Chapter 15: The Respiratory System

Why do animals cough, sneeze, yawn, sigh, and hiccup?

A cough is a protective reflex stimulated by irritation or foreign matter in the trachea or bronchi. Moist coughs, also known as productive coughs, help an animal clear mucus and other matter from the lower respiratory passages. They are generally beneficial to the animal, and we usually do not try to eliminate them with medications. Dry coughs, also known as nonproductive coughs, are generally not beneficial and are often treated with cough-suppressant medications. A sneeze is similar to a cough, but the irritation originates in the nasal passages. The burst of air is directed through the nose and mouth in an effort to eliminate the irritant or irritants. A yawn is a slow, deep breath taken through a wide-open mouth. It may be stimulated by a slight decrease in the oxygen level of the blood, or it may result from boredom, drowsiness, or fatigue. Yawns can even occur in humans by the power of suggestion, such as seeing someone else yawn or even thinking about yawning. (Did you just yawn?) A sigh is a slightly deeper-than-normal breath. It is not accompanied by a wide-open mouth like a yawn. A sigh breath may be a mild corrective action when the blood level of oxygen gets a little low or the carbon dioxide level gets a little high. It may also serve to expand the lungs more than the normal breathing pattern does. Anesthetized animals are often manually given deep sigh breaths periodically to keep their lungs well expanded. This is done to prevent the partial collapse of the lungs, which can occur in anesthetized animals as a result of respiratory system depression caused by general anesthetic drugs. Hiccups are spasmodic contractions of the diaphragm accompanied by sudden closure of the glottis, causing the characteristic "hiccup" sound. Although hiccups can result from serious conditions such as nerve irritation, indigestion, and central nervous system damage, most of the time they are harmless and temporary and occur for unknown reasons.

Describe how the mechanical respiratory control system maintains a normal, rhythmic, resting breathing pattern.

Breathing is controlled by the medulla oblongata of the brain stem in an area called the respiratory center. Within the respiratory center are individual control centers for functions such as inspiration, expiration, and breath-holding that receive input from stretch receptors in the lungs that indicate when the lungs reach preset inflation and deflation limits. These centers send nerve impulses out to the respiratory muscles at a subconscious level, directing when and how much they contract. Therefore, the voluntary respiratory muscles are controlled by nerve impulses from a subconscious part of the brain.

What is the difference between breathing and respiration?

Breathing, also known as ventilation, is the process of drawing air into the lungs and blowing it back out again. Respiration is the process of exchanging oxygen and carbon dioxide.

What is the difference between internal respiration and external respiration? Which one occurs in the lungs?

External respiration occurs in the lungs. It is the exchange of oxygen and carbon dioxide between the air inhaled into the lungs and the blood flowing through the pulmonary (lung) capillaries. Internal respiration, on the other hand, occurs everywhere in the body. It is the exchange of oxygen and carbon dioxide between the blood in the capillaries all over the body (the systemic capillaries) and all of the cells and tissues of the body. Internal respiration is the real "business end" of respiration. It is the means by which the body's cells receive the oxygen they need and get rid of their waste (carbon dioxide). Without external respiration, however, there would be no oxygen in the blood for the cells to absorb and no way for the cells to dump the carbon dioxide.

What are the main muscles of expiration? How do they push air out of the lungs?

Internal intercostal muscles and abdominal muscles. The fibers of the internal intercostals run deep and at right angles to the external intercostal muscles. When they contract, they rotate the ribs backward, which decreases the size of the thorax and helps push air out of the lungs. When abdominal muscles contract, they push the abdominal organs against the caudal surface of the diaphragm. This pushes the diaphragm back into its full dome shape and also decreases the size of the thorax.

What is the primary function of the respiratory system?

Its primary function is to bring oxygen (O2) into the body and carry carbon dioxide (CO2) out of it.

Which main pulmonary blood vessel contains bright red, high-oxygen blood: the pulmonary artery or the pulmonary vein? Why?

Pulmonary vein

What are secondary functions of the respiratory system?

Secondary functions include voice production, body temperature regulation, acid-base balance regulation, and the sense of smell.

How is the larynx involved in the straining process that aids functions such as defecation?

