Respiration includes the movement of air into and out of the lungs, the exchange of gases between the air and the blood, the transport of gases in the blood, and the exchange of gases between the blood and tissues.

Functions of the Respiratory System

• The respiratory system exchanges oxygen and carbon dioxide between the air and blood, regulates blood pH, produces sounds, moves air over the sensory receptors that detect smell, and protects against some microorganisms.

Anatomy of the Respiratory System

Nose and Nasal Cavity

• The bridge of the nose is bone, and most of the external nose is cartilage.
• The nasal cavity warms, humidifies, and cleans the air.
• The external nares open to the outside, and the internal nares lead to the pharynx.
• The nasal cavity is divided by the nasal septum into right and left parts.
• The paranasal sinuses and the nasolacrimal duct open into the nasal cavity.
• Hairs just inside the external nares trap debris.
• The nasal cavity is lined with pseudostratified epithelium with cilia that traps debris and moves it to the pharynx.

Pharynx

• The nasopharynx joins the nasal cavity through the internal nares and contains the opening to the auditory tube and the pharyngeal tonsils.
• The oropharynx joins the oral cavity and contains the palatine and lingual tonsils.
•  The laryngopharynx opens into the larynx and the esophagus.

Larynx

• The larynx consists of three unpaired cartilages and six paired ones. The thyroid cartilage and cricoid cartilage form most of the larynx. The epiglottis covers the opening of the larynx during swallowing.
• The vestibular folds can prevent air, food, and liquids from passing into the larynx.
• The vocal folds (cords) vibrate and produce sounds when air passes through the larynx. The force of air movement controls loudness, and changes in the length and tension of the vocal folds determines pitch.

Trachea

• The trachea connects the larynx to the primary bronchi.

Bronchi

• The primary bronchi extend from the trachea to each lung.

Lungs

• There are two lungs.
• The airway passages of the lungs branch and decrease in size.
• The primary bronchi form the secondary bronchi, which go to each lobe of the lungs.
• The secondary bronchi form the tertiary bronchi, which go to each bronchopulmonary segment of the lungs.
• The tertiary bronchi branch many times to form the bronchioles.
• The bronchioles branch to form the terminal bronchioles, which give rise to the respiratory bronchioles, from which alveolar ducts branch.
• Alveoli are air sacs connected to the alveolar ducts and respiratory bronchioles.
• Important features of the tube system.
• The epithelium from the trachea to the terminal bronchioles is ciliated to facilitate removal of debris.
• Cartilage helps to hold the tube system open (from the trachea to the bronchioles).
• Smooth muscle controls the diameter of the tubes (especially the bronchioles).
• The alveoli are formed by simple squamous epithelium, and they facilitate diffusion of gases.

Pleural Cavities

• The pleural membranes surround the lungs and provide protection against friction.

Lymphatic Supply

• The lungs have superficial and deep lymphatic vessels.

Ventilation and Lung Volumes

Changing Thoracic Volume

• Inspiration occurs when the diaphragm contracts and the external intercostal muscles lift the rib cage, thus increasing the volume of the thoracic cavity. During labored breathing additional muscles of inspiration increase rib movement.
• Expiration can be passive or active. Passive expiration during quiet breathing occurs when the muscles of inspiration relax. Active expiration during labored breathing occurs when the diaphragm relaxes and the internal intercostal and abdominal muscles depress the rib cage to forcefully decrease the volume of the thoracic cavity.

Pressure Changes and Air Flow

• Respiratory muscles cause changes in thoracic volume, which cause changes in alveolar volume and pressure.
• During inspiration, air flows into the alveoli because atmospheric pressure is greater than alveolar pressure.
• During expiration, air flows out of the alveoli because alveolar pressure is greater than atmospheric pressure.
• Lung Recoil
• The lungs tend to collapse because of the elastic recoil of the connective tissue and surface tension of the fluid lining the alveoli.
• The lungs normally do not collapse because surfactant reduces the surface tension of the fluid lining the alveoli and pleural pressure is less than alveolar pressure.

