Respiration is the process by which oxygen from the lungs is carried by the blood to the tissues; and carbon dioxide formed in the tissues by metabolic activity is carried by the blood to the lungs and is expired out.
Primarily the human respiratory organs is meant for the oxygenation of blood. Two numbers of lungs are the chief organs of the respiratory system, are called the left and right lung. Oxygen of the air reaches the lungs and by passing through a series of respiratory passages, also removes the carbon dioxide from the blood.
The external nares, which opens into the left and right nasal cavities and the air gets entry into the body through it. The nasal cavities also have olfactory areas which act as the end organs foe smell. The posterior ends of the nasal cavities have the openings called the internal nares. The internal nares opens into the pharynx, which is a single cavity and not divided into right and left halves. The nasal cavities open into the nasopharynx is the upper part, where as the middle part is the oropharynx and the lower part is the laryngopharynx. Air passes the nose enters the nasopharynx and passes down through the oropharynx and the laryngopharynx. However in the system of human respiratory organs the air can also passes through the mouth directly in to the oropharynx and then to laryngopharynx. Air from the laryngopharynx get entry into a box like structure organ called the larynx . Apart from being a respiratory passage voice also produce from this organ.
The Larynx is continuous with a tube namely the trachea which passes through the lower part of the neck in to the upper part of the thorax. The trachea is a cartilaginous and membranous tube about 10 or 11 cm long. It is flattened posteriorly; its external diameter from side to side is about 2 cm in adult male and 1.5 cm in the adult female. At the level of lower border of the manubrium sterni the trachea bifurcates in to the right and left principal bronchi , which carry air to the right and left lungs. In the lung each principal bronchus divides, like the branches of a tree, into many smaller bronchi to more smaller bronchi that ultimately end in microscopic tubes namely the bronchioles . The alveoli are the microscopic sac-like structures, where the bronchioles are open. The walls of the alveoli contain a reach network of blood capillaries where the blood is separated from the air in the alveoli by a very thin membrane through which oxygen can pass into the blood and carbon dioxide can pass into the air of the alveoli and so the continuous process performs by the human respiratory organs.
There is a partition between thorax and abdomen called the diaphragm . It is a dome-shaped muscle with vertically running fibres at the centre ending in the central tendon. The muscle is attached behind to fixed structures like the lower ribs at the sides. Another important set of respiratory muscles are the intercostals muscles that occupy the intercostals spaces. There have also several other muscles attached to the thoracic wall may help in the performance of pumping of air in and out of the lungs for breathing.
The process of human respiratory system involves four stages, such as
(a) Ventilation, which means the passage of air in and out of lungs during inspiration and expiration.
(b) Intrapulmonary gas-mixing, that means the distribution of oxygen-rich inspired air with the air which is already present in the lungs.
(c) Diffusion, it means the gas transfer across alveolo-capillary membrane due to tension gradient.
(d) Perfusion, which means flow of adequate quantity of blood through the lungs so the the diffused gasses may are carried away.
So all the human respiratory organs are acts as an organised unit for the continuous process of respiratory system.