Breathing
Breathing consists of two phases, inspiration and expiration. During inspiration, the diaphragm and the intercostal muscles contract. The diaphragm moves downwards increasing the volume of the thoracic (chest) cavity, and the intercostal muscles pull the ribs up expanding the rib cage and further increasing this volume. This increase of volume lowers the air pressure in the alveoli to below atmospheric pressure. Because air always flows from a region of high pressure to a region of lower pressure, it rushes in through the respiratory tract and into the alveoli. This is called negative pressure breathing, changing the pressure inside thelunsg relative to the pressure of the outside atmosphere. In contrast to inspiration, during expiration the diaphragm and intercostal muscles relax. This returns the thoracic cavity to it's original volume, increasing the air pressure in the lungs, and forcing the air out.
Air enters the body through the nose where nasal hairs act as a coarse filter to remove large particles from inspired air. Within each nasal cavity there is a swell body on the nasal fossae (ridges in the nasal epithelium) that, when engorged with blood, swells and decreases the flow of air on that side. This happens every 20-30 minutes on one side or the other thus allowing the respiratory epithelium to recover from desiccation. Allergic reactions or inflammation can cause abnormal enlargement of the swell bodies in both nasal fossae thereby obstructing airflow through the nose. The structures of the nose, mouth and pharynx act to warm and moisten the air before it enters the larynx. From the larynx, air is conducted through the trachea, bronchi, and bronchioles before reaching the thin membranes of the alveoli where gas exchange occurs.
Inhalation is initiated by the contraction of both the diaphragm and the external intercostal (between the ribs) muscles, which cause expansion of the vertical and horizontal dimension of the thorax (including the lungs) respectively. Whereas inspiration is an active process, expiration is passive. As the respiratory muscles relax, the elastic recoil of the lungs, chest wall and the abdominal viscera combined with gravity cause the thoracic volume to decrease and air moves out from the lungs. Forced expiration on the other hand is active, involving contractions of the abdominal and internal intercostal muscles.
The stretchability of the thorax and lungs (elastic work), viscosity (tissue resistance work) and the amount of resistance the air encounters within de respiratory tract (airway resistance work) are the main parameters affecting the amount of work performed by the respiratory muscles. The airway resistance largely depends on the diameter of the passageways. In normal situations the energy expenditure of the respiratory muscles is only a few percent of the total energy expenditure.
Contraction of the diaphragm increases the size of the thoracic cavity causing pulmonary expansion. Increasing the size of the chest cavity creates a negative intrapleural pressure causing air inflow. Contraction of the diaphragm produces a 75% change in intrathoracic volume during resting inspiration when its range of motion might be about cm. The maximum range of movement of the diaphragm is 10 cm . In labored breathing, the external intercostals contract to elevate lower ribs and push the sternum outward, thus increasing the volume of chest. The muscles of the neck are accessory muscles of respiration. The elasticity of the lung prevents over distention of the chest cavity. Bronchi and bronchioles increase in diameter and alveolar ducts enlarge. Alveoli do not increase in size. Increases in the size of the airways decreases the resistance to airflow. In the upright position, the base of the lungs are better ventilated than the apices.
Expiration is normally passive due to the elastic recoil. In labored breathing, the internal intercostal muscles contract and pull the rib cage downward. Abdominal muscles also aid in active expiration by pulling the rib cage down and increasing abdominal pressure which forces the diaphragm up. Even when at rest the lungs are continually attempting to achieve a smaller relaxed size due to the elastic nature of the tissue.
At rest a person breathes 12-15 times per minute, 500 ml per breath and therefore 6--8 L/min is inspired and expired. Each minute, 250 ml of oxygen enters the body and 200 ml of CO 2 is excreted. Some 250 different volatile substances have been found in human breath.
The upper respiratory tract
Air enters the respiratory tract through the nose and the nasal cavity. The walls of the latter are formed by conchae and grooved passageways called meatures in order to increase the surface. This surface is composed of a highly vascularized mucous membrane which function is to warm and humidify the incoming air. Then the air passes through the pharynx (throat) into the larynx.
The lower respiratory tract
The larynx (voicebox) is the part of the tract were sounds can be produced. It includes the epiglottis, which covers the glottis (entrance of the larynx) during swallowing to prevent food or drink from entering the lower respiratory tract, the thyroid cartilage (Adam's apple), and the vocal folds. The trachea (windpipe) joins the larynx to the two primary bronchi. It is covered with a mucous membrane containing ciliated cells (cilia) moving mucus trapped foreign substances away from the lungs. The primary bronchi (one to left and the other to the right) divide into smaller secondary bronchi which lead into the different lobes of the lungs. There they branch into tertiary bronchi which in turn branch into bronchioles. The latter are completely surrounded by smooth muscle tissue in order to regulate airway resistance. The terminal bronchioles divide into alveolar ducts at which numerous alveoli are situated. The alveolar wall is only a single-cell layer thick and contacts with the single-cell thick wall of the capillaries. At this level the actual gas exchange takes place (external respiration).
Lung Volumes
- Tidal Volume (TV) - Amount of air inspired or expired (500 ml) at rest in normal breathing.
- Inspiratory Reserve Volume (IRV) - Amount of air taken in with maximum inspiratory effort ( 3 L in men and 2 L in women) over the tidal volume.
- Expiratory Reserve Volume (ERV) - Amount of air taken in with maximum expiratory effort ( 1 L in men and 0.7 L in women) over the tidal volume.
- Reserve Volume (RV) - Air left in the lungs after maximal expiratory effort ( 1.2 L in men and 1.1 L in women).
- Vital Capacity (VC)- Maximum amount of air expired after maximal inspiration. Used clinically as an indicator of pulmonary function. In healthy adults the vital capacity is generally 4.8 L in males and 3.1 L in females.