In this lesson, we will discuss:
the definition of gaseous exchange
characteristics of gaseous exchange surfaces
the mechanism of breathing in humans
the structure of the respiratory system in humans
Gaseous exchange is the physical process by which gases move by diffusion across a surface. (Remember that diffusion is the net movement of particles from an area of high concentration to an area of low concentration due to the random movement of particles). In most organisms, it is how oxygen and carbon dioxide (the respiratory gases) move in opposite directions across their respiratory membranes (eg the alveoli), between the air or water of their external environment and the body fluids of their internal environment. As we discussed in a previous post on respiration, oxygen is needed by cells to extract energy from organic molecules like glucose. Carbon dioxide is produced as a waste product and must be excreted.
Gaseous exchange surfaces, the membranes or surfaces across which gaseous exchange occurs, tend to have similar characteristics since they want to accomplish similar functions as efficiently as possible. These features include:
They have a large surface area relative to the volume of the organism.
They are thin, so they have a short diffusion pathway, allowing gases to diffuse through them more quickly.
They have a moist surface where gases can dissolve first before they diffuse in our out.
They are able to maintain the diffusion gradient down which the gases can diffuse. That is, gaseous exchange surfaces ensure that oxygen is always diffusing in and carbon dioxide is always diffusing out. They do this by maintaining a concentration gradient, where oxygen is less concentrated in the internal environment and carbon dioxide is less concentrated in the external environment. For example, in humans, the gaseous exchange surface of the alveoli in the lungs have a rich supply of blood so that the diffused gases are very quickly transported to and from the body's cells. Therefore, the concentration gradient is maintained.
So, organisms need to have a way to take in oxygen and get rid of carbon dioxide.
Humans have a high metabolic rate so they require a constant oxygen supply, and therefore a fast rate of gaseous exchange. To help with this there are two key features in the human system:
A blood transport system with red cells containing haemoglobin (to carry oxygen)
A means of ventilation to get the gases to and from the gaseous exchange surface.
In humans, gaseous exchange occurs in the lungs, which are composed of millions of tiny sacs known as alveoli. However, the lungs cannot function alone. The lungs are a part of the respiratory system, which overall assists in a process known as ventilation, better known as breathing. As stated above, ventilation is how gases are transported to and from the gaseous exchange surface (the alveoli).
The two lungs are surrounded by the ribs, which form the chest cavity. In between the ribs are the intercostal muscles, and below the lungs is the diaphragm. Breathing, or ventilation, is brought about when the intercostal muscles and diaphragm contract and relax to cause air to move into and out of the lungs.
.Each of the lungs is contained within two pleural membranes which contain pleural fluid in between. A bronchus leads into each lung from the trachea. The trachea branches off into smaller branches known as bronchioles that have the tiny sacs known as alveoli at their ends. Each of the lungs receives deoxygenated blood from the heart through the pulmonary artery, oxygenates it through diffusion at the alveoli, and sends it back to the heart to be pumped around the body through the pulmonary vein (We discuss more about transport in humans in this post).
The following diagram shows the process of inhalation and expiration:
As the air is inhaled, it is warmed in the nasal cavity, then cleaned and moistened by mucus that lines the nasal passages and trachea. This is all done to ensure that blockages do not form in the trachea or bronchi, and also so that colder air doesn't damage the lungs. You should also know about the cilia, which are tiny hairs that use a rhythmic beating motion to keep the airways clean of mucus and dirt.
The air now travels through the bronchi and bronchioles until is reaches the alveoli, where gaseous exchange occurs between the air and the blood in the capillaries.