Straining begins with the animal holding the glottis closed while applying pressure to the thorax with the breathing muscles. This stabilizes the thorax and allows the abdominal muscles to effectively compress the abdominal organs when they contract. Without the closed glottis, contraction of the abdominal muscles merely forces air out of the lungs (exhalation).

How do the physical characteristics of the alveoli and the capillaries that surround them facilitate the exchange of gases between the air in the alveoli and the blood in the capillaries?

Structurally, the alveoli are tiny, thin-walled sacs that are surrounded by networks of capillaries. The wall of each alveolus is composed of the thinnest epithelium in the body—simple squamous epithelium. The capillaries that surround the alveoli are also composed of simple squamous epithelium. The main physical barriers between the air in the alveoli and the blood in the capillaries are the very thin epithelium of the alveolus and the adjacent, equally thin epithelium of the capillaries. These two thin layers allow oxygen and carbon dioxide to freely diffuse back and forth between the air and the blood. Each alveolus is lined with a thin layer of fluid that contains a substance called surfactant. Surfactant helps reduce the surface tension (the attraction of water molecules to each other) of the fluid. This prevents the alveoli from collapsing as air moves in and out during breathing.

When a piece of lung from a dead newborn animal is dropped into water, it sinks. What conclusion can be drawn about whether the newborn animal was born dead and never breathed or took some breaths before dying?

The animal was born dead. If it had been born alive and had breathed, the lung piece would have floated.

What is the hilus of the lung and why is it important?

The hilus is a small, well-defined area on the medial side of each lung where air, blood, lymph, and nerves enter and leave the lung. It is the only area of the lung that is "fastened in place." The rest of the lung is free within the thorax.

Why are the hyaline cartilage rings important to the function of the trachea?

The hyaline cartilage rings prevent the trachea from collapsing each time the animal inhales

What is the basic difference between the functions of the mechanical and chemical respiratory control systems?

The mechanical system sets routine inspiration and expiration limits, and the chemical system monitors the levels of certain substances in the blood and directs adjustments in breathing if they get out of balance.

What is the mediastinum and what organs and structures are located there?

The mediastinum is the area between the lungs that contains most of the rest of the thoracic contents: the heart, large blood vessels, nerves, trachea, esophagus, and lymphatic vessels and lymph nodes.

How do the pharynx and larynx work together to keep swallowed material from entering the trachea? What role does the epiglottis play in that process?

The pharynx and larynx work together to prevent swallowing from interfering with breathing, and vice versa. The seemingly simple act of swallowing actually involves a complex series of actions that stops the process of breathing, covers the opening into the larynx, moves the material to be swallowed to the rear of the pharynx, opens the esophagus, and moves the material into it. Once swallowing is complete, the opening of the larynx is uncovered and breathing resumes. The epiglottis is the structure that temporarily covers the opening to the larynx so that food can pass over it and into the rear of the pharynx and esophagus.

Why is negative intrathoracic pressure important to breathing? What happens if it is lost?

The pressure within the thorax is negative with respect to atmospheric pressure and creates a partial vacuum within the thorax. That partial vacuum pulls the lungs tightly out against the thoracic wall. The soft, flexible nature of the lungs allows them to conform closely to the shape of the inside of the thoracic wall. Pleural fluid between the lungs and the thoracic wall provides lubrication. As the thoracic wall goes, so go the lungs. The lungs follow passively as movements of the thoracic wall and diaphragm alternately enlarge and reduce the volume of the thorax. The whole system functions like a bellows as it pulls air into the lungs (inspiration) and blows it back out (expiration). Without this negative pressure, the lungs would collapse and not be able to fill. In addition, blood return to the heart would be decreased.

Describe how the respiratory and digestive passageways "switch places" in the pharynx.

The respiratory passageway (nasal passage) starts out dorsal to the digestive passageway (mouth) rostrally, but further caudally, the respiratory passageway (larynx) is ventral to the digestive passageway (esophagus).

By what mechanisms is inhaled air warmed, humidified, and filtered as it passes through the nasal passages? How do the turbinates aid these processes?