Changing Alveolar Volume

• Increasing thoracic volume results in decreased pleural pressure, increased alveolar volume, decreased alveolar pressure, and air movement into the lungs.
• Decreasing thoracic volume results in increased pleural pressure, decreased alveolar volume, increased alveolar pressure, and air movement out of the lungs.

Pulmonary Volumes and Capacities

• There are four pulmonary volumes: tidal volume, inspiratory reserve, expiratory reserve, and residual volume.
• Pulmonary capacities are the sum of two or more pulmonary volumes and include vital capacity and total lung capacity.
• The forced expiratory vital capacity measures the rate at which air can be expelled from the lungs.

Gas Exchange

• The respiratory membranes are thin and have a large surface area that facilitates gas exchange.
• The components of the respiratory membrane include a film of water, the walls of the alveolus and the capillary, and an interstitial space.
• Respiratory Membrane Thickness
• Increases in the thickness of the respiratory membrane results in decreased gas exchange.

Surface Area

• Small decreases in surface area adversely affect gas exchange during strenuous exercise; and, when the surface area is decreased to one-third to one-fourth of normal, gas exchange is inadequate under resting conditions.

Partial Pressure

• The pressure exerted by a specific gas in a mixture of gases is reported as the partial pressure of that gas.
• Oxygen diffuses from a higher partial pressure in the alveoli to a lower partial pressure in the pulmonary capillaries. Oxygen diffuses from a higher partial pressure in the tissue capillaries to a lower partial pressure in the tissue spaces.
• Carbon dioxide diffuses from a higher partial pressure in the tissues to a lower partial pressure in the tissue capillaries. Carbon dioxide diffuses from a higher partial pressure in the pulmonary capillaries to a lower partial pressure in the alveoli.

Gas Transport in the Blood

Oxygen Transport

• Most (98.5%) oxygen is transported bound to hemoglobin. Some (1.5%) oxygen is transported dissolved in plasma.
• Oxygen is released from hemoglobin in tissues when the partial pressure for oxygen is low, the partial pressure for carbon dioxide is high, pH is low, and temperature is high.
• Carbon Dioxide Transport and Blood pH
• Carbon dioxide is transported as bicarbonate ions (70%), in combination with blood proteins (23%), and in solution in plasma (7%).
• In tissue capillaries, carbon dioxide combines with water inside the erythrocytes to form carbonic acid that dissociates to form bicarbonate ions and hydrogen ions. This reaction promotes the transport of carbon dioxide.
• In lung capillaries, bicarbonate ions combine with hydrogen ions to form carbonic acid. The carbonic acid dissociates to form carbon dioxide that diffuses out of the erythrocytes.
• As blood carbon dioxide levels increase, blood pH decreases; as blood carbon dioxide levels decrease, blood pH increases. Changes in ventilation change blood carbon dioxide levels and pH.
• Rhythmic Ventilation

Respiratory Areas in the Brainstem

• The medullary respiratory center (dorsal respiratory and ventral respiratory groups) establishes rhythmic ventilation.
• The pontine respiratory group is involved with the switch between inspiration and expiration.

Generation of Rhythmic Ventilation

• Inspiration begins when stimuli from many sources, such as receptors that monitor blood gases, reaches a threshold.
• Expiration begins when the neurons causing inspiration are inhibited.

Modifications of Ventilation

Nervous Control of Ventilation

• Higher brain centers allow voluntary control of ventilation. Emotions and speech production affect ventilation.
• The Hering-Breuer reflex inhibits the respiratory center when the lungs are stretched during inspiration.
• Touch, thermal, and pain receptors can stimulate
  ventilation.

Chemical Control of Ventilation

• Chemoreceptors in the medulla oblongata respond to changes in blood pH. Usually changes in blood pH are produced by changes in blood carbon dioxide.
• Carbon dioxide is the major chemical regulator of respiration. An increase in blood carbon dioxide causes a decrease in blood pH, resulting in increased ventilation.
• Low blood levels of oxygen can stimulate chemoreceptors in the carotid and aortic bodies, resulting in increased ventilation.

Effect of Exercise on Respiration

• Input from higher brain centers and from proprioceptors stimulates the respiratory center during exercise.

Respiratory Adaptations to Exercise

• Training results in increased minute volume at maximal exercise because of increased tidal volume and respiratory rate