Respiratory System

General Information The chief function of the respiratory system is to supply body tissues with oxygen and eliminate carbon dioxide. The respiratory system consists of the nasal cavities, pharynx, larynx, trachea, bronchi, and lungs. The nasal cavities, pharynx, larynx, trachea, and bronchi constitute the airway.

Nose The nose has two nasal cavities, which open on the face through the anterior nasal apertures called the nares. The nasal septum separates the cavities. The anterior portion of the septum is composed of hyaline cartilage. The posterior portion is composed of bone called the vomer and the perpendicular plate of the ethmoid bone.

Nose Each nasal cavity has three mucosa-covered structures called the superior, middle, and inferior conchae. The nasal cavities are lined with hairs that trap dust and foreign particles. Epithelial cells lining the nasal cavities secrete mucus, which collects foreign particles which are then moved by cilia toward the pharynx, where the particles and mucus can be removed by swallowing, sneezing, or spitting. The paranasal sinuses, which surround and drain into the nasal cavities, are located in the frontal, sphenoid, and maxillary bones.

conchae

Pharynx The pharynx is also known as the throat. Acts as a passageway for air and food. It also functions in speech, changing shape to allow phonation of vowel sounds. The entire pharynx is composed of striated muscle and lined with mucous membrane. The pharynx has three divisions… Nasopharynx Oropharynx Laryngopharynx

Larynx The larynx is called the voice box. It is a triangular organ in the front of the neck. It extends from the 4th to the 6th cervical vertebrae, attaching to the hyoid bone. The larynx is composed of muscle and numerous cartilages The true vocal cords are a pair of horizontal folds that project into the laryngeal cavity. They are separated by a space called the glottis. The epiglottis, which overhangs the larynx, prevents food from entering the lungs.

Trachea The trachea is also called the windpipe. It is a membranous tube measuring 10 to 12.5 cm long. On entering the mediastinum, the trachea branches into the right and left main bronchi at the 5th thoracic vertebrae. Dorsally, the trachea contacts the esophagus. The trachea has a series of C-shaped cartilage rings to strengthen the trachea and prevents it from collapsing during inspiration. The trachea is lined with ciliated pseudostratified columnar epithelium containing mucus-secreting goblet cells, which trap and propel inhaled debris upward to the pharynx for removal through coughing.

Bronchi The right and left primary bronchi branch from the trachea. The secondary bronchi are smaller passageways that branch from the primary bronchi. The right primary bronchi divides into three secondary bronchi and the left branches into two secondary bronchi. The secondary bronchi branch into tertiary bronchi which then branch into smaller bronchioles which branch into progressively smaller tubes until they branch into alveolar ducts, which terminate in clusters called alveoli. Alveoli are tiny air sacs lined with thin squamous epithelium. They are surrounded by capillaries where oxygen and carbon dioxide are exchanged.

Lungs The lungs are paired, cone-shaped organs that fill the pleural divisions of the thoracic cavity. They extend from the root of the neck to the diaphragm. The right and left lungs are separated by the heart and other mediastinal structures. Each lung is enclosed in a pleura, a protective, double-layered serous membrane. The parietal pleura lines the wall of the thoracic cavity. The visceral pleura covers the lung directly. The space between the two membranes contains fluid, which lubricates the lungs as they expand and contract.

Lung Lobes The right lung is divided into three lobes. The left lung, smaller than the right, is divided into two lobes. It contains a concavity called the cardiac notch, which is molded to accommodate the heart.

Breathing Inspiration occurs when the diaphragm contracts, moving downward, and the intercostal muscles expand the size of the chest cavity. The increase in the size of the chest cavity reduces pressure within it and air moves from the outside high pressure to the low pressure inside the lungs. Expiration occurs when the diaphragm and intercostal muscles relax and therefore reduce the size of the chest cavity. This increases air pressure and air moves out. The elastic recoil of lung tissue aids in expiration.

Gas Exchange Air exerts a total pressure of 760 mm Hg at sea level…. Here in Albuquerque is ~640 mm Hg and so partial pressure of oxygen is 134 mm Hg Air contains about 21% oxygen. The partial pressure of oxygen is 21% of the atmospheric pressure 760 mm Hg. So the partial pressure of oxygen is 159.6 mm Hg.

Gas Exchange Gas concentrations in inspired air… Nitrogen = 79% Oxygen = 21% Water vapor = .5% Carbon dioxide = .04% Gas concentrations in alveolar air… Nitrogen = 74.9% Oxygen = 13.6% Water vapor = 6.2% Carbon dioxide = 5.3% Gas concentrations in expired air… Oxygen = 15.7% Carbon dioxide = 3.6%  

Gas Diffusion In diffusion substances move from an area of higher concentration to an area of lower concentration. A gas diffuses from an area with a high partial pressure of the gas to one with a lower partial pressure.

Oxygen Transport Oxygen is transported by the blood in two different methods. About 3% of oxygen is dissolved in blood plasma The remaining 97% is chemically bound with hemoglobin (carries 4 oxygens). Oxygen uptake by hemoglobin is most efficient in the lungs where the oxygen concentration is high. Oxygen release occurs most readily in the tissues where the oxygen concentration is low. Lower blood pH and higher temperatures of actively metabolizing tissue, which occurs during vigorous exercise, also enhances oxygen release from hemoglobin.

Hemoglobin with 4 sites for O2

CO2 Transport Carbon dioxide is transported by three different methods. A small amount is dissolved in the blood plasma Some is loosely combined with amino groups in the hemoglobin molecule.

CO2 Transport Most of the carbon dioxide is converted in erythrocytes to bicarbonate by the enzyme carbonic anhydrase. Erythrocyte uptake of carbon dioxide begins in the capillaries. In the erythrocyte, the hemoglobin releases oxygen to supply the tissues. At the same time, carbon dioxide diffuses from the tissues into the erythrocytes where enzymes help turn carbon dioxide and water into carbonic acid. H2O + CO2  H2CO3.

CO2 Transport Then carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3 -). The hemoglobin molecules take up the hydrogen ions. The remaining bicarbonate ions accumulate until their concentration in the erythrocyte exceed that in the plasma, then the excess will diffuse out into the plasma. Simultaneously, chloride ions diffuse into the erythrocyte to replace the bicarbonate ions. Then the bicarbonate ions combine with plasma sodium and form sodium bicarbonate. At the lungs the whole process reverses and carbon dioxide diffuses into the alveoli and is exhaled.

Hb= hemoglobin

Control of Respiration A control center in the brain stem, called the respiratory center, regulates the rate and depth of respiration. This center discharges impulses to neurons that innervate the diaphragm and intercostal muscles. Neurons in the respiratory center are stimulated directly by increased arterial concentrations of carbon dioxide and hydrogen ions. During exercise, more carbon dioxide is produced. This stimulates the respiratory center to increase respiratory rate and depth.

Control of Respiration Chemoreceptors in the aortic arch and carotid sinus detect levels of carbon dioxide and convey impulses to the respiratory center. Receptors in the lungs respond to stretching as the lungs inflate, sending impulses to the respiratory center to inhibit further inspiration. The cerebral cortex sends impulses to the respiratory center in response to strong emotions, such as anxiety, fear, and anger. Some sensory stimuli, such as irritating vapors may cause reflex inhibition of respiration.