The function of the respiratory system is to bring oxygen to the blood and to remove the carbon dioxide. The respiratory system is composed of two parts:
The Respiratory Portion consists of :
Functions of the conducting portion are to provide a route for the air to reach the lungs and also for conditioning the air. Air-conditioning during its passage through the conducting portion includes :
Respiratory epithelium is a pseudostratified columnar ciliated epithelium with goblet cells. The epithelium lining the smaller tubes of the bronchial tree becomes simple cuboidal epithelium. The goblet cells of the respiratory epithelium secrete mucus, which traps dust and particulate matter, which is propelled by the ciliary action towards the pharynx. The mucus moistens and lubricates the ciliary surface and provides a barrier to prevent much of the dust entering the lungs. The amount of mucus increases greatly in cases of infections of the respiratory tract.
NOSE and NASAL CAVITY
The external nostrils (nares) and vestibule have coarse hairs to filter large particles. The nose is divided by the nasal septum into two chambers. Three incomplete plates of bone (conchae or turbinates) divide each chamber into three smaller chambers (superior, middle and inferior). The surface of these chambers is lined with respiratory epithelium, apart from the superior chamber, which is lined by a specialized olfactory (smell) receptor (Regio olfactoria).
Regio olfactoria
The olfactory region (about 10 cm2) is lined by a highly specialized sensory "epithelium", which is in fact composed of neurons and glia. This is unique as neurons are usually not in direct contact with the external environment.
The olfactory "epithelium" consists of :
Most food that we eat is "tasted" by chemoreception of the olfactory organ. The taste buds of the tongue can only distinguish between sweet, bitter, acid and salt. People suffering from colds are unable to distinguish the aromas of food and typically do not have an appetite.
The lamina propria of the conchae, including those of the Regio olfactoria, possesses large vascular venous plexuses. These blood vessels are thin-walled and possess both longitudinal and circular smooth muscle. These vessels can engorge with blood (similar to erectile vessels of the penis). Erotic stimulation can cause engorgement. Many animals, such as dogs, where the sense of smell is much greater than that of humans, have much more extensive olfactory areas and typically use their olfactory sense to detect hormonal secretions in other dogs. It is possible that in humans this function no longer plays a significant role as we hide our natural smells by bathing and use of deodorants. These blood vessels (also known as "swell bodies") engorge periodically on alternate sides of the nose to reduce the rate of airflow and protect the epithelium from desiccation.
A further function for the rich vascular blood system of the lamina propria is to warm inspired air, especially in cold climates.
The nose leads to the nasopharynx, the first part
of the pharynx. The development of the palate in evolution was of utmost
biological significance as it enabled the separation of the route for air
(nasopharynx) from the route for feeding (oropharynx). People with incomplete
palates (cleft palate) suffer from many clinical problems and palate malformations
need to be surgically corrected as soon as possible.
EPIGLOTTIS
The epiglottis is a flexible flap-like structure forming the uppermost part of the larynx. It is thought to have a passive role in preventing food or fluids from entering the larynx.
The epiglottis is lined by stratified squamous epithelium. The epithelium of the upper (lingual) surface is usually thicker, than the lower (laryngeal) surface, which continues as respiratory epithelium. The main support for the epiglottis is a plate of elastic cartilage, surrounded by perichondrium. Small exocrine glands are common in the lamina propria.
LARYNX
The larynx is an irregular tube connecting the pharynx to the trachea. The larynx has two functions:
The true vocal cords consist of:
Lymphatic nodules are common in the lamina propria of the larynx, especially in the area of the false vocal cords.
TRACHEA
The trachea is a short tube (about 10 cm long) extending from the larynx to the bifurcation at the beginning of the two primary bronchi. The trachea is lined with typical respiratory epithelium. In the lamina propria there are about 20 C-shaped rings of hyaline cartilage (the open ends are posterior). The open ends of each ring are connected by a bundle of smooth muscle. A dense fibroelastic ligament continuous with the perichondrium of each ring connects adjacent rings. This ligament prevents overdistention of the lumen. In histological preparations as a result of contraction of the smooth muscle, the muscle may be displaced and the C-shape of the rings may be distorted.