The scroll-like twists and turns of the turbinates tremendously increase the surface area of the nasal lining. They allow it to function as a combination radiator and humidifier. The air is warmed by the blood flowing through the complex of blood vessels just beneath the nasal epithelium and humidified by the mucus and other fluids that lie on the epithelial surface. The filtering function of the nasal passages helps remove particulate matter, such as dust and pollen, from the inhaled air before it reaches the lungs. The filtering mechanism relies on the many twists and turns of the nasal passages produced by the turbinates, the mucous layer on the surface of the nasal epithelium, and the cilia that project up into it. Air easily passes along the tortuous path of the nasal lining as it is inhaled, but particles of dust and other debris do not negotiate the twists and turns as readily and become trapped in the mucous layer. The beating of the cilia "sweeps" the mucus and the trapped foreign material back to the pharynx, where it is swallowed.

Describe the basic structure of the bronchial tree in the lung

The trachea bifurcates to form two main bronchi. Each main bronchus enters the lung and divides into some fairly large branches, which divide into smaller and smaller branches called bronchioles. Bronchioles continue to subdivide down to the smallest air passageways, the microscopic alveolar ducts. These ducts end in groups of alveoli arranged like bunches of grapes, the alveolar sacs

Why are the smooth pleural surfaces important to the process of breathing?

These surfaces are lubricated with pleural fluid and ensure that the surfaces of the organs, particularly the lungs, slide smoothly along the lining of the thorax during breathing.

Describe the basic processes by which oxygen moves from the air in the alveoli into the blood in the alveolar capillaries and how carbon dioxide moves in the other direction.

When that air is inhaled down into the alveoli of the lungs, it is only a couple of thin epithelial layers away from the blood in the surrounding capillaries. That alveolar capillary blood contains very little oxygen but a high level of carbon dioxide because it gave up its oxygen to the body's cells and picked up their carbon dioxide as it flowed through the systemic circulation. As this low-oxygen, high-carbon-dioxide blood circulates right next to an alveolus containing high-oxygen, low-carbon-dioxide air, oxygen diffuses from the alveolar air (area of high concentration) into the blood of the alveolar capillary (area of low concentration). At the same time, carbon dioxide diffuses from the blood (area of high concentration) into the alveolus (area of low concentration). The differences in the concentrations of the gases (the concentration gradient) stay fairly constant because as the blood picks up oxygen and dumps carbon dioxide, it flows away and is replaced by more low-oxygen, high-carbon-dioxide blood. At the same time, the air in the alveoli is refreshed with each breath.

When does the chemical respiratory control system kick in and override the mechanical control system?

Chemical receptors in blood vessels (the carotid and aortic bodies located in the carotid artery and aorta, respectively) and in the brain stem constantly monitor various physical and chemical characteristics of the blood. Three characteristics important to the control of the breathing process are (1) the CO2 content, (2) the pH, and (3) the O2 content of the arterial blood. If any of these varies outside preset limits, the chemical control system signals the respiratory center to modify the breathing process to bring the errant level back into balance. If the chemical control system detects a rise in the blood level of CO2 and a decrease in the blood pH (blood becomes acidic), it signals the respiratory center to increase the rate and depth of respiration so that more CO2 can be eliminated from the lungs. If the CO2 level falls too low, which is usually accompanied by a rise in the blood pH level, the opposite occurs (that is, respiration is decreased to allow the CO2 level to rise back into the normal range). The effects of variations in the blood O2 level are not as clear cut as the CO2 effects. If a slight decrease in the blood O2 level (hypoxia) occurs, the chemical control system signals the respiratory center to increase the rate and depth of breathing so that more O2 will be taken in. If, however, the blood O2 level drops below a critical level, the neurons of the respiratory center can become so depressed from the hypoxia (lack of oxygen) that they cannot send adequate nerve impulses to the respiratory muscles. This can cause breathing to decrease or stop completely.

What are the main muscles of inspiration? How do they cause air to be drawn into the lungs?

Diaphragm and external intercostal muscles. When the diaphragm contracts (flattens), the thoracic cavity enlarges and the lungs expand. The fibers of the external intercostal muscles are oriented obliquely so that when they contract, they rotate the ribs upward and forward, thus increasing the size of the thoracic cavity.