Lung Structure and Ventilation

Function of the Respiratory System

The primary function of the respiratory system is to exchange gases with the environment. The respiratory system also plays roles in communication, olfaction, pH balance, and regulation of blood pressure. The lungs aid in the mobilization of abdominal contents and act as a respiratory pump to promote venous return and lymph flow
The primary function of the respiratory system is gas exchange. During inspiration (or inhalation), the act of breathing in, oxygen is taken from the environment and into the lungs. Gas exchange occurs between each alveolus (plural, alveoli) in the lungs and capillary beds, networks of tiny blood vessels, that surround the alveoli. As blood flows through the capillaries and around the alveoli, carbon dioxide moves out of the bloodstream and into the alveoli. Oxygen taken into the lungs through the act of breathing moves out of the alveoli and into the bloodstream. Carbon dioxide is released into the environment during expiration (or exhalation), the act of breathing out.

Gas Exchange at the Alveolus

Gas exchange occurs in the alveoli in the lungs. Carbon dioxide (CO2) moves from the capillaries to the alveolus and oxygen (O2) moves from the alveolus to the capillaries.
The brain uses signals from chemoreceptors, specialized nerve cells that detect changes in chemicals, to regulate the respiratory system. There are three regions where chemoreceptors that regulate respiration are found: the brain, the aortic arch of the heart, and in the carotid arteries of the neck. They detect carbon dioxide and/or oxygen levels in the bloodstream. When carbon dioxide levels in the blood rise, the pH of the blood falls, or becomes more acidic. Chemoreceptors detect this change and the brain triggers the respiratory system to increase the rate of breathing.

The goal of both the cardiovascular system and the respiratory system is to deliver oxygen to the tissues and remove carbon dioxide. The respiratory system brings oxygen-rich air to the capillaries where oxygen enters the bloodstream. Carbon dioxide moves from the capillaries to the lungs and is released to the atmosphere during expiration. The respiratory system also aids the cardiovascular system through blood pressure regulation.

The respiratory system is involved in communication by allowing people to produce sound. As air passes through the nose or mouth through the pharynx and then the larynx, or voice box, the movement of air vibrates the vocal cords which produces sound. The vibration creates sound waves that then move back through the pharynx, nose, and mouth. In order for sufficient sound to be produced, the lungs have to be strong enough to provide adequate airflow to move the vocal cords.

Olfaction, or the sense of smell, occurs as air passes through the nasal passageways. Olfactory sensory neurons at the top of the nasal cavity have olfactory sensor proteins in their cell membranes that detect smells through subtle chemical changes in the inspired air. The cells are centered in the olfactory bulb which transmits the information to the olfactory nerve.

In addition, breathing functions as a type of pump that impacts venous return and lymph flow. During expiration, the vena cava (the large vein carrying oxygen-deficient blood back to the heart) is compressed, decreasing venous return and lymph flow. During inspiration, the vena cava is no longer compressed and both venous return and the flow of lymph is increased. Also during inspiration, blood flow from the lungs to the left atrium of the heart is increased, resulting in increased left ventricular filling. Increased ventricular filling leads to increased cardiac output. Movement of the diaphragm changes the pressures and space within the abdominal cavity. In this way, breathing helps to mobilize abdominal contents.