The structure of the breathing system

The structure of the breathing system

The nose

The nasal cavity runs from the nostrils to the nasopharynx and is separated into two halves by the nasal septum.
The roof of the nasal cavity is formed by four ventral skull bones, the floor is formed by the hard palate, and the sides of the nasal cavity are formed by the three turbinate bones.
Skin bearing hairs, is found immediately behind the nostrils and the remainder of the cavity is lined with cilliated epithelium which contains mucous glands (goblet cells). The hair and cilia help to filter the air of dust particles and the mucous functions in keeping the air moist as well as preventing inhaled bacteria from multiplying in the nasal passages.
The upper part of the nasal cavity functions as the olfactory region, the area where smell is sensed. The olfactory cells are located in the mucous membrane lining the upper turbinate bone.
Some of the skull bones and most of the nasal bones have small cavities which are lined by a mucous membrane and are known as the sinuses which store excess mucus.
The many blood capillaries in the upper nasal cavity assist in the warming of inspired air.
The pharynx
The pharynx is found at the back of the nose cavity and mouth, ending in the larynx. It is divided into two parts:
The nasopharynx (found above the soft palate) is lined with mucous membrane. The two nasal apertures (from the nasal cavities) enter here. The two eustaliah tubes (from the middle ear) enter into the lateral walls of the nasopharynx.

The oral pharynx (found at the back of the mouth). The oral pharynx or throat functions as a passage for inspired air and also for the swallowing of food. Two adenoids (masses of lymphatic tissue) are found here, the tonsils.

The larynx
The larynx is the part of the respiratory system containing the vocal cords for voice production. It is larger in adult males than it is in adult females. The larynx communicates with the oral pharynx above and the trachea below. It is composed of a number of laryngeal cartilages with their associated ligaments and muscles.
One such structure is the leaf-shaped epiglottis found just below the back of the tongue, overhanging the opening of the oral pharynx into the larynx. Its purpose is to prevent food from entering the larynx.

The trachea
The trachea is continuous with the larynx above and divides to form two bronchi below. It runs in front of the oesophagus and is reinforced by 15-20 c-shaped cartilaginous rings connected to each other by smooth muscle.
Each ring is incomplete posteriorly so that the soft oesophagus does not wear away each time food is swallowed. The c-shaped cartilages prevent the collapses of the windpipe. The trachea is lined with cilliated mucous epithelium.

The bronchi
The trachea divides into 2 bronchi, to separate the lungs and allow air to enter. The right bronchus starts to divide into bronchioles before it enters the right lung, the left bronchus divides into bronchioles inside the left lung. Bronchi resemble the trachea but as they subdivide into smaller and smaller bronchioles the cartilaginous rings become incomplete and eventually disappear.
The bronchioles
The bronchioles are formed by subdivisions of the bronchi.

The alveolus

The alveolus is a 'lung bubble' with a wall that consists of one layer of squamous epithelial cells. It is surrounded by blood capillaries and it is here that external gaseous exchange is going to take place.

The lungs
The two lungs lie in the thoracic cavity and are well protected by the ribcage, sternum, the spine and the muscles of the body.
The pleural membranes Further protection is offered by the two pleural membranes that surround each lung. The outer pleural membrane lines the chest cavity. The inner pleural membrane covers the outside of each lung. Between the two membranes a pleural cavity is found and this is filled with fluid to prevent friction during breathing movements.
The external structure of the lung The lung is conical in shape. The base of the lung is in contact with the diaphragm. On the inner side (the medial side) of each lung there is an indent, where the bronchus and the blood vessels enter and leave the lung. The left lung consists of 2 lobes.
The right lung consists of 3 lobes. The lungs are pink in colour and appear spongy and porous.

The internal structure of the lung

The bronchi divide and subdivide into fine tubes called bronchioles (the reinforcing cartilage rings eventually disappear - leaving only smooth muscle). The bronchioles terminates in a sac. The sac divides into smaller sacs called alveoli (250 million per lung). The walls of the alveoli consist of a single layer of flattened squamous epithelial cells.

The pulmonary artery from the heart, this artery divides into a left and right branch and enters each lung carrying deoxygenated blood. The arteries subdivide to form capillaries around the walls of each alveolus.
The pulmonary veins the capillary network around each alveolus unites to give rise to a venule. All of the venules unite to give rise to two pulmonary veins which leave the lung and return oxygenated blood to the left side of the heart for distribution throughout the body via the aorta.

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