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PUBH 3100 Human Disease and Prevention

Week 2 – The Human Body – How It Works: The Respiratory System

(ON-SCREEN GRAPHIC ---Films for the Humanities & Sciences)

(ON-SCREEN GRAPHIC ---The Human Body – How It Works: The Respiratory System)

[MUSIC PLAYING]

NARRATOR: Oxygen is vital to all human beings and breathing the air around us usually supplies our bodies with the oxygen we need. But as altitude increases, the atmosphere gets thinner, breathing becomes more difficult. Mountain climbers, passengers and crews on airliners, and those venturing out into space, all must bring their own oxygen supplies or suffer the effects of oxygen deprivation. Why do we breathe? How do we breathe? And why is thin air so physiologically challenging? You'll find the answers to these and other questions as we explore the human respiratory system.

(ON-SCREEN GRAPHIC ---Why Do We Breathe?)

Why is oxygen so important to our bodies?

(ON-SCREEN GRAPHIC ---Timothy Shaw, PhD – Professor of Human Anatomy Bethel University)

TIMOTHY SHAW: Respiration is a process that involves the exchange of gases between the air around us and the cells within our body. Oxygen is required for cellular respiration. This is the process where food molecules, like glucose, are broken down in the presence of oxygen to produce a molecule called adenosinetriphosphate, also known as ATP. ATP is the form of energy that can be used by our cells. In the process of creating ATP, the waste products of carbon dioxide; water and heat are formed. If oxygen isn't available, ATP has difficulty being produced. So one of the main functions of the respiratory system is to provide adequate oxygen, so cells can make ATP and fuel their various functions.

NARRATOR: Certain tissues in the body have greater oxygen demands than others. For example, the brain is very sensitive to oxygen deprivation and will quickly suffer diminished function. On the other hand, muscle tissue has a relatively high tolerance for oxygen deficiencies. In fact, active muscle tissue can continue to function during temporary oxygen deficits due to a process called lactic acid fermentation.

TIMOTHY SHAW: The lactic acid fermentation process provides an additional source of ATP to support muscle work without the need for oxygen. But, producing ATP in this way comes at a price. The accumulation of lactic acid occurs. This is a natural byproduct of the process of producing ATP without oxygen. This accumulation of lactic acid results in muscle soreness and pain and muscle fatigue.

(ON-SCREEN GRAPHIC ---Anatomy of the Respiratory System)

NARRATOR: The respiratory system can be divided into three main regions based on function; the upper respiratory tract where air enters and leaves, the lower respiratory tract that conducts air to and from the alveoli of the lungs, and the alveoli themselves, the tiny air sacs where gas exchange occurs. When someone breathes in, air enters the upper respiratory tract. Air is directed to the nasal cavity, where it is warmed and humidified before going to the lungs.

The nasal cavity is lined with a mucous membrane. The mucous produced in this membrane trap smaller particles that come in with the air. Then, hair-like projections, called cilia, move the particle-laden mucus toward the throat, where it is swallowed. The nasal cavity is surrounded by sinuses, which are also lined with mucous membranes. Mucous produced by the sinuses drains into the nasal cavity.

We can breathe through both our nose and mouth. In either case, air is directed into the pharynx, or throat, which divides at its lower extent, allowing food and drink to pass to the esophagus and stomach, and inhaled air to be directed to the larynx, or voice box and then on to the trachea, or windpipe. The epiglottis, a small flap of tissue, prevents foods and liquids headed for the esophagus from entering the larynx.

The trachea, like the sinus and nasal cavities, is lined with mucous-producing cells and ciliated epithelial cells that push particle-laden mucous upward to the pharynx. From there, it's swallowed and digested in the stomach. The lower respiratory system begins where the trachea splits into the right and left primary bronchi that supply air to the right and left lungs. Within a short distance, the left primary bronchus splits into two secondary branches, while the right splits into three.

TIMOTHY SHAW: The difference in the number of secondary branches on the right and the left side is due to the fact the right lung has three lobes, while the left lung has only two lobes. This is because the heart is placed slightly to the left within the chest cavity, and the left lung is slightly smaller to make room for the heart.

NARRATOR: A thin two-layer membrane, called the pleura, completely surrounds both lungs. The inner layer is called the visceral pleural membrane, and is in contact with the outer surface of the lung. The outer parietal pleural membrane lines the inner wall of

the thoracic cavity. A thin space is formed between these two layers, called the pleural cavity. The two pleural membranes secrete a lubricating fluid into the pleural cavity that helps reduce friction during breathing.

The bronchi continue to branch into smaller and smaller tubes or branches, known as bronchioles. The final series of bronchioles ends at the alveolar ducts, which lead to the microscopic air sacs, called alveoli. The alveoli are referred to as the lung's surface, and they are the site for the exchange of respiratory gases between the air and the blood. The outer surface of each alveolar sac is covered with pulmonary capillaries, the tiny vessels that will pick up oxygen and deliver carbon dioxide to the alveoli.

The distance between the air in the alveoli and the blood in the capillaries is very short. To travel this distance, oxygen molecules pass through the wall of the alveolus through the interstitium, the fluid-filled space between cells, and finally through the wall of the capillary. Carbon dioxide travels the same distance in reverse.

