Respiratory System PDF

The respiratory system The function of the pulmonary system The pulmonary system has three primary functions: (1) to ventilate the alveoli; (2) to allow gases to diffuse into and out of the blood; and (3) to perfuse the lungs so that the organs and tissues of the body receive blood that is rich in...

The respiratory system The function of the pulmonary system The pulmonary system has three primary functions: (1) to ventilate the alveoli; (2) to allow gases to diffuse into and out of the blood; and (3) to perfuse the lungs so that the organs and tissues of the body receive blood that is rich in oxygen and low in carbon dioxide. These functions are influenced by chemical and neural input to the nervous system, which controls the movement of the chest wall muscles. Each of these components of the pulmonary system contributes to one or more of these functions, and we explore them in more detail below. The mechanics of breathing The physical aspects of inspiration and expiration are known collectively as the mechanics of breathing and involve: (1) major and accessory muscles of inspiration and expiration; (2) elastic properties of the lungs and chest wall; and (3) resistance to airflow through the conducting zone. Alterations in any of these properties increase the work of breathing or the metabolic energy needed to achieve adequate ventilation of alveoli and gas exchange. Major and accessory muscles The major muscles of inspiration are the diaphragm and the external intercostal muscles (muscles between the ribs) (see [Fig. 24.12](https://www.clinicalkey.com.au/nursing/f0120) ). The diaphragm is a dome-shaped muscle that separates the abdominal and thoracic cavities. When it contracts and flattens downwards, it increases the volume of the thoracic cavity. This creates a slight negative pressure, which draws air into the lungs through the upper airways and trachea. Contraction of the external intercostal muscles elevates the anterior portion of the ribs and increases the volume of the thoracic cavity by increasing its front-to-back (anterior-posterior) diameter. Although the external intercostals may contract during quiet breathing, inspiration at rest is usually achieved by the diaphragm only. The work of breathing is determined by the muscular effort required for ventilation --- that is, the energy expenditure. Normally very low, the work of breathing may increase considerably in diseases that disrupt the equilibrium between forces exerted by the lung and chest wall. More muscular effort is required when lung compliance decreases (e.g. pulmonary oedema), chest wall compliance decreases (such as in obesity or spinal deformity) or airways are obstructed by bronchospasm or mucus plugging (e.g. asthma or bronchitis). An increase in the work of breathing can result in a marked increase in oxygen consumption and can eventually lead to an inability to maintain adequate ventilation. DESCRIPTION AND MEASUREMENT ------------------------------------ ----------------------------------------------------------------------------------------------------------- **Respiratory volumes** Tidal volume (V ^T ^) The amount of air inspired or exhaled in a normal resting breath Inspiratory reserve volume (IRV) The maximal amount of air that can be inhaled after tidal volume Expiratory reserve volume (ERV) The maximal amount of air that can be exhaled after tidal volume Residual volume (RV) The amount of air that remains in the lungs after a maximal exhalation **Respiratory capacities** Inspiratory capacity (IC) The amount of air inspired by maximum inspiratory effort after tidal volume, expiration: IC = V ^T ^+ IRV Vital capacity (VC) The amount of air that can be forcibly expired after a maximal inspiration: VC = V ^T ^+ IRV + ERV Functional residual capacity (FRC) The amount of air left in the lungs after normal tidal expiration: FRC = ERV + RV Total lung capacity (TLC) The volume of air occupying the lungs after maximum inhalation: TLC = V ^T ^+ IRV + ERV + RV The **respiratory centre** in the brainstem controls ventilation by transmitting impulses to the respiratory muscles, causing them to contract and relax. The respiratory centre is composed of several groups of neurons: the dorsal respiratory group, the ventral respiratory group, the apneustic centre and the pneumotaxic centre. The exact nature of each group is not entirely understood; however, it is thought that the automatic ventilatory rhythm is set by the dorsal respiratory group (located in the medulla), which receives afferent