6.4. The respiratory system

All living things get their energy through a biochemical process that takes place within their cells. Some organisms can produce energy without oxygen. Humans, however, do require oxygen to produce energy. Human bodies are adapted to carry oxygen from the atmosphere to body cells and to eliminate the waste products resulting from the energy-producing process.

The respiratory system and the excretory system are involved in these function s. Through the respiratory system, oxygen is inhaled and diffused into the blood. In addition, carbon dioxide is diffused from the blood into the lungs and is exhaled. Carbon dioxide is the chief waste product of cellular activity. Other cellular wastes are eliminated by the excretory system.

The respiratory system consists of the organs of breathing. However, breathing is only one part of respiration. Respiration is the process by which the body takes in oxygen, uses it to produce energy, and then eliminates some waste products of the cellular activity. Three subprocesses are involved in respiration. They are external respiration, internal respiration, cellular respiration. In external respiration, or breathing, the body exchanges gases between the atmosphere and the blood. Internal respiration is the diffusion of gases between the blood or tissuefluid and body cells. Cellular respiration is the process by which cells break down glucose molecules in the presence of oxygen to form the energy molecule ATP.

The Lungs and Breathing

The major breathing organs are two lungs, located in the tho­racic cavity. The lungs are spongy, cone-shaped, saclike or­gans. Each lung weighs about 600 g. The right lung has three main divisions, or lobes, and is slightly larger than the left lung, which has two lobes. Both lungs are encased in a tough membrane that also lines the thoracic cavity. This double mem­brane, the pleura, secretes a lubricating fluid that allows the lungs to move smoothly. Inflammation of the pleura can lead to fluid buildup in the thoracic cavity. This condition is called pleurisy.

Breathing begins when the diaphragm, the dome-shaped muscle below the chest cavity, contracts and moves downward. The intercostal muscles between the ribs also contract, causing the rib cage to move up and out. Together, these muscle con­tractions cause the chest cavity to enlarge. When the chest ex­pands, the air pressure in the chest cavity drops. Air pressure outside the body is then greater than that inside the chest cavity. Air then flows into the lungs from outside the body, equalizing the pressure. This part of the breathing process is called inspira­tion or inhalation.

When the air pressure has been equalized, it causes the dia­phragm and intercostal muscles to relax and return to their nor­mal positions. This in turn reduces the size of the chest cavity. As the size of the chest decreases, the air pressure inside the chest cavity gradually becomes greater than the air pressure out­side the body. Air then leaves the lungs, again equalizing the pressure. This part of the breathing process is called expiration or exhalation.

The Pathway of Air

Air enters the body through two openings in the nose called nostrils. From there the air flows into the nasal cavities, two spaces in the nose. The cavities are separated by a cartilage and bone partition called the septum. The cavities are lined with mucous tissue that contains many blood vessels. The mucous tissue warms and moistens the incoming air. Moisture must be present for diffusion of gases to take place within the lungs. Cilia and hairs also line the cavities and filter foreign particles from the air. The cilia move constantly, carrying these particles outward toward the nostrils.

Air travels from the nasal cavities into the back side of the pharynx, a tube at the rear of the nasal cavities and mouth. The pharynx is a common passageway for both food and air. While air must get into the cartilage-ringed trachea, or windpipe, at the front of pharynx, food must get to the esophagus at the back side of the pharynx. Therefore, food and air cross each other's paths. If food entered the air passageway, the person would choke. To ensure that food does not enter the air passageway, the body makes involuntary adjustments. During the process of swallowing, a flap of tissue called the epiglottis closes over the glottis, or the upper part of the trachea. At the same time, the soft palate closes off the nasal cavities. During inhalation, the glottis is open to allow air to enter the trachea.

At the top of the trachea is the larynx, or voice box. Two ligaments called vocal cords are stretched across the larynx. The larynx is called the voice box because sound is produced when air is forced between the cords. The amount of tension in the cords determines the pitch of a sound. Nine cartilage rings connected by ligaments hold the mucus-lined larynx open dur­ing inhalation and against the pressure from food passing through the adjacent esophagus. The largest of the cartilage rings appears as the Adam's apple in the throat.

The trachea descends to a point near the middle of the breastbone. There it divides into two branches called bronchi. Bronchi walls consist of muscle supported by carti­lage and are lined with mucus and cilia. The bronchi reach deep into the lungs, subdividing about 25 times into smaller and smaller passageways. The first 10 subdivisions are called sec­ondary bronchi. The remaining subdivisions are microscopic-sized tubes called bronchioles. Bronchiole walls consist of smooth muscle and are lined with mucus and cilia. The continuous beating of the cilia in the bronchi and bronchioles carries foreign particles and excess mucus into the pharynx. This material may then be expelled by being swal­lowed or coughed out.

The smallest bronchioles branch into tiny ducts, which end in clusters of tiny bulges. These bulges are air sacs called alve­oli. Each lung has more than 300 million alveoli. Each alveolus measures from 0.1 to 0.2 mm in diameter. The total surface area provided by the alveoli is estimated at about 70 m².

Exchange of Gases

Alveoli are completely surrounded by capillaries. The actual exchange of gases occurs when oxygen in the air of the alveoli diffuses into the blood in the capillaries. In turn, the carbon dioxide in the blood diffuses into the air of the alveoli. The epithelial tissue forming the walls of both the alveoli and capil­laries is only one cell thick. Together, the walls of an alveolus and an adjacent capillary measure only 0.0004 mm. The oxygen in inhaled air dissolves in the mucus on the lining of the alveoli.

In the blood, most oxygen combines with hemoglobin to form oxyhemoglobin. Oxygen from the oxyhemoglobin diffuses into body cells and is used in metabolism, the chemical and physical activities within cells. Metabolism includes the build­ing up and breaking down of complex molecules and the releas­ing of energy during the breakdown. As a result of metabolism, oxygen concentration in the body cells is low, but carbon diox­ide concentration is high.

Carbon dioxide, a metabolic byproduct, diffuses from body cells into the blood. Carbon dioxide is transported in the blood in three ways. About 5 percent dissolves in the plasma. About 25 percent enters the red blood cells and combines with hemo­globin. With help from a special enzyme, the remainder—or about 70 percent—combines with water in the red blood cells to form carbonic acid:

CO₂ + H₂O H₂CO₃

(carbon dioxide) (water) (carbonic acid)

Almost immediately, carbonic acid separates into hydrogen ions (H⁺), which combine with hemoglobin, and bicarbonate ions (HCO₃^-), which diffuse into the plasma.

H₂CO₃ H⁺ +HCO₃^-

As a result of this chemical process, most carbon dioxide is transported in the plasma as bicarbonate ions.

When blood reaches the lungs, chemical reactions occur that reverse the process, releasing carbon dioxide:

H ⁺ +HCO₃^- H₂CO₃

CO₂+H₂O

The carbon dioxide diffuses from the blood into the lungs. The carbon dioxide is exhaled along with water vapor.

Regulation of Breathing

Many factors influence the control of breathing, in­cluding carbon dioxide and oxygen levels in the blood. The level of carbon dioxide in the blood plays a vital role in regu­lating breathing. Carbon dioxide affects blood acidity. Certain nerve cells are sensitive to changes in blood acidity. These nerves send messages to the breathing center at the base of the brain. hen the carbon dioxide level in the blood is high, the messages cause the breathing center to trigger speedup in breathing rate. Conversely, a low carbon dioxide level reduces the stretch receptors in the lungs. When the lungs expand sufficiently, the stretch receptors send messages to the breathing center. The breathing center then sends messages that make the muscles relax. Stretch receptors thus operate as another kind of breathing control mechanism.