(ON-SCREEN GRAPHIC ---The Diffusion of Gas Molecules)

NARRATOR: Diffusion is the process that allows these respiration gases to be exchanged through the walls of the alveoli and the capillaries. But what is diffusion? And what makes it work in the lungs?

TIMOTHY SHAW: Diffusion is, very simply, the process by which the molecules move randomly and spontaneously from a region of higher concentration to one of lower concentration until equilibrium has been attained. At that point, diffusion causes no more net movement of the gases. For diffusion to be effective in the lungs, a difference must exist in the partial pressures of oxygen and carbon dioxide between the blood within the capillary and the air within the alveoli.

NARRATOR: In the lungs, diffusion of oxygen is always from the alveoli to the blood, because the partial pressure of oxygen in the alveoli is greater than the partial pressure of oxygen in the blood. The reverse is true for carbon dioxide. But there are several factors that affect the rate of oxygen diffusion into the blood. According to Fick's Law, an increase in the surface area for gas exchange will cause an increase in the rate of oxygen diffusion. So, for example, a person with a damaged lung or damaged lung tissue has a reduced surface area for gas exchange. Because of that, this person also has a reduced rate for oxygen uptake.

Fick's Law also allows us to quantify the impact of high altitude on oxygen uptake. At sea level, the partial pressure of oxygen is about 120 millimeters of mercury. At the top of the highest mountains, it can be reduced to less than 20. This large drop in pressure reduces the rate at which oxygen enters the blood.

TIMOTHY SHAW: Another factor affecting oxygen diffusion is the thickness of the diffusion barrier, in other words, the distance that oxygen molecules must travel from the alveoli to the blood capillaries. For example, in a condition called pulmonary edema, fluid collects within the alveoli, but also within the interstitium of the respiratory membrane. This increases the distance between the wall of the alveolus and the wall the capillary. This increased distance means that oxygen molecules must travel further to reach the blood from the alveolus, and slows down the diffusion rate. Individuals with pulmonary edema cannot take up oxygen as efficiently.

(ON-SCREEN GRAPHIC ---How Do We Breathe?)

NARRATOR: Air moves in and out of the lungs by a transport process known as bulk flow.

TIMOTHY SHAW: The term, bulk flow, describes how fluid-like substances, like the air, move from areas of higher concentration to areas of lower concentration. In trying to understand how air moves in and out the lungs, this is a kind of a convenient device here. The orange balloons are to illustrate the lungs. The bell jar is to illustrate the pleural cavity that surrounds the lungs within the thorax. And this rubberized membrane is to represent our diaphragm.

Because volume and pressure are inversely related, increasing volume decreases pressure. So when the diaphragm is lowered, we lower the pressure around the lungs. This functions to pull the walls of the lungs. When the walls of lungs are pulled out, the pressure inside the lungs decreases, and air fills them.

When the diaphragm is elevated, this is when we exhale. The volume around the lungs decreases and the pressure increases. Without this decreased pressure around the lungs, they are no longer pulled out, and the intrinsic elasticity within them closes them, decreasing the volume in the lung, forcing the air to move out of them.

NARRATOR: This relationship between a volume of gas and its pressure is described by Boyle's Law, "When temperatures is constant, the pressure of a gas changes inversely according to its volume." In other words, when the volume of the container holding the gas increases, the pressure of the gas within the container decreases.

To take a breath, the lung volume must increase. This is achieved by the diaphragm, the muscular bottom of the chest cavity, and the intercostal muscles located between the ribs. Here's how the process works. Contraction of the diaphragm causes it to push downward, pulling the lung tissue with it. Contraction of the external intercostal muscles causes the rib cage to expand upward and outward, also pulling the lung tissue

with it. Both of these actions serve to increase lung volume, with a corresponding decrease in pressure inside the lung.

Consequently, outside air flows into the lung. Once air pressure in the lungs equals the outside pressure, air flow into the lungs stops. Relaxation causes the diaphragm and intercostal muscles to return to their original positions, decreasing the volume of the lung. As a result, air flows out of the lung. Air that remains in the lungs after exhaling is called dead air.

TIMOTHY SHAW: Incoming fresh air, with its higher oxygen content, mixes with what we would call the dead air, that's been left in the lungs and has a lower oxygen content. According to Fick's Law, the resulting decrease in the partial pressure of oxygen in the dead air and fresh air mix, also reduces the rate of oxygen diffusion into the blood.

NARRATOR: Often during intense exercise, when the body needs more oxygen, another set of muscles, called the internal intercostals, are activated. Contraction of these muscles decreases the chest volume further, forcing more dead air out of the lungs. This allows for a greater volume of new oxygen laden air to enter.

(ON-SCREEN GRAPHIC ---Preventing Lung Collapse)

NARRATOR: For the lungs to work properly, it's important to keep them inflated. The dead air remaining after we exhale helps to do that. But there are other factors that keep our lungs inflated. Remember the pleura and the fluid-filled pleural cavity? The pressure within the pleural cavity is at about four millimeters of mercury below that of the atmosphere. Between breaths, the negative pressure here prevents the elastic lung tissue from completely collapsing in on itself.

