Heart Circulation and Gas Excange

The human body cells need oxygen in order to function and the respiratory system allows this to happen. Consisting of air passages, pulmonary vessels, the lungs and breathing muscles, it supplies fresh oxygen to the blood which can be fed to other parts of the body.
The ribs, rib muscles and diaphragm all play a part in allowing air into and out of the lungs, (oxygen going into the body and carbon dioxide leaving the body). This is known as ventilation. When we breathe in we inhale and when we breathe out we exhale.
Breathing in is an active process, which means it uses energy, and the process is as follows: the external intercostal muscles contract, whilst the internal muscles relax; the ribs are pulled upwards and outwards which increases the volume of the chest (thoracic cavity) and the muscles of the diaphragm contract which causes it to flatten, (this also increases the volume of the thoracic cavity). The volume increase in the thorax results in reduced pressure in the lungs, so atmospheric pressure is now greater than pulmonary pressure; therefore air is forced into the lungs.
This process is known as Inspiration.
Expiration is the largely passive process of breathing out, (it requires little energy). The process begins with the contraction of the internal intercostal muscles, whilst the external intercostal muscles relax. The ribs move down and inwards, which decreases the thorax volume. The diaphragm muscles then relax allowing a decrease in the volume of the thorax. The decreased volume increases the pressure in the lungs, so the pulmonary pressure is now greater than the atmospheric pressure, thus is forced out of the lungs.
Gas exchange takes place through a gas exchange surface, (also known as a respiratory surface) and is the process of getting oxygen by taking air into the blood, and the removal of carbon dioxide waste from the blood into the air. The removal of carbon dioxide from the blood is essential for the well-being of the body. A build-up of carbon dioxide waste can result in a a variety of symptoms including headaches, increased blood pressure, reduced nerve and brain function, confusion and lethargy. If left untreated, high levels of carbon dioxide in the blood can lead to damage to the internal organs such as the brain.

The gas exchange system is made up of a respiratory surface - membrane lining the alveoli (air sacs) in the lungs. (Pickering, 2000, p.76) Situated at the ends of the bronchioles, (a set of tubes which allow air to reach the respiratory surface) alveoli are found in the lungs in their millions and are extremely small. Due to their large number, they allow a large surface area for gas exchange, which means that a large amount of gas can diffuse at once. The gases move by diffusion from where they have a high concentration to a low concentration: oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses from the blood into the air in the alveoli,- so blood supply is needed as part of the gas exchange system, (carried by the pulmonary artery and pulmonary vein) to carry the dissolved gases to and from the respiratory surface.
Further explanation of the process of ventilation: Fig. 1
The movement of air into and out of the alveoli is due to changes in the volume of the thoracic cavity produced by the muscles of ventilation. The diaphragm and the external intercostal muscles are the primary inspiratory muscles, whereas the intercostal and abdominal muscles are the primary expiratory muscles.
Inspiration
The expansion of the thoracic cavity during respiration causes intrapleural pressure, which is pressure exerted within the pleural cavity itself to decrease. (Norman, (1999) defines the pleural cavity as the closed space within which the lung grows). According to Stanfield and Germann (2006) this increases the transpulmonary pressure, (the negative pressure within the pleural cavity walls; this negative force acts as a suction to keep the lungs inflated) that is due to the difference between the intra-alveolar pressure and the intrapleural pressure. This increase in transpulmonary pressure causes the lungs to expand. The increase in the volume of the alveoli that accompanies this expansion decreases intra-alveolar pressure. The pressure gradient between atmospheric pressure and intra-alveolar pressure causes air to flow into the lungs. Air flow continues until intra-alveolar pressure equals atmospheric pressure.

Expiration
During normal breathing expiration is a passive process in which the muscles of inspiration relax and the elastic properties of the chest wall and lungs cause lung volume to decrease. This decrease in lung volume causes an increase in intra-alveolar pressure and creates the pressure gradient that causes an outflow of air. Further explanation of the process of gas exchange: Fig. 2
The process of gas exchange is respiration and takes place on two levels:
Internal respiration: Oxygen used by the mitochondria to produce ATP, with carbon dioxide produced as a waste product.
External respiration: the oxygen and carbon dioxide exchange between the air and body tissues. Air is drawn into body in order to provide oxygen to the lungs and the internal or used air is then expelled from the lungs to remove the carbon dioxide. From the diagram we see the movement of air (O2) into the lungs and carbon dioxide, (CO2) out of the lungs, (this is known as pulmonary ventilation). The exchange of O2 and CO2 gases between air and blood takes place through the capillaries. Air, (O2) is taken into the body and CO2 is produced by respiring cells in the tissues. It is then diffused out of the tissues and transported to the lungs where it is breathed out.