input (sensory neuron) from **peripheral chemoreceptors** in the carotid and aortic bodies and from several different types of receptors in the lungs (see below). The ventral respiratory group (also located in the medulla) contains both inspiratory and expiratory neurons and is almost inactive during normal, quiet ventilation, becoming active when increased ventilatory effort is required. The pneumotaxic centre and apneustic centre, situated in the pons (above the medulla), do not generate primary rhythm but, rather, act as modifiers of the rhythm established by the medullary centres. It should be noted that the pattern of breathing can be influenced by many factors, such as emotions, pain and disease. Mechanical control Three types of lung receptors send impulses from the lungs to the dorsal respiratory group: -   Irritant receptors are found in the epithelium of all conducting airways. They are sensitive to noxious aerosols (vapours), gases and particulate matter (e.g. inhaled dusts), which cause them to transmit impulses to the medullary centre to initiate a cough reflex. When stimulated, irritant receptors also lead to bronchoconstriction and an increase in ventilatory rate. -   Stretch receptors are located in the smooth muscles of airways and are sensitive to increases in the size or volume of the lungs. When stimulated, ventilatory rate and volume will decrease, an occurrence sometimes referred to as the Hering-Breuer reflex. This reflex is active in newborns and assists with ventilation. In adults, this reflex is active only at high tidal volumes (such as with exercise) and may protect against excess lung inflation. -   J-receptors are located near the capillaries in the alveolar septa (lining or membranes). They are sensitive to increased pulmonary capillary pressure, which results in rapid shallow breathing, hypotension and bradycardia. Chemical control Chemoreceptors monitor the pH, carbon dioxide level in the arterial blood (PaCO ~2~ , with the lower case 'a' referring to arterial) and oxygen level in the arterial blood (PaO ~2~ ). There are two groups of chemoreceptors: (1) central chemoreceptors located in the brainstem near the respiratory centres; and (2) peripheral chemoreceptors located in the aortic and carotid bodies (see [Fig. 24.16](https://www.clinicalkey.com.au/nursing/f0160) ). **Central chemoreceptors** monitor arterial blood indirectly by sensing changes in the pH (hydrogen ion content) of cerebrospinal fluid. They are sensitive to hydrogen ion concentration, or the amount of acid, in the cerebrospinal fluid. The pH of the cerebrospinal fluid reflects arterial pH because carbon dioxide in arterial blood can diffuse across the blood--brain barrier (the capillary wall separating blood from cells of the central nervous system) into the cerebrospinal fluid, until the carbon dioxide level is equal on both sides. Carbon dioxide that has entered the cerebrospinal fluid combines with water to form carbonic acid (a weak acid), which subsequently dissociates into hydrogen ions that are capable of stimulating the central chemoreceptors. In this way, carbon dioxide regulates ventilation, through its impact on the pH (hydrogen ion content) of the cerebrospinal fluid. The structure of the pulmonary system -   The pulmonary system consists of the lungs, airways, chest wall and pulmonary and bronchial circulation. -   Air is inspired and expired through the conducting zone, which include the nasopharynx, oropharynx, trachea, bronchi and bronchioles to the sixteenth division. -   The respiratory zone consists of structures that are involved in gas exchange. These include the respiratory bronchioles, alveolar ducts and alveoli. -   The chief gas-exchange units of the lungs are the alveoli. The membrane that surrounds each alveolus and contains the pulmonary capillaries is called the alveolar--capillary membrane. -   The gas-exchange airways are served by the pulmonary circulation, a separate division of the circulatory system. The bronchi and other lung structures are served by a branch of the systemic circulation called the bronchial circulation. -   The chest wall, which contains and protects the contents of the thoracic cavity, consists of the skin, ribs and intercostal muscles, which lie between the ribs. -   The chest wall is lined by a serous membrane called the parietal pleura; the lungs are encased in a separate membrane called the visceral pleura. The area where these two pleurae come into contact and slide over one another is called the pleural space. -- -- -- --