Lungs - The Respiratory System
Around 21% of our atmospheric air is Oxygen (O2), which is essential for human life because the chemical activities that take place in our cells can only occur in its presence. This oxygen is involved in the series of chemical changes that result in the release of energy from nutrient materials. It is essential for all cellular activities.
This respiratory system, see representation of respiratory system, is the system by which oxygen is taken into the body from the external environment and carbon dioxide a waste product of cell metabolism is excreted. It consists of a series of tubes or respiratory passages that carry the air from our nose (or mouth) through to our lungs and into the network of blood capillaries therein. The respiratory passages consist of the nose, the pharynx, the larynx (voice box), the trachea, two bronchi and a large number of bronchial tubes that subdivide and lead to millions of tiny air sacs called Alveoli.
Blood Flow Between Heart and Lungs
We have two lungs, a left and a right one, the left one being slightly smaller than the right due to the space being taken by the heart. The right lung consists of three lobes, superior, middle and inferior, the left lung only has two, it has no middle lobe. Each lobe is made up of a large number of Lobules.
Lobules consist of terminal bronchioles, respiratory bronchioles, alveolar ducts and alveoli; see Lung Lobule diagram.
So what occurs within the lungs?
The Heart answer mentioned the Pulmonary and Systemic circulation systems; it is the pulmonary system that links the Heart with the Lungs.
The above diagram is my attempt to show both the bronchial tree and blood flow through the lungs.
The Pulmonary Artery divides into two branches each of which conveys blood to a lung. Each of these branches divide again and again eventually becoming a dense capillary network that surround the walls of the alveoli. The walls of the alveoli and the walls of the capillaries each consist of only one layer of flattened epithelial cells. It is here between two very fine membranes that the exchanges of gasses take place.
Capillary Network around Alveoli
Exchange of Gasses betweel Alveolar Air and Capillary Blood
Blood arriving at the lungs contains high levels of Carbon Dioxide (CO2) and low levels of Oxygen (O2). Atmospheric air arriving at the Alveolus has a higher level of O2 and a lower level of CO2. Hence CO2 diffuses downhill from the blood capillaries into the alveolus and O2 diffuses downhill from the alveolus to the blood capillaries. Hence the blood leaving in the Pulmonary vein contains less CO2 and is refreshed with fresh molecules of Oxygen.
How does this fresh Air get into our lungs?
This is called the Mechanism of Respiration, the process by which the lungs expand to take air in and contract to expel it. This cycle occurs approximately 15 times per minute and consists of three stages: -
The expansion of the chest during Inspiration is caused by muscle activity, partly voluntary and mainly involuntary, voluntary control usually occurs during speaking, singing and other such activities. During normal respiration, the Intercostal muscles and the Diaphragm are used but during deep breathing or when encountering any breathing difficulty, muscles of the neck, shoulder and abdomen assist.
There are 11 pairs of intercostal muscles, they occupy the spaces between the 12 pairs of ribs, and they are arranged in layers, the external and internal intercostals muscles. The diaphragm is a dome shaped structure that separates the thoracic and abdominal cavities. Working together, the intercostals muscles and the diaphragm change the capacity of the thoracic cavity, and hence the lungs. During inspiration the muscles contract to expand the cavity and the lungs fill with fresh air, during expiration the muscles relax, retracting the cavity and exhaling waste gasses.
During quiet breathing, the amount of air passing in and out of the lungs is around 500ml; this is known as the tidal volume. During maximal inspiration, extra air can be inhaled; this extra is known as inspiratory capacity.
Functional Residual capacity is the amount of air left in the passages and alveoli at the end of quiet expiration. The tidal air mixes with this so as to cause only small changes to the composition of alveolar air. Because blood is continually flowing and the alveoli never completely empty then a continuous, uninterrupted exchange of gasses can occur thus keeping the concentration of blood gasses at a constant level.
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