Organs of the Respiratory System
Nose, Nasal Cavity, and Paranasal Sinuses
Trachea, Bronchiole Tree, and Lungs
Respiratory Air Volumes and Capacities
Respiratory Center and Factors Affecting Breathing
Alveoli, Respiratory Membrane, and Diffusion Across the Respiratory Membrane
Oxygen Transport and Carbon Dioxide Transport
The respiratory system functions to move air in and out of the lungs. This process is called ventilation or breathing. This system also functions to deliver oxygen to the blood stream and to remove carbon dioxide from the blood stream. This exchange of oxygen and carbon dioxide is called external respiration.
Click here to see the organs of the respiratory system
The nose is made of bones and cartilage and the skin covering them. The openings of the nose are called nostrils. The hairs of the nostrils prevent large particles from entering the nose through air.
The nasal cavity is simply the hollow space behind the nose. The nasal cavity is divided into a left and right portion by the nasal septum. There are structures called nasal conchae that extend from the lateral walls of the nasal cavity. Most of the nasal cavity is lines with a mucous membrane that acts to warm and moisten air as it passes through the nasal cavity.
The nasal cavity is also lines with cells that possess cilia. As mucus traps dust and other particles in the nasal cavity, the cilia push the mucus toward the throat where it is swallowed. The enzymes of the stomach will then destroy the particles in the dust.
Paranasal sinuses are air-filled spaces within skull bones that open into the nasal cavity. The paranasal sinuses reduce the weight of the skull and also give your voice a certain tone. When your paranasal sinuses are "stopped up" the tone of your voice changes.
The pharynx is an organ of the respiratory system as well as the digestive system. Air flows through all parts of the pharynx.
The larynx is more commonly called the voice box. It sits superior to and is continuous with the trachea. It functions to conduct air in and out of the trachea. It also houses the vocal cords.
The larynx is mostly made of cartilage and muscle tissue. There are 3 cartilages in the larynx. The largest cartilage is called the thyroid cartilage and it forms the anterior wall of the larynx. A smaller cartilage called the epiglottic cartilage forms the framework of the epiglottis. The epiglottis is the flap-like structure that closes off the larynx during swallowing. The third cartilage of the larynx is called the cricoid cartilage and it forms most of the posterior wall of the larynx.
Vocal cords stretch between the thyroid cartilage and the cricoid cartilage. The opening between the vocal cords is called the glottis. The upper vocal cords are called false vocal cords because they do not produce sounds. The lower vocal cords are called true vocal cords because they can be manipulated to produce sounds.
If the true vocal cords are stretched, the voice becomes higher in pitch. When the true vocal cords are relaxed, the voice becomes lower in pitch. Men have thicker vocal cords which is why their voices are deeper than female voices.
Click here to view the structure of the larynx
Click here to view vocal cords
The trachea is more commonly called the windpipe. It is a tubular organ made of rings of cartilage and smooth muscle that extends from the larynx to the bronchi. It is lines with cells that possess cilia. These cilia constantly move mucus up to the throat.
The distal end of the trachea branches and starts a series of tubes called the bronchial tree. The first branches off the trachea are called primary bronchi. The branches off primary bronchi are called secondary bronchi. The secondary bronchi branch into tertiary bronchi. Tertiary bronchi branch into bronchioles. At the ends of bronchioles are air sacs called alveoli.
The alveoli are very thin sacs made of simple squamous epithelial cells. The alveoli are surrounded by capillaries. Blood in the capillaries release carbon dioxide into the alveoli and air in the alveoli releases oxygen into the blood of the capillaries. This is how oxygen gets into the blood and carbon dioxide gets out.
Click here to view the trachea and bronchial tree
The lungs are cone-shaped organs that contain connective tissue, the bronchial tree, nerves, lymphatics and many blood vessels. The right lung is larger than the left lung. The right lung is divided into three lobes and the left lung is divided into two lobes.
