Animal cells require O2 for aerobic respiration. If cells are not directly exposed to the outside environment, then some mechanism must provide gas exchange to internal cells, delivering O2 and removing waste CO2. The movement of gases into and out of the entire organism is called respiration. (This term, respiration, is also used to describe cellular respiration, the process of producing ATP within the mitochondria of cells.) The following gas exchange mechanisms are found in animals:

1. Direct with environment. Some animals are small enough to allow gas exchange directly with the outside environment. Many of these animals, such as the Platyhelminthes (flatworms), typically have large surface areas, and every cell either is exposed to the outside environment or is close enough that gases are available by diffusion through adjacent cells. In larger animals, such as the Annelida (segmented worms), gas exchange through the skin is augmented by a distribution system (a circulatory system) just inside the skin.

2. Gills. Gills are evaginated structures, or outgrowths from the body, that create a large surface area over which gas exchange occurs. Inside the gills, a circulatory system removes the oxygen and delivers waste CO2. In some animals, such as polychaete worms (Annelida), the gills are external and unprotected. In other animals, the gills are internal and protected. In fish, for example, water enters the mouth, passes over the gills, and exits through the gill cover, or operculum. Countercurrent exchange between the opposing movements of water and the underlying blood through blood vessels maximizes the diffusion of O2 into the blood and CO2 into the water.

3. Tracheae. Insects have chitin-lined tubes, or tracheae, that permeate their bodies. Oxygen enters (or CO2 exits) the tracheae through openings called spiracles; diffusion occurs across moistened tracheal endings.

4. Lungs. Lungs are invaginated structures, or cavities within the body of the animal. Book lungs, occurring in many spiders, are stacks of flattened membranes enclosed in an internal chamber.

Gas exchange in humans occurs as follows:

1. Nose, pharynx, larynx. Air enters the nose and passes through the nasal cavity, pharynx, and larynx. The larynx (“voice box”) contains the vocal cords.

2. Trachea. After passing through the larynx, air enters the trachea, a cartilage-lined tube. When the animal is swallowing, a special flap called the epiglottis covers the trachea, preventing the entrance of solid and liquid material.

3. Bronchi, bronchioles. The trachea branches into two bronchi (singular, bronchus), which enter the lungs and then branch repeatedly, forming narrower tubes called bronchioles.

4. Alveolus. Each bronchiole branch ends in a small sac called an alveolus (plural, alveoli).

Each alveolus is densely surrounded by blood-carrying capillaries.

5. Diffusion between alveolar chambers and blood. Gas exchange occurs by diffusion across the moist, sac membranes of the alveoli. Oxygen diffuses into the moisture covering the membrane, through the alveolar wall, through the blood capillary wall, into the blood, and into red blood cells. Carbon dioxide diffuses in the opposite direction.

6. Bulk flow of O2. The circulatory system transports O2 throughout the body within red blood cells. Red blood cells contain hemoglobin, iron-containing proteins to which O2 bonds.

7. Diffusion between blood and cells. Blood capillaries permeate the body. Oxygen diffuses out of the red blood cells, across blood capillary walls, into interstitial fluids (the fluids surrounding the cells), and across cell membranes. Carbon dioxide diffuses in the opposite direction.

8. Bulk flow of CO2. Most CO2 is transported as dissolved bicarbonate ions (HCO3 –) in the plasma, the liquid portion of the blood. The formation of HCO3 –, however, occurs in the red blood cells, where the formation of carbonic acid (H2CO3) is catalyzed by the enzyme carbonic anhydrase.

Following their formation in the red blood cells, HCO3 – ions diffuse back into the plasma. Some CO2, however, does not become HCO3 –; instead, it mixes directly with the plasma (as CO2 gas) or binds with the amino groups of the hemoglobin molecules inside red blood cells.

9. Bulk flow of air into and out of the lungs (mechanics of respiration). Air is moved into and out of the lungs by changing their volume. The volume of the lungs is increased by the contraction of the diaphragm (a muscle under the lungs) and the intercostal muscles (muscles between the ribs). When the lung volume increases, the air pressure within the lungs decreases. This causes a pressure difference between the air in the lungs and the air outside the body. As a result, air rushes into the lungs by bulk flow. When the diaphragm and intercostal muscles relax, the volume of the lungs decreases, raising the pressure on the air, causing the air to rush out.

10. Control of respiration. Chemoreceptors in the carotid arteries (arteries that supply blood to the brain) monitor the pH of the blood. When a body is active, CO2 production increases. When the CO2 that enters the plasma is converted to HCO3 – and H+, the blood pH drops (becomes more acidic). In response, the chemoreceptors send nerve impulses to the diaphragm and intercostal muscles to increase respiratory rate. This results in a faster turnover in gas exchange, which, in turn, returns blood pH to normal. The regulation of the respiratory rate in this manner is an example of how homeostasis is maintained by negative feedback.