In human beings, many organs take part in the process of respiration that organs are called as respiratory organs. We call them organs of respiratory system. The main organs of human respiratory system are : Nose, Nasal passage (or nasal cavity), Trachea, Bronchi, Lungs and Diaphragm.
The human respiratory system begins from the nose. Our nose has two holes in it which are called nostrils. There is a passage or hole in the nose behind the nostrils which is called nasal passage (or nasal cavity). The air for respiration is drawn or goes into our body through the nostrils or nares present in the nose. This air then goes into nasal. The nasal passage is separated from the mouth cavity (buccal cavity or oral cavity) by a heart, Bony palate so that we can breathe in air even when we are eating food (and the mouth cavity is filled with food). The nasal passage or the internal tube is lined with fine hair and mucus (Mucus is secreted by the glands inside the nasal passage). When the oxygen of air passes through the nasal passage, the dust particles and other impurities present in it are trapped by nasal hair and mucus so that clean air goes into the lungs. The part of throat between the mouth in windpipe is called pharynx. From the nasal passage, air enters into pharynx and then goes into the windpipe (or trachea).
The trachea is a tube which is commonly known as wind pipe. The air coming from the nostrils during breathing passes through trachea(The trachea, colloquially called the windpipe, is a cartilaginous tube that connects the pharynx and larynx to the lungs, allowing the passage of air ). Trachea does not collapse even when there is no oxygen of air in it because it is supported by rings of soft bones called cartilage. The upper end of trachea has a voice box called larynx. The trachea runs down the neck and divides into two smaller tubes called ‘bronchi’ (Bronchi are the main passageway into the lungs. When someone takes a breath through their nose or mouth, the air travels into the larynx ) at its lower end. The singular of bronchi is bronchus. The two bronchi are connected to the two lungs. The lungs lie in the chest cavity or thoracic cavity which is separated from abdominal cavity by a muscular derivation partition called diaphragm. The lungs are covered by two thin membranes called pleura(A pleura is a serous membrane which folds back onto itself to form a two-layered membranous pleural sac). The lungs are enclosed in a collection of ‘rib cage’ made of bones called ‘ribs’.
Each bronchus (a bronchus is a passage of airway in the respiratory system that conducts air into the lungs) divides in the lungs to form a large number of still smaller tubes called ‘bronchioles’. The pouch-like air-sacs at the ends of the smallest bronchioles are called ‘alveoli’ (singular alveolus). The walls of alveoli are very thin and very small and they are surrounded by very thin blood capillaries. It is in the alveoli that oxygen is take into the body and carbon dioxide is eliminated. In other words, it is in the alveoli that gaseous exchange takes place. The human lungs have been designed to maximise the exchange of gases as follows: There are millions of alveoli in the the lungs. The presence of millions of alveoli in the lungs provides a very large area for the exchange of gases. And the availability of large surface area maximise the exchange of gases. For example, if all alveoli from the two lungs of the humans are unfolded, they would give an area of about 80 square metres (which is nearly the size of a tennis court). The diaphragm is a sheet or a thin cover of muscle below the lungs. It help in ‘breathing in’ and ‘breathing out’.
When we breathe oxygen of air, the diaphragm and muscles attached to the ribs contract due to which our chest cavity expands. This expansion movement of the chest when we breathe that increases the volume inside the chest cavity. Due to increase in volume, the air pressure decreases inside the chest cavity and air from outside (being at higher pressure) rushes into the lungs through the nostrils, Treachea and bronchi. Alveoli are surrounded by very thin blood vessels called capillaries that carrying blood in them. So, the oxygen of air diffuses out from the alevolus wall (alveoli walls) into the blood. The oxygen is carried by blood to all the parts of the body (This oxygen is carried by a red pigment called haemoglobin present in blood).
As the blood passes through the tissue of the body, the oxygen present in it diffuses into the cells (due to its higher concentration in the blood). This oxygen combines with the digested food (glucose) present in the cells to release energy. Carbon dioxide gas or the CO2 gas is produced as a waste product during respiration in the cells of the body tissues. This carbon dioxide diffuses into the blood (due to its higher concentration in blood tissues). Blood carries the carbon dioxide or CO2 back to the lungs where it diffuses into the alveoli. When we breathe out oxygen of air, then the diaphragm and the muscles attached to the ribs relax due to which our chest cavity contacts and becomes smaller. This contraction or this pressure in the movement of the chest pushes out carbon dioxide from the alveoli of the lungs into the Trachea, nostrils and then out of the body into air in the atmosphere.
Please note that during the breathing cycle, when air is taken in (or inhaled) and let out (or exhaled), the lungs always contain a certain residual volume of air so that there is sufficient time ‘for the oxygen to be absorbed’ into the blood and ‘for the carbon dioxide to be released’ from the blood. Another point to be noted is that carbon dioxide is more soluble in water (than oxygen), so it is mostly transported in dissolved form in our blood.