AUSTRALIAN
NATUROPATHIC NETWORK
Serving
the community since 1998
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
| Departments
» Medical Sciences » A&P » Respiration by Peter Stevenson |
Introduction
Respiration is the exchange of gasses between the atmosphere, blood, and cells. It consists of three basic phases: pulmonary ventilation, external respiration, and internal respiration1. The function of the respiratory system is to provide oxygen to the cells of the body and remove carbon dioxide. This paper discusses the path oxygen takes as it is brought into the body and used at the cellular level.
Pulmonary Ventilation Inspiration
Pulmonary ventilation is the noticeable act of breathing: the exchange of air between the atmosphere and the lungs. The first step for the oxygen molecules on its journey to the cells.
The inspiration of air is a process initiated by contraction of the diaphragm and the external intercostal muscles. Contraction of the diaphragm increases the vertical division of the thoracic cavity, whilst the external intercostals pull the ribs superiorly and push the sternum anteriorly. This action pulls the superficial serosal membrane surrounding the lungs (the parietal pleura) outwards. The inner serosa (visceral pleura) is pulled outward owing mainly to adherence to the parietal layer. This results in the expansion of the lungs and a drop in pressure due to the increase in volume. The pressure differential between the atmosphere and within the lungs causes air to rush in.
As the air enters the lungs it passes through a number of anatomical structures. These are described in Table 1.
| Structure | Function |
| Nose | Warm, filter and
humidify air as it enters the body
Location for sensory organ of smell Aid with speech |
| Nasopharynx | Allows passage of air
Contains openings for Eustacian tube and permits equalisation of pressure in middle ear |
| Oropharynx | Passageway for air and food |
| Laryngopharynx | Passageway for air and food |
| Larynx | Connects Laryngopharynx
to trachea
Aids in voice production and contains vocal folds and ventricular folds Contains Epiglottis which prevents food entering the trachea |
| Trachea | Passageway for air to
primary bronchi
Filters air, cilia on walls of trachea move foreign bodies towards the mouth |
| Primary bronchi | Branches left and right from trachea connects to secondary bronchi |
| Secondary bronchi | Passageway for air to
each lobe of the lungs joins to tertiary bronchi
Three for the right lung (three lobes) Two for the left lung (two lobes) |
| Tertiary bronchi | Passageway for air from secondary bronchi to bronchioles |
| Bronchioles | Continue to branch into smaller tubes until they become terminal bronchioles |
| Terminal bronchioles | Continue to divide into microscopic branches called respiratory bronchioles |
| Respiratory bronchioles | Continue to divide into alveolar ducts |
| Alveolar ducts | Deliver air to alveoli |
| Alveoli | Place of exchange between Oxygen and blood stream |
Table 1 Anatomical structures on route to the lungs
Once at the alveoli the oxygen in the air can be exchanged with the blood stream. Around the alveoli are a network of capillaries (pulmonary capillaries) which have a single cell thickness, and it here that the oxygen molecules contained in the air are transferred into the blood stream through a process of diffusion.
External Respiration
The exchange of oxygen and carbon dioxide between the alveoli and the blood is called external respiration. Deoxygenated blood is pumped from the right ventricle of the heart through to the lungs. The partial pressure of oxygen in the deoxygenated blood is lower than that of the oxygen in the air in the alveoli. Owing to this difference there is a net diffusion of oxygen from the alveoli across the cell membranes and into the blood. At the same time carbon dioxide present in the blood diffuses into the alveoli because there is an imbalance in the partial pressures in the opposite direction.
Factors that affect the rate of diffusion include:
The partial pressure differential the greater the difference the faster the oxygen diffuses.
The surface are provided for the exchange the greater the surface area the greater the rate.
The thickness of the alveolar membrane the thicker the membrane the slower the rate.
The solubility of CO2 and O2 CO2 has greater solubility than O2 and thus diffuses out of the blood faster than the O2 diffuses in, for this reason if diffusion is impaired the affects of lack of oxygen (hypoxia) occur before the affects of carbon dioxide retention (hypercapnia).
Transportation of Oxygen
Once the oxygen is in the blood stream it must some how be transported to the cells. Oxygen does not easily dissolve in water so only a little is carried in the blood plasma. The remainder is transported by combining with haemoglobin in red blood cells. Each 100ml of oxygenated blood contains about 20 ml of oxygen: 19.7 ml combined with the haemoglobin and 0.3 ml dissolved in the plasma1.
Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhemoglobin. A number of factors determine how much oxygen combines with the haemoglobin, chief among them is the partial pressure of oxygen in arterial blood which is mainly determined by lung function2. As the partial pressure of oxygen increases so to the oxygen saturation of the haemoglobin.
Dissociation curves, which show the relationship between partial pressures and haemoglobin saturation, have an S shape. The flat upper portion of these curves reflects that significant drops in partial pressure can be tolerated without a great reduction in saturation. However, the curve becomes steep and almost linear at a partial pressure of 60 mmHg and below this the saturation levels drop of significantly2.
Other factors influence the affinity of the haemoglobin for oxygen, these include:
The pH of the blood - at higher levels of acidity haemoglobin has less affinity for oxygen,
The partial pressure of carbon dioxide in the blood at higher pCO2 oxygen is released by haemoglobin more readily,
The temperature of the blood at higher temperatures the oxygen dissociates with the haemoglobin more readily,
The amount of 2,3-bisphospoglycate (BGP) in the blood the higher the levels of this substance the greater the dissociation of oxygen.
The oxygen now has its ride to the cell.
Internal Respiration
Internal respiration involves the exchange of oxygen from systemic blood capillaries into cells, and again this process relies on diffusion and partial pressure for exchange to take place.
Oxygenated blood travels from the lungs to the left atrium of the heart and is pumped through the body by the left ventricle. The oxygenated blood enters the arteries eventually arriving at the tissue capillaries where it reaches the cells.
The partial pressure of oxygen in the cells is much lower than that of the oxygenated blood; thus the oxygen dissociates with the haemoglobin and diffuses into the cell. At the same time the reverse is true for carbon dioxide, and it diffuses back into the blood stream.
Once inside the cell the oxygen is used by the mitochondria in the Krebs cycle for the production of ATP. This reactions produces carbon dioxide and water, and life can continue.
Conclusion
The three steps of getting oxygen to the cell are pulmonary ventilation, external respiration, and internal respiration. These processes go on continually in the human body, and if they stop we die.
References
Tortora, G.J., Grabowski, S.R., Principles of Anatomy and Physiology - 8th Edition, Harper Collins, NY, 1996.
Willis Hurst, J (Ed.), Medicine for the Practicing Physician 3rd Edition, Butterworth-Heinemann, USA, 1992.