BRONCHIAL TREE
The tubes bringing air to the lungs continually divide into smaller tubes (trachea, primary bronchi, secondary bronchi, terminal bronchioles) and are often described as the bronchial tree. In humans this has the general appearance of an oak tree. The bronchial tree can be well demonstrated if latex casts are made of the respiratory tubes and ducts.
BRONCHI
The trachea divides into the two primary bronchi, which enter the lung at the hilum. These primary bronchi divide into smaller secondary bronchi (3 in the right lung, 2 in the left lung). The extrapulmonary bronchi have a similar histological appearance to that of the trachea. The intrapulmonary bronchi (lobar bronchi) have irregular plates of hyaline cartilage (instead of C-shaped rings). In transverse section these appear as small oval or crescent-shaped plates or islands of cartilage. As the bronchi become smaller, the bands of smooth muscle in the wall become more prominent. Contraction of this smooth muscle after death typically causes the mucous membrane of the bronchi to appear to have longitudinal folds. The lamina propria of the bronchi has abundant elastic fibers.
BRONCHIOLES
Terminal bronchioles
The first and larger bronchioles are the terminal bronchioles. These have a diameter less than 1mm and lack cartilage in their walls. They lack glands in the mucosa. Goblet cells are virtually absent and if present are only found in very small numbers in the initial segments. A simple ciliated columnar or cuboidal epithelium lines the terminal bronchioles. (In the initial segments of the largest bronchioles, it may be pseudostratified). Clara cells (non-ciliated secretory cells) are dome-like cells located between the ciliated cells. The function of Clara cells is still unknown. They secrete glycosaminoglycans in response to chemical irritation (xenobiotics).
The lamina propria of the terminal bronchioles contains relative large amounts of smooth muscle and elastic fibers. These smooth muscles contract and severely restrict air-flow during asthmatic attacks. Asthmatics use drugs that stimulate the sympathetic innervation of the smooth muscle and cause their relaxation resulting in distention of the bronchiolar diameter.
Respiratory bronchioles
These are short tubes regarded as areas of transition between the conducting and respiratory portions of the respiratory system. The diameter of the respiratory bronchioles is about 0.5mm. The lining epithelium is simple cuboidal and non-ciliated. They have no cartilage in their walls. They lack goblet cells.
Respiratory portion of the lung
The functional unit of the lung (primary lobule) includes the :
The alveolar ducts are short tubes into which open
numerous alveoli. "Knobs" of smooth muscle can be seen separating adjacent
alveoli. The only support for the alveolar ducts and alveoli is a rich
matrix of elastic and collagen fibers.
The terminal part of the alveolar duct opens into an
atrium (alveolar sac) into which open many alveoli. The sac-like
alveoli are the final elements of the bronchial tree. Each alveolar
sac is surrounded by a rich network of blood capillaries. It has been estimated
that each adult lung has about 300 million alveoli, with a total surface
area for gas exchange of 70-80 square meters.
Adjacent alveoli may be connected by small openings with diameters of about 10-15um (alveolar pores). These pores may help equalize air pressure in adjacent alveoli.
The alveolar "wall"
The alveolar "wall" is composed of three main cell types :
Alveolar macrophages
Alveolar macrophages ("dust cells") are mobile
phagocytic cells in the lung that seek and collect particulate matter that
has managed to reach the lungs. These belong to the Mononuclear Phagocyte
System. Alveolar macrophages are much more prominent in lungs or smokers
or people from cities with industrial pollution.
Olfactory epithelium Olfactory epithelium Regio Olfactoria Fila Olfactoria
Respiratory epithelium Bowman's glands Epiglottis Epiglottis
Epiglottis Epiglottis-Elastic Cartilage Larynx-Vocal Cord False Vocal Cord - Larynx
Larynx Vocal Cord Trachea Trachea Trachea
Trachea Secondary Bronchus Terminal Bronchiole Terminal Bronchiole
Respiratory Brochiole Alveolar Duct Alveolar Duct Alveoli
Elastin in Alveoli Alveolar Wall Impregnation Fetal Lung Alveolar Macrophages
