Unit 12: The Respiratory System

What is Respiration? Respiration is the word used to describe the process of gas exchange in living organisms, including plants and animals. Animals take in oxygen and give off carbon dioxide, while plants take in carbon dioxide and give off oxygen. The gases in animals are carried by their blood, while in plants, there are different cells, making up xylem and phloem, that specialize in gas exchange.

Simple Organisms In order for a gas, like oxygen or carbon dioxide, to pass into or out of a cell, it must be dissolved in a liquid first. Since the simplest organisms (bacteria and fungi) usually live in damp or wet areas, they can exchange gas directly with the liquid they’re in contact with.

Complex Organisms More complex organisms have a harder time with gas exchange for several reasons. First, many don’t live in watery environments. Second, their bodies are too thick for each cell to exchange gas directly with the external environment.

Complex organisms have specialized body areas to accommodate for these problems. For example, body tissues involved in gas exchange (nasal passages, mouth, lungs) are kept moist to dissolve gases. These areas also have large surface areas to increase the volume of gases entering and exiting the organism. The tissues are usually thin so gases can cross through easily. Also, many complex organisms have organs that work together to form a transport system.

Respiration in Plants Gas exchange in plants happens in three major areas, each of which is specialized for its environment. Gas exchange in roots occurs when, through active transport, gas found in the spaces between soil particles is taken in through root hairs.

Gas exchange in stems occurs in xylem cells, found on the outside of the stems. Gas exchange in leaves occurs through small openings, usually on the underside of the leaf, called stomata. The actual exchange of gases occurs in the palisade and spongy layers of the leaves, where the majority of photosynthesis takes place.

Respiration in Animals Depending on the habitat, animals have many features that aid in respiration. Fish have gills, worms have mucusy skin, and humans have lungs. All of these features perform the same vital role - gas exchange.

STOP!

Respiration in Humans When you inhale, O2 enters the nose or mouth, passes to the pharynx, and moves to the trachea. The opening of the trachea is protected by a flap called the epiglottis. If food goes into the trachea instead of the esophagus, the epiglottis blocks it, causing you to cough it up.

Branching from the trachea are two large tubes called the bronchi. The bronchi branch and re-branch until they form bronchioles, which end in many small sacs called the alveoli. Gas exhange occurs in the alveoli. Groups of alveoli greatly increase the surface area of the lungs for gas exchange, so the human body is assured an adequate supply of oxygen.

The lungs, since they are thin and delicate organs, are located deep inside the body cavity and are protected by the ribs. The alveoli completely infiltrate the lungs to carry out gas exchange.

Oxygen travels through the bloodstream by way of red blood cells (RBCs). RBCs contain a substance called hemoglobin. Hemoglobin is a protein that is rich in iron, so that it will bind to and carry oxygen. CO2 can bond with hemoglobin, water, and plasma, so it moves around easily.

CO2-rich blood reaches the alveoli through capillaries that come from the pulmonary arteries. Since CO2 concentration is low and O2 concentration is high, CO2 will diffuse into the alveoli and be exhaled. Simultaneously, O2 will diffuse into the blood and return to the heart through the pulmonary veins on the left side of the heart.

From there, the blood is pumped to the rest of the body. Gas exchange occurs throughout the body cells as the gases move across the cell membrane from high to low concentration.

A Breath of Fresh Air A single breath is the act of drawing air from the environment into your body and forcing it back out. Inhaling requires the action of the rib muscles, which must stretch to allow the lungs to inflate, and a flat muscular sheet called the diaphragm.

The diaphragm is located across the base of the rib cage, and must also stretch at this time. During an inhalation, the diaphragm pulls downward and the rib muscles move upward and outward.

Air rushes into the trachea and lungs because air pressure is less in the chest than it is outside the body. This causes the muscles to relax as the air pressure stabilizes. The diaphragm relaxes, pushing upward, and the rib muscles pull in and down, forcing air back out. This is exhaling.

Summary of Respiration O2-rich air enters the body through the nose or mouth and travels through the pharynx down the trachea to reach the bronchi. The bronchi branch to form smaller tubes called bronchioles, which end in alveoli. The alveoli are arranged throughout the lungs, which inflate with the intake of air. The inflation and change in air pressure causes the rib muscles to stretch and the diaphragm to pull downward.

In the lungs, specifically the alveoli (the site of gas exchange), the CO2 is traded for O2 that is carried to the rest of the body by the RBCs, which contain hemoglobin. Eventually, the blood will reach the lungs again, where, because of a decrease in air pressure, the CO2 will be forced out of the body during exhalation. Blood that is rich in O2 is usually symbolized by the color red and CO2-rich blood is shown by blue arrows or vessels.