Anatomy & Physiology of the Respiratory System

The respiratory system is situated in the thorax, and is responsible for gaseous exchange between the circulatory system and the outside world. 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. Move the pointer over the coloured regions of the diagram; the names will appear at the bottom of the screen)

The lungs are divided into lobes; The left lung is composed of the upper lobe, the lower lobe and thelingula (a small remnant next to the apex of the heart), the right lung

is composed of the upper, themiddle and the lower lobes.

Mechanics of Breathing

To take a breath in, the external intercostal muscles contract, moving the ribcage up and out. Thediaphragm moves down at the same time, creating negative pressure within the thorax. The lungs are held to the thoracic wall by the pleural membranes, and so expand outwards as well. This creates negative pressure within the lungs, and so air rushes in through the upper and lower airways.

Expiration is mainly due to the natural elasticity of the lungs, which tend to collapse if they are not held against the thoracic wall. This is the mechanism behind lung collapse if there is air in the pleural space (pneumothorax).

Physiology of Gas Exchange

Each branch of the bronchial tree eventually sub-divides to form very narrow terminal bronchioles, which terminate in the alveoli. There are many millions of alveloi in each lung, and these are the areas responsible for gaseous exchange, presenting a massive surface area for exchange to occur over.

Each alveolus is very closely associated with a network of capillaries containing deoxygenated blood from the pulmonary artery. The capillary and alveolar walls are very thin, allowing rapid exchange of gases by passive diffusion along concentration gradients. CO2 moves into the alveolus as the concentration is much lower in the alveolus than in the blood, and O2 moves out of the alveolus as the continuous flow of blood through the capillaries prevents saturation of the blood with O2 and allows maximal transfer across the membrane.

Respiratory physiology Pulmonary ventilation - movement of air into and out of lungs External respiration - The process by which gases are exchanged between the lungs and the pulmonary vasculature. O2 diffuses from the alveoli into the blood, and CO2 diffuses from the blood to the alveoli. Diffusion is a passive process in which gases move across a membrane from an area of higher to lower concentration. In the lungs the membrane is the alveolar-capillary network Internal respiration - The process by which gases are exchanged between the pulmonary vasculature and the body's tissues. Oxygen from the lungs diffuses from the blood into the body tissue. CO2 diffuses from the tissues into the blood. This blood in then carried back to the right side of the heart for re-oxygenation Hypoxia - reduced oxygenation of cells in tissues Hypoxemia - reduced oxygenation of arterial blood (PaO2). Hypoxemia can lead to tissue hypoxia, however tissue hypoxia has other causes and can occur in light of normal arterial oxygenation (low CO or cyanide poisoning) Cyanosis, a bluish coloring of the skin, caused by an unusually high percentage of deoxygenated hemoglobin in the blood. Deoxygenated hemoglobin is purplish, in contrast to the bright red oxyhemoglobin Anoxia - when O2 supply completely shut off, anoxia and tissue death may result .Pulmonary Ventilation Boyles Law - as pressure of gas decreases it volume expands; as pressure increases its volume decreases. Intrapulmonary pressure drives air exchange - decreased pressure, air in. Diaphragm and external and internal intercostals - used for normal quite breathing Accessory muscles - active during active inspiratory and expiratory movements of forced breathing hyperventilation is defined as increased alveolar ventilation. This leads to a decrease in PaCo2. There are many causes of hyperventilation including hypoxemia, head injury, and anxiety or panic attack. hypoventilation is defined as a decreased alveolar ventilation. This leads to a increased PaCo2. Some common causes of hypovetilation include chest wall restriction, altered neurologic control of

breathing, and obstructed airway. Gas exchange Dalton’s Law - In a mixed gas the individual gases exert a pressure proportional to their abundance in the mixture. Partial pressure - the pressure contributed by a single gas is the partial pressure Alveolar ventilation - the amount of air reaching the alveoli each minute. O2 and CO2 exchange ooccurs at the interface of the alveolus and pulmonary capillaries (respiratory membrane) Diffusion of these gases depends on several factors. Two important factors are the thickness and surface area of the respiratory membrane. Interstitial edema and fibrotic changes cause the respiratory membrane to change in thickness. The surface area of the respiratory membrane becomes smaller with emphysema as alveoli are destroyed. If the surface area is decreased more than 25% then the patient can have diffusion problems and shortness of breath even at rest Blood entering peripheral capillaries delivers oxygen and absorbs CO2 at the tissues Po2 - over the range of O2 pressures normally present in the body, a small change in plasma Po2 will mean a large change in the amount of O2 bound or released. At alveolar Po2 the hemoglobin are almost fully saturatted; at the peripheral tissues it retains a substantial O2 reserve When low plasma Po2 continues for an extended period of time, red blood cells generate more 2,3 (BPG) bisphosphoglycerate, which reduces hemoglobins affinity for oxygen. CO2 production - aerobic metabolism in the peripheral tissues generates CO2. About 7 percent of the CO2 transported in the blood is dissolved in the plasma; 23 percent is bound as carbaminohemoglobin; the rest is converted to cardbonic acid which dissociates into H+ and HCO3-