• Distinguish between the concepts of breathing and respiration
  • Describe the muscular processes involved in breathing
  • Explain the concept of respiratory capacity and respiratory efficiency


Breathing is the mechanism by which mammals ventilate their lungs. This ventilation relies on the principle that air flows from higher pressure to lower pressure.

Breathing involves two muscular structures to control air pressure in lungs:

  • intercostal muscles: muscles associated with the ventral (front) surface of the rib cage
  • diaphragm: muscle layer that separates the region of the lungs (thoracic cavity) from the region of the stomach and liver (abdominal cavity)

The diaphragm is found in all mammals and its prime function is to assist in ventilation of the lungs. The intercostal muscles and diaphragm can both produce breathing movements but they usually work together. Breathing begins when the intercostal muscles and diaphragm contract. The intercostal muscles expand the rib cage and the diaphragm moves down in the thoracic cavity. This increases the volume of the thoracic cavity and because the thoracic cavity is relatively airtight this decreases the air pressure within the cavity. The decreased prssure draws the flexible walls of the lungs outward into the thoracic cavity causing the lungs to expand. Because of this expansion the air pressure inside the lungs decreases and so is lower than in the outside environment. Air enters the lungs, moving from a region of higher pressure to lower pressure.

The reverse muscular movement expels air from the lungs, by relaxing the diaphragm and external intercostal muscles and contracting the internal intercostal muscles to pull the rib cage back into place. This creates higher pressure in the thoracic cavity causing the lungs to shrink, increasing pressure inside them and pushing the air out.

Exchange of GasesEdit

External respiration takes place in the lungs. The alveoli and adjacent capillaries each have walls only a single cell thick so they allow the diffusion of oxygen and carbon dioxide. For inhaled oxygen to enter the bloodstream it must first dissolve in the fluid lining each alveolus. Usually the oxygen concentration of inhaled air is greater than in the blood of the capillaries entering the lungs and the carbon dioxide concentration is greater in the blood than in the inhaled air so oxygen diffuses across the capillary wall into the bloodstream and carbon dioxide moves from the capillaries across the alveoli into the lung.

Composition of inhaled air:

  • Oxygen: 20.94%
  • Carbon dioxide: 0.04%
  • Nitrogen and trace gases: 79.02%

Composition of exhaled air:

  • Oxygen: 16.49%
  • Carbon dioxide: 4.49%
  • Nitrogen and trace gases: 79.02%

Lung CapacityEdit

Under normal conditions regular breathing does not use up the full capacity of the lungs. As the body's needs increase the volume of air drawn in can also increase.

  • tidal volume: the volume of air inhaled and exhaled in a normal breathing movement
  • inpiratory reserve volume: additional volume of air that can be taken in beyond a regular tidal inhalation
  • expiratory reserve volume: additional volume that can be forced out of the lungs beyond a regular tidal exhalation
  • vital capacity: total volume of gas that can be moved in and out of the lungs (tidal volume + inspiratory reserve volume + expiratory reserve volume = vital capacity)
  • residual volume: amount of gas that remains in the lungs and passageways of the respiratory system after a full exhalation; this volume never leaves the respiratory system otherwise the lungs and respiratory passageways would collapse

The rate at which oxygen can be transferred to the blood is called respiratory efficiency. Some animals have respiratory systems with special adaptations to help increase respiratory efficiency, for example facilitated diffusion which can speed the transfer of oxygen across cell membranes in mammals.

Counter-current flowEdit

Water flows from the front of the fish to the back and the blood vessels lining the gills are arranged to flow back to front; this is called counter-current flow and plays an important role in increasing the efficiency of the fish respiratory system. The counter-current arrangement makes the most of the oxygen gradient between the respiratory medium and the blood. As blood flows past the gills the most oxygen-depleted blood meets the most oxygen depleted respiratory medium first. The concentration of oxygen in the blood is still lower than that of the water so oxygen will tend to diffuse into the blood. As blood flows toward the head of the fish it becomes richer in oxygen but continues to meet water with higher oxygen concentration. The level of oxygen in the blood never reaches the level in the water so even as blood is about to leave the gill it continues to pick up oxygen.

The Respiratory System in BirdsEdit

Much of the efficiency of a bird's respiratory system comes from a series of air sacs that branch out from the two lungs. These air sacs permeate most of the cavities of the bird including some of the cavities of the bones. The anterior air sacs are located generally on the anterior side of the lungs (between the lungs and trachea). Posterior air sacs are located on the posterior side of the lungs. All air sacs have tubes connecting them to the lungs. No exchange of gases takes place in these air sacs, however the regular expansion and contraction of both sets of air sacs serves to ventilate the lungs in a highly efficient manner. In the mammalian lung air enters the lungs and reaches the dead-end sacs of alveoli where gas exchange take place which means that much of the air that makes contact with the respiratory surface is residual air from the last act of inspiration. Birds, however, have a respiratory system that allows for true circulation of the air, meaning fresh air is continually moving along the lung surface whether the bird is inhaling or exhaling. Birds also have a counter-current blood flow to further increase the efficiency of the system.