• List the steps in the path taken by air as it moves from the outside of the animal to the internal gas exchange site in the lungs
  • Describe the role played by each part of the respiratory tract
  • Explain how the mammalian respiratory system is adapted to reduce water loss

Respiration in mammals is broken into different categories:

Breathing- is separated into two actions, 1) inspiration or the act of taking in air and 2) expiration or the act of exhaling air

External respiration- is the act of exchanging the gases Oand CO between the air and the blood

Internal respiration- is the act of exchanging O2 and CO2 between blood and the cells in the body

Cellular respiration- this is the complex steps of chemical reactions that take place mostly in the mitochondria


Different mammals have different respiration systems but all of them have lungs and require a moist atmosphere in which to exchange gases. The human body is a good example of how mammals have adapted their systems to their surroundings.

The Path of Air in the Human Body

The gateways to our respiratory system are commonly known as the nose and mouth.These gateways lead to the pathways and meet in a chamber known as the pharynx. After the pharynx, there are two passageways; the open one is the oesophagus which leads to the stomach, and the closed (or guarded) one is the trachea and leads to the lungs. The trachea is guarded by a trapdoor called the epiglottis which prevents unknown substances, such as food, from entering the lungs. After the epiglottis, the larynx (or voice box) resides in a widened chamber. The larynx is what enables us to talk and produce sounds. After the larynx the trachea braches into two airways called the bronchi. One of the bronchi leads into the left lung and the other leads into the right lungs. Each of the bronchus branches off into smaller and smaller airways, of which the smallest are called bronchioles. The bronchioles each end in a minuscule air-filled sac called the alveolus. There are millions of alveolus and they resemble bunches of grapes. The alveoli are a single cell thick and are where gas exchange takes place in humans.

In short:

Air travels through the nose and mouth, pharynx, epiglottis, larynx, trachea, bronchi, bronchioles and end in the alveoli.

There are other factors which aid in respiration.

Our lungs both fit snugly within our chest cavity and are protected by the ribs. Below the lungs there is a thick dome shaped muscle called the diaphragm that looks like the floor for the ribcage. The diaphragm moves up and down along with the muscles between the ribs to help us breathe.

The Nose and Mouth

The gateways to the respiratory system play an important role in breathing. When air is breathed in through the nostrils, the bristly little hairs can screen and trap dust and other particles from the air. When there are particles too small to be trapped by the hairs they can be trapped further along in the nasal passages in mucus. A mucous membrane is a sticky lining that covers the entirety of the respiratory system. The mucus adds moisture to the air that you breathe so that it is suitable for the lungs to take in.

Air enters through the nostrils into a large chamber called the nasal cavity. The nasal cavity is divided into half by the nasal septum Three flat spongy plates called the nasal conchae or turbinates stick out into the cavity and warm the air Bones that surround the nasal cavity have hollow spaces called sinuses, which make the bones lighter. The uvula is a flap of flesh that can seal off the nasal cavity and is located at the back of the roof of the mouth. When this does not close off fast enough, you can sneeze or laugh out fluid that you are drinking. If you breathe through your mouth, your throat feels cooler and drier. The mouth is not equipped to clean, warm or add moisture to the air you breathe. However, breathing through the mouth can bring in extra oxygen when you are exercising or when your nose is stuffed up from a cold. Snoring can happen if you breathe through your mouth when you are sleeping. This happens as the uvula hangs down into the passageways and makes a rumbling/vibrating noise.

The Airways

The airways of the respiration system include the tonsils, pharynx, larynx, epiglottis, trachea, bronchi, and bronchioles.

Pharynx- Air passes by the tonsils in the pharynx. The tonsils produce disease-fighting white blood cells. The pharynx leads to the larynx.

Larynx- The larynx is a chamber that is shaped like a triangular box and has walls made of cartilage. Sometimes this is seen in the front of the neck and is called the Adam’s apple. Air must pass through the larynx to produce speech and sounds.

Trachea- The trachea is flexible and strong. It is 10-13 centimetres long and approximately 2.5 centimetres in diameter. The larynx extends from the larynx down into the chest and its walls are thick and muscular. It is strengthened by C-shaped rings of cartilage that you can feel on the front of your neck. These rings keep your throat open when you move and can tighten is something solid slips down by accident.

Bronchi- The trachea divides into two narrow tubes called the bronchi. The bronchi are also reinforced by cartilage rings to prevent closure. One bronchus goes into each lung and divides into smaller and smaller tubes.

Bronchioles- The bronchioles are the smallest air tubes and are approximately 1millimetre in diameter. They have no cartilage support and are very elastic. They have strands of muscles that may contract to narrow their openings and slow the flow of air.

The Lungs

The lungs are where external respiration takes place. The lungs look similar to two greyish pink balloons. The right lung divides into three lobes, while the left lung has only two lobes to accommodate the heart as well. The two lungs weigh 1 kilogram. The lungs are covered by two membranes called the pleurae. One is attached to the chest wall and the other surrounds the lungs. The pleural membranes protect the lungs so that movement is allowed while we breathe. The space that is in between them is filled with a fluid that sticks them together. When the chest muscles make the chest cavity expand, the pleurae pulls and makes the lungs expand as well.

The bronchi in the lungs divide 20 times, creating bronchioles. These bronchioles have bunches of alveoli at the ends. There are 300-400 million alveoli in the lungs. There is a network of capillaries on each of the alveolus so that gas can pass freely between the alveoli and the capillary walls. The alveoli require a non-stick coating called surfactant to keep the walls from sticking together and makes it easier to inflate the sacs. This decreases the effort needed for breathing to take place.

In a newborn baby the lungs are a pale pink color, but the lungs grow darker as we age. Although our filtering system works to keep out most particles, it cannot stop them all. Eventually particles make it into the airways and cause scarring. A miner’s lungs may look black after inhaling dust for years, but an Eskimo’s lungs may stay pink their entire lives as they live in a dust free environment.

The Chest

The ribs are 12 pairs of flat bones that are hinged at the spine, they curve to the front of the chest and most are attached to the sternum by flexible cartilage. The sternum is a long flat bone in the center of our chest and is also known as the breastbone. All of the ribs are connected to spine and rib above it by cartilage except for the last two. This allows for all the ribs to move at the same time when you breathe.

The muscles between the ribs are known as intercostal muscles. There are two types of intercostal muscles, interior and exterior. When we breathe in, the muscles contract and shorten, this pulls the ribs closer together and lifts the rib cage, increasing the lung volume. The muscles relax and the ribcage is pulled back into place when we exhale.

The diaphragm curves upward into the chest cavity just below the ribs and follows the curved line of the ribcage. The diaphragm separates the lungs and the heart from the other organs in the body. The stomach, liver and pancreas are located just below the diaphragm.

Structure Description Function
Nasal cavities Hollow spaces in nose Filter, warm, moisten air
Pharynx Chamber connecting oral and nasal cavities to larynx Connection to surrounding regions
Glottis Opening to larynx Air passage to larynx
Larynx Organ containing vocal cords Sound production
Trachea Flexible tube linking larynx and bronchi Passage of air to bronchi
Bronchi Tracheal divisions to lungs Passage of air to lungs
Bronchioles Branched tubes from bronchi to alveoli Passage of air to each alveolus
Lungs Soft spngy organs in thoracic cavity Gas exchange