By Frank H. Osborne, Ph. D. Respiration Principles of Biology By Frank H. Osborne, Ph. D. Respiration

Respiration Breathing is generally called respiration. We use respiration to distinguish breathing from cellular respiration, which is the oxidation of glucose for energy production. All higher organisms require O2. Unless the O2 diffuses into the body, they must have a mechanism and a system for bringing in O2 and eliminating the CO2 that results from cellular respiration.

Respiration Plants and the lower members of the animal kingdom can obtain their O2 and eliminate their CO2 by diffusion. They do not require lungs or gills. Higher animals, including humans, require a breathing apparatus in order to exchange these gases.

Unicellular Organisms In unicellular organisms, gases exchange with the surrounding medium by diffusion across the moist cell membrane of the cell. The membrane must be moist. It is more difficult for gases to diffuse across a dry membrane. Besides, a dried out organism is usually dead. No special breathing apparatus is required by these organisms.

Green Plants Gases enter and exit the air spaces of leaves via the stomates. The openings of stomates are controlled by guard cells.

Green Plants Older stems are usually impermeable to gases. Some trees have lenticels that let air into the cambium. In many large trees, there are enough cracks in the bark so that air is available for the cambium and living phloem cells.

Green Plants Roots can exchange gases with the soil. This is done via the moist cell membranes of the cells in the lower parts of the roots.

Coelenterates Hydra does not have any specialized breathing apparatus. It only has two cell layers and therefore does not need one. Each cell layer is in contact directly with the exterior aqueous environment. Gas exchange is across the moist cell membranes of the individual cells.

Coelenterates Hydra. With only two body layers, no special breathing equipment is required.

Higher Marine Animals Gills are used by marine worms, molluscs (clams), and fishes. The gill lamellae (layers) have a large area for exchange of gases. The gases are transported to the rest of the body by the circulatory system. In the crustacea (lobsters) and echinoderms (starfishes), different systems are used.

Diagram of clam

Diagram of fish

Diagram of lobster

Diagram of starfish

Gill Function The afferent vessel carries blood toward the gill (afferent and approach both begin with "a"). This blood is high in CO2 and low in O2. The blood in the gill exchanges gases with the water. The CO2 leaves the gill and enters the water while the O2 leaves the water and enters the gill. Then, the efferent vessel carries blood away from the gill (efferent and exit both begin with "e"). This blood is high in O2 and low in CO2.

Gill Function

Insects and Spiders Each cell in the body of an insect or spider is in contact or near a breathing tube known as a trachiole. A group of trachioles will be combined to form a larger breathing tube called a trachea. These animals have numerous tracheas that exit through holes on the side of the body called spiracles. These spiracles are used for gas exchange, the "spir" part is also found in the word "respiration."

Grasshopper

Frog In the frog and other amphibians, about half of the gas exchange takes place across the skin. The other half of the respiration is done using lungs. Frogs do not have a diaphragm so they must force air into the lungs. This is called positive pressure breathing. Humans have a diaphragm so their breathing is by negative pressure.

Frog

Birds Birds have a complicated respiratory system consisting of lungs and air sacs in the bones. Air is cycled through the air sacs in the hollow spaces in the bones.

Birds

Human Respiration The human respiratory system consists of the lungs, bronchi and trachea. The lungs are located in the thoracic (chest) cavity, the area in the chest above the diaphragm. The heart is also located in the chest cavity.

Human Lungs

Human Respiration In humans, the diaphragm contracts and makes the chest cavity larger. This creates a partial vacuum in the chest cavity. Air rushes in because the pressure is lower in the chest cavity when the diaphragm contracts. The air that enters the lungs from the outside is high in O2 and low in CO2.

Human Respiration Inspiration is the term used for inhaling air. When the diaphragm relaxes, the pressure in the chest increases and forces the air out. The air that leaves the lungs is low in O2 and high in CO2. Expiration is the term used for exhaling air.

Breathing

Control of Breathing Control of breathing is done by the nervous system. The brain stem has a CO2 sensor that detects the concentration of CO2 in the blood. When CO2 increases as a result of exercise, for example, the breathing rate and heart rate increase. When there is little CO2 in the blood, the nervous system slows the breathing rate.

Gas Exchange in the Lungs Gases are exchanged in the air sacs (alveoli) of the lungs. Air makes its way down to the air sacs where O2 passes through the capillary walls by diffusion, and into the blood stream where it enters the red blood cells. Blood leaving the alveoli is high in O2 and low in CO2.

Gas Exchange in the Lungs Simultaneously, CO2 and some water leave the blood stream and enter the alveoli of the lungs. These respiratory gases pass up through the respiratory system to the outside. All air exchange in the lungs is external respiration and occurs through the moist surfaces of the cell membranes of the air sacs of the lungs and the capillaries of the blood stream. Cellular respiration is internal respiration.

Gas Exchange in the Lungs

Principles of Biology Respiration The End Principles of Biology Respiration