volume
Vital capacity is the amount of air that can be forced out of the lungs after maximal respiration. The volume is around 4,800cm3. It is the most air you can exhale after taking deepest breath you can. It can be up to ten times more than you would normally exhale.  

Taking Residual volume
Residual volume is the amount of air left in the lungs after maximal respiration, which is when you breath out as hard as you can. The volume is around 1,200cm3 for an average male. If you then exhale as much as possible only the residual volume remains. There is also some air left in the lungs to stop the lungs from collapsing.

Total lung capacity
Total lung capacity is the volume of air contained in the lungs after maximal inspiration. The volume is usually between 4,00cm3 and 8,000cm3, with 6000cm3 for an average sized male.
Spirometry means the measure of breath. It is the most common of the pulmonarary function test, measuring lung function and specifically the measurement of the amount of volume and speed or flow of air that can be inhaled and exhaled. Spirometry is an important tool used for generating pneumotachographs. This is an apparatus for recording the rate of air flow to and from the lungs.  This is helpful in assessing conditions such as asthma.  

Transport of gases and gaseous exchange

The main function of the respiratory system is gaseous exchange. This refers to the process of Oxygen and Carbon Dioxide moving between the lungs and blood

The oxygen absorbed into the blood in the capillaries combines with haemoglobin in the red blood cells form oxyhaemoglobin. The concentration of red blood cells and their haemoglobin affects the amount of oxygen taken up by blood. The red blood cells typically make up 45 per cent of blood volume. These concentrations increase during exercise as more fluid moves from the plasma to the tissues and more water is lost from the plasma as sweat. Long term endurance training can result in an increase in red blood cells and therefore haemoglobin. 

Carbon Dioxide
Carbon dioxide is a waste product of aerobic metabolism and collects in tissues. It is carried to the veins by the cardio vascular system. It is cleared from the tissues by the blood in veins. It is carried by the haemoglobin in red blood cells and diffused into the lungs where it is expired when you breathe out. 

Haemoglobin is a large protein that can combine reversibly with oxygen. Haemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues to the lungs. Haemoglobin also plays a part in maintaining the shape of red blood cells.

Oxyhaemoglobin is when oxygen attaches to haemoglobin. Haemoglobin forms an unstable reversible bond with oxygen. Blood carries oxyhaemoglobin to tissue  where the oxygen is released during a process known as tissue respiration. Oxyhaemoglobin is bright red and in oxygen unloaded form it is called deoxyhemoglobin and is purple-blue.

Control of breathing

Neural control                                                                                                                                                                                Neurones conduct nerve impulses from the medulla and pons which are both part of the brain stem. The medulla is the lower part of the brain stem responsible for controlling several major autonomic functions of the body and pons are located on the brain stem and responsible for providing linkage between the upper and lower levels of the central nervous system.  In the pons there is the pneumotaxic centre and the apneustic centre. The pneumotaxic centre limits the contraction of the inspiratory muscles and prevents the lungs from overinflating. The appneustic centre stimulates the inspiratory center, prolonging the contraction of inspiratory muscles. Neurones from two areas of the medulla are critical in respiration. These are the DRG, the dorsal respiratory group and the VRG, the ventral respiratory group. The VRG is responsible for rhythm generation it is located in the medulla and contains both inspiratory and expiratory neurons. It is almost in active during normal respiration but is active when increased ventilation. The DRG is a cluster of inspiratoty nerve cells found in the medulla. It sends impulses to the diaphragm and inspiratory intercostals muscles. The DRG also receives impulses form chemoreceptors.        

Chemical control                                                                                                                     
Levels of carbon dioxide and oxygen are constantly changing. In order to monitor this, the body has receptors called chemoreceptors in the medulla, aortic arch and carotid arteries. Chemoreceptors are receptors sensitive to various chemicals in solution, aortic arch is the major artery that has three branches to deliver oxygenated blood throughout the body and carotid arteries feed the head and brain with oxygenated blood. Chemoreceptors in the carotid arteries and aorta, detect the levels of carbon dioxide in the blood and causes these receptors to stimulate the respiratory centre to increase the inspiratory rate. It then sends nervous impulses to the external intercostal muscles and the diaphragm via the phrenic nerve to increase breathing rate and the volume of the lungs during inhalation. Stretch receptors in the walls of bronchi and bronchioles are activated when the lungs expand to their limit. These receptors signal the respiratory center to discontinue stimulation of the inspiratory muscles which allow expiration to begin, this response is called the inflation reflex.

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