Respiratory System - NCLEX-PN

Many gases, such as room air, are composed of several gasses (in the case of room air, primarily nitrogen, oxygen, and several trace gasses). Partial pressure is the amount of pressure that each gas contributes. To find the total pressure of a gas mixture, one would find the sum of all partial pressures of each individual gas.

Gas exchange in the lung occurs exclusively in the alveoli, the tiny grape-like clusters of sacks at the terminus of the bronchioles of the lung. The gas contained in the larger structures of the lung - the trachea, bronchi, and bronchioles, is termed "dead space" as it does not participate in gas exchange.

Gas exchange in the alveoli occurs by the process of simple diffusion. The blood that enters the lung capillaries is relatively low in oxygen and high in , in comparison to the room air that is drawn into the alveoli during respiration. The separation between the capillaries and the alveolar epithelium is thin enough that these gasses can exchange across their pressure gradients by the process of simple diffusion.

The main trigger for the impulse to breathe is high carbon dioxide (decreased blood pH). In response to a decrease in blood pH, the medullary respiratory center triggers the diaphragm and the external intercostal muscles to increase breathing rate.

Myoglobin, an oxygen-binding protein in the muscle tissue, can bind with one molecule of oxygen. Hemoglobin, the main oxygen-binding metalloprotein in red blood cells, can bind four molecules of oxygen.

While hyperventilation or tachypnea may be triggered by various forms of acidosis, they will not cause a decrease in blood pH, but rather push the blood into a more alkaline state. This occurs due to loss of carbon dioxide during respiration, followed by a compensatory decrease in bicarbonate ions. The result is in an elevation in the pH of the blood. This condition is known as respiratory alkalosis.

There are five lobes total in the human lungs: the right lung has three lobes (the upper lobe, the middle lobe, and the lower lobe) while the left lung has two lobes (the upper lobe and the lower lobe).

The topmost part of the lungs is the apex. This area extends into the neck above the 1st rib and is the location auscultated for a diagnosis of a pancost tumor. The lingula of the lung is a small flap of the lowest part of the upper lobe of the left lung. The hilum is the entry point of the lung for the bronchi, pulmonary artery and vein, and nerves. The trachea is not a part of the lung at all; it is the cartilaginous tube that connects the pharynx and larynx to the lung.

The largest airway structure is the trachea. This branches into two smaller bronchi, which enter the left and right lung and bifurcate further into smaller bronchioles. The bronchioles give way into the smallest structures of the lung, the tiny grape-like clusters of alveoli.

The main mediator of air flow during respiration is the bronchiole. These tiny airways are wrapped in smooth muscle, which allows them to contract or relax in order to restrict or increase air flow in the lung. The trachea and bronchioles are cartilaginous rather than muscular, and are unable to constrict, while the wall of the alveolar sac is also devoid of smooth muscle, made up only of epithelial cells, capillaries, and connective tissue.

The system by which debris is removed from the lung is termed the mucociliary escalator. This is made up of two parts: the mucus produced by respiratory goblet cells, and the wave-like movement of cilia covering the bronchioles and bronchi of the lung. Bacteria and particles of debris are caught in the mucus coating the cilia, which is then passed in waves upward toward the trachea. Once it clears the trachea and enters the pharynx, it can either be coughed out or swallowed.

There are two types of alveolar cells: type I cells and type II cells. Each cell type has a unique function within the alveolus, and the dysfunction of either cell type results in serious respiratory pathology. Type I cells are simple squamous epithelial cells that account for about 95% of all alveolar cells. They are the primary cells responsible for diffusion of gasses across the respiratory membranes. Type I alveolar cells are responsible for secreting surfactant, which facilitates the diffusion of substances across the epithelium.

Type II alveolar cells secrete pulmonary surfactant, a lipoprotein complex that is essential for lung function. This surfactant acts to break up the surface tension of fluid coating the lung airspaces, allowing for alveolar compliance and reducing the buildup of fluid in the lung. Lack of surfactant can lead to atelectasis, or collapse of part of the lung.

The most important muscle of respiration is the diaphragm, followed by the intercostals. The sternocleidomastoid and scalenes are considered accessory muscles of respiration.

The diaphragm is a thin sheet of muscle that separates the thoracic cavity from the abdominal cavity. At rest, it is drawn upward into a dome-like shape under the lungs. When the diaphragm contracts, the dome flattens out, which increases the size of the thoracic cavity. The negative pressure allows the lungs to expand and fill with air, causing an increase in total lung volume (inhalation).

Dead space is the volume of air that remains in the trachea, bronchi, and bronchioles during respiration. It does not enter the alveoli and so does not participate in gas exchange. In the average adult, the amount of air left in the dead space after expiration is .

Tidal breathing is the term for relaxed inhalation and exhalation (as opposed to maximum inspiration and forced exhalation). The average volume of air breathed into and out of the respiratory system during tidal breathing is .

Carbon dioxide has paracrine effects in the airway, causing the smooth muscle of bronchioles to relax. When the partial pressure of carbon dioxide in the alveoli increases, the bronchioles dilate. This allows for increased ventilation.

There are many factors that can cause bronchioconstriction. Some of them are normal physiological responses, such as parasympathetic input or decreased partial pressure of carbon dioxide in the alveoli. Other causes are pathological, such as allergic bronchioconstriction or asthma triggered by cold air.

The diaphragm is innervated by the phrenic nerve. This nerve exits the spine at C3-C5. In the case of spinal cord trauma, if injury occurs above C3, the diaphragm is no longer able to function on its own and breathing assistance is generally required.