TIMOTHY SHAW: If the pleural cavity is punctured from the outside, air from outside of the body will be pulled into the pleural cavity when one tries to inhale. This will equalize the pressure between the two areas. When the pressure is equalized, it will prevent a decrease in pressure surrounding the lungs, which is necessary to cause the lungs to inflate. When this occurs, the situation severely compromises as a person's ability to breathe.

NARRATOR: Another factor that prevents lung collapse is the production of surfactants on the inner surface of the alveoli. Whenever water and air are in contact, water molecules are arranged in a pattern that creates tension at the water's surface. This is the same surface tension that allows water striders to walk on the surface of a pond or stream.

TIMOTHY SHAW: Within the lungs, the inner surface of the alveoli is moist and in contact with air that fills the lungs. As the alveoli contract, like they do when the lungs deflate, surface tension increases in the walls of the alveoli, and this tends to pull the alveolar walls towards collapse. To counter this tendency, the alveoli have type II alveolar cells, which secrete molecules, called surfactants. Surfactants coat the alveolar surfaces and physically disrupt the arrangement of the water molecules. This reduces the surface tension and prevents the alveoli from collapsing.

(ON-SCREEN GRAPHIC ---Meeting Changing Oxygen Demands)

NARRATOR: The medulla oblongata of the brain stem controls our breathing through inspiratory and expiratory neurons. These neurons extend down the spinal cord and connect with other neurons that carry signals to the diaphragm and intercostal muscles. When the inspiratory neurons fire, they stimulate the diaphragm and external intercostal muscles to contract, so we inhale. When the inspiratory neurons stop firing, these muscles relax, and we exhale.

Typically, the expiratory neurons fire only when it's necessary to exhale more deeply, for example, during vigorous activity, as explained earlier. Together, the inspiratory and expiratory centers are responsible for our rhythmic breathing. There are a couple of other mechanisms that alert these centers to the need to increase breathing rates or volume. Peripheral chemoreceptors, located in the carotid arteries and aortic arch, are sensitive to the oxygen levels of the blood.

When oxygen levels drop, peripheral chemoreceptors fire, stimulating the inspiratory neurons of the medulla to increase the depth of breathing and the breathing rate.

TIMOTHY SHAW: Central chemoreceptors located in the brain stem perform a role similar to peripheral chemoreceptors. Only, in this case, they're sensitive to carbon dioxide. Carbon dioxide, of course, is a waste product of cellular respiration. When carbon dioxide levels increase, as they do during exercise, for example, the central chemoreceptors signal the brain stem to increase the breathing rate, and in so doing, eliminate the excess CO2.

NARRATOR: During exercise, skeletal muscles require more oxygen to make ATP, their source of energy. The respiratory system is designed to ensure that the blood flowing to those muscles is adequately saturated with oxygen to meet their increased needs. To initiate the respiratory response, the surface of the brain, called the cerebral cortex, responds to changes in joint and muscle activity. By relaying signals to the respiratory centers of the brain stem, the inspiratory center signals the respiratory muscles to increase both the depth of breathing and the breathing rate.

High altitude environments pose a particular challenge to both respiratory and circulatory functions. The reduced oxygen pressure immediately reduces oxygen levels in the blood, a condition called hypoxia. Symptoms include fatigue, lack of appetite, distorted vision, headache, confusion, nausea, and dizziness. After a day or two, most people begin to acclimate to the high altitude. But in some cases, high altitude exposure becomes life threatening.

TIMOTHY SHAW: High altitudes result in a specific kind of pulmonary edema, known as high altitude pulmonary edema, or HAPE. In this condition, fluid from the capillaries leaks into the interstitial spaces that separate the capillaries from the alveoli, and fills the alveoli themselves. As this occurs, it inhibits the ability of the alveoli to exchange oxygen. The effect is as if HAPE victims are actually beginning to drown in their own body fluids.

NARRATOR: Respiration is essential for life. Without it, our cells are deprived of the oxygen they need for cellular respiration. And without cellular respiration, our tissues can't perform their essential life functions.

TIMOTHY SHAW: For our respiratory health, as well as for the health of all the other body systems, the most important things we can do are to eat right, to exercise regularly, to get enough sleep, and most importantly to refuse the use of tobacco products and alcohol and other addictive drugs.

(ON-SCREEN GRAPHIC ---

Based on the Book Series The Human Body – How It Works

The Respiratory System Chelsea House Publishers

Executive Producer – Craig Claudin Producer/Director – Christine Dean

Script Development – Christine Dean Script Consultant – Timothy Shaw, PhD – Professor of Human Anatomy Bethel University

Narrator – Erin Mathe Videographers – Tim Lewis and Matt Bjur

Editor – Matt Bjur Animator – Chris Parrish-Taylor and Matt Bjur

Graphics – Matt Bjur Illustrations – Britta Bjur

Thanks to the staff and students of Thomas Jefferson High School Bloomington, MN for

their assistance in producing this program series.

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ALL RIGHTS RESERVED