Breathing or pulmonary ventilation is divided into two events - inspiration and expiration.
During inspiration or inhalation oxygen-rich air eventually flows into the alveoli of the lungs. Air flows into the airways during inspiration because the thoracic cavity enlarges. When the thoracic cavity enlarges pressure decreases in the cavity. The atmospheric pressure outside the body is greater than the pressure inside the cavity. Air flows from an area of high pressure to an area of low pressure. The events that lead to inspiration are as follows:
1) The diaphragm contracts. When the diaphragm contracts it becomes flat. This increases the amount of space in the thoracic cavity.
2) The intercostal muscles raise the ribs; this further enlarges the thoracic cavity.
Click here to view the events of inspiration
During expiration or exhalation carbon dioxide-rich air flows out of the airways. Air flows out because the thoracic cavity becomes smaller which increases the pressure inside the cavity. When the pressure inside the cavity becomes greater than atmospheric pressure air flows out. The events that lead to expiration are as follows:
1) The diaphragm relaxes. When the diaphragm relaxes it domes up into the thoracic cavity decreasing the space in the cavity.
2) The intercostal muscles lower the ribs; this further decreases the size of the thoracic cavity.
Click here to view events of expiration
During different intensities of breathing, different volumes of air move in and out of the lungs. These volumes are called respiratory volumes and can be measured to assess the healthiness of the respiratory system. Respiratory capacities can be calculated by adding certain respiratory volumes together.
The following are different types of volumes and capacities:
1) Tidal volume - the amount of air that moves in or out of the lungs during a normal breath.
2) Inspiratory reserve volume - amount of air that can forcefully inhaled following a normal inhalation.
3) Expiratory reserve volume - amount of air that can be forcefully exhaled following a normal exhalation.
4) Residual volume - volume of air that always remains in lungs even after a forceful expiration.
5) Vital capacity - tidal volume + inspiratory reserve volume + expiratory reserve volume; the total amount of air that can be forcefully exhaled after the deepest inhalation possible.
6) Total lung capacity - Vital capacity + residual volume; total amount of air the lungs can hold.
The respiratory center is a group of neurons in the pons and medulla oblongata.
The medulla oblongata contains the rhythmicity area which controls the rhythm of breathing as well as the depth of breaths. The pons contains the pneumotaxic area which controls breathing rate.
Increase in the amount of carbon dioxide or hydrogen ions in the blood will increase the rate and depth of breathing.
An inflation reflex also helps to regulate the depth of breathing. Stretch receptors in plueral membranes are activated when the lungs are stretched past a certain point. This triggers the depth of breathing to decrease to prevent over inflation of the lungs.
Fear and pain normally increase breathing rate.
Breathing rapidly and deeply is called hyperventilation. Hyperventilation decreases the amount of carbon dioxide in the blood.
Alveoli are the air sacs of the lungs where gases (oxygen and carbon dioxide) are exchanged. The exchange of these gases occurs across the respiratory membrane.
The respiratory membrane consists of the wall of an alveolus and the wall of a capillary. Oxygen diffuses across the membrane from the air in the alveolus to the blood in the capillary. Carbon dioxide diffuses across the membrane from the blood in the capillary to the air in the alveolus.
Once oxygen gets into the blood stream, most of it binds to hemoglobin in red blood cells. Recall that hemoglobin bound to oxygen is called oxyhemoglobin and is bright red. However, some oxygen stays dissolved in plasma and is carried there.
When carbon dioxide gets into the blood stream, most of it reacts with water to form bicarbonate ions. So most carbon dioxide is actually carried in the blood as the bicarbonate ion. When the bicarbonate ion reaches the respiratory membranes of the lungs, an enzyme changes it back into carbon dioxide and water again. Now the carbon dioxide can diffuse out of the blood. Some carbon dioxide can bind to hemoglobin and be transported that way. Still other molecules of carbon dioxide will simply stay dissolved in plasma.
Click here to view oxygen and carbon dioxide transport in the blood