Respiratory system
The respiratory system is responsible for gaseous exchange between the circulatory system and the atmosphere. Air is taken in via the upper airways (the nasal cavity, pharynx and larynx) through the lower airways (trachea, primary bronchi and bronchial tree) and into the small bronchioles and alveoli within the lung tissue.Upper respiratory tract
Mouth, nose, nasal cavity and pharynx are the organs of the upper respiratory tract. The functions of this part of the system are to warm, filter and moisten the inhaled air.
The nasal cavity is divided into two equal sections by the nasal septum.
The pharynx: is a chamber shared by the digestive and respiratory systems i.e. the pharynx is a passageway for food and drink as well as air.
Lower respiratory tract
The lower respiratory tract includes the larynx, the trachea, the right and left primary bronchi and all the constituents of both lungs. The air passages are lined with mucous membrane composed mainly of ciliated epithelium. Cilia constantly clean the tract and carry foreign material upwards for swallowing or expectoration.
The larynx: consists of nine pieces of cartilage tissue. The thyroid cartilage is more commonly known as the Adam’s apple. The epiglottis is a leaf-shaped piece of elastic cartilage attached to the top of the larynx. Its function is to protect the airway from food and water.
The trachea: the trachea (windpipe) contains 15–20 C-shaped cartilage rings that protect the trachea to prevent it from collapsing or overexpansion as pressure changes within the respiratory system.
Bronchi and bronchioles: The human trachea divides into two main bronchi, the left and the right. The segmental bronchi divide into many primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into and terminate in tiny air sacs called alveoli.
The lungs: The lungs are two coned-shaped organs that almost fill the thorax. They are protected by a framework of bones, the thoracic cage, which consists of the ribs, sternum and vertebrae. The lungs are divided into distinct regions called lobes. There are three lobes in the right lung and two in the left. Each lung is surrounded by two thin protective membranes called the parietal and visceral pleura.
Functionally, the bronchial tree is divided into 3 zones (figure 2 right) :
1. Conduction :;one : This extends from the trachea down to the terminal bronchioles (the 16th generation of the bronchial tree). lt receives blood supply from -the -bronchial «arteries (which are branches from the aorta) and since no gas exchange occurs in this zone, it is called the anatomic dead space.
2. Respiratory zone : This consists of the respiratory bronchioles, conducts air and also exhale because these bronchioles contain some alveoli in their walls. This includes the alveolar ducts, atria and alveolar sacs, and it receives blood supply from the pulmonary arteries. It is the main site of gas exchange, which occurs in the alveoli. The latter are located mainly in the alveolar sacs, and few are also present in the alveolar ducts and respiratory bronchioles.
Inspiration
Breathing is called inhalation. Just before breathing (inhalation) the pressure in the lungs equals the atmospheric pressure (760 mmHg). Thus for air to flow into the lungs, the pressure inside the alveoli must be lower than the atmosphere, thus for air to flow into the lungs. This is achieved by increasing the volume of the lungs and this is achieved by expansion of the thorax by the action of respiratory muscles.Expiration
Breathing out (exhalation) is also as a result of pressure gradient. At rest the normal exhalation is a passive process as no respiratory muscles are involved in the process. The process results from elastic recoil of the chest walls and the lungs.
Muscles of Respiration
The Muscles of respiration can be divided into those used during inspiration and expiration.
Inspiration
Diaphragm - Thin, dome-shaped sheet of muscle inserted at the lower ribs. Contraction of the diaphragm moves the abdomen downward and forward increasing the vertical dimensions of the chest cavity. The ribs are also lifted outward increasing the diameter of the thorax.
External Intercostal Muscles - Connect adjacent ribs and under contraction, the ribs move upward and forward.
Accessory Muscles of Inspiration
scalen muscles - Elevate the first two ribs
sternomastoids - Raise the sternum (may contract vigorously during exercise)
Expiration
Muscles in the Abdominal Wall:
rectus abdominus
internal and external obliques
transversus abdominus
Internal Intercostal Muscles - Pull ribs downward and inward (opposite to external intercostal muscles)
Respiratory Rate
Respiratory rate (also known as ventilation rate) is the number of breaths a person takes during one minute.
Normal Respiratory Rate by Age:
0 – 30 days 30 – 50 cycles per minute
1 – 5 years 20 – 30 cycles per minute
Adult 16 – 20 cycles per minute
Tachypnea: is the increased respiratory rate above the normal range.
Bradypnea: is the decreased respiratory rate below the normal range.Apnea: is the complete cessation of respiration.