Lesson 2

The Respiratory System

How do mountain climbers, astronauts, and undersea

explorers survive when they are in environments

where there is little or no oxygen to breathe?

Your respiratory system is made up of a pair of lungs,

a series of passageways into your body and a thin sheet

of skeletal muscles called the diaphragm. When you hear

the word respiration, you probably think of breathing. Breathing

means taking air into and out of the lungs. Breathing allows the

blood to absorb oxygen and dispose of carbon dioxide.

Respiration includes all the mechanisms involved in getting

oxygen to the cells of your body and giving off carbon

dioxide as a waste product.

Let us find out how the air that you breathe enters your

body through the different organs of the respiratory system.

The human respiratory system consists of the lungs and the system of air tubes that carry air to and from the lungs. Lungs are the most important organs for gas exchange in land-breathing animals. Each lung consists of small chambers, or air sacs, surrounded by capillaries. These air sacs provide a huge respiratory surface for the diffusion of oxygen and carbon dioxide into and out of the blood.

Let us study the parts of the human respiratory system and their functions.

The human respiratory system is composed of the upper and lower respiratory tract.

Upper Respiratory Tract

nasal cavity

The Nose

The nose is the gateway of the respiratory system.

Air normally enters the respiratory system through

small openings called nostrils, which lead into

hollow spaces in the nose called the nasal cavity.

Hairs called cilia, lining the nasal cavity prevent

the entrance of foreign particles. The walls of the

nasal passages, like the rest of the air passageways

in the respiratory system, are lined with mucous

membranes made up mainly of ciliated epithelial cells.

Some epithelial cells secrete mucus, a sticky fluid that

traps bacteria, dust, and other particles in the air we breathe in.

The PharYnx mucus also moistens the air that we breathe.

Just below the mucous membrane is a rich supply of capillaries.

As air passes through the nose, it is warmed

by the blood in these capillaries. Thus, the nasal

passages serve to filter, moisten, and warm inhaled

air before it reaches the delicate lining of the lungs.

Although you can breathe through your mouth,

you lose these advantages if you do not regularly breathe through your nose. Breathing through ■ our mouth can happen when you have a severe cold.

Pharynx

From the nasal cavity, air passes into the pharynx, or throat, which is located behind the mouth cavity. The adenoids and tonsils are lymphoid tissues found in the throat. They are part of the body’s defense against infection. The pharynx is the common passageway for food and air. It acts like a station where the food tube and the air tube meet. Food that is being swallowed is prevented or blocked from entering the air tube by a thin flap of muscle, called the epiglottis, that closes the air tube. This is the reason why you cannot breathe while you are swallowing.

Lower Respiratory Tract

Larynx -

From the pharynx, air passes into the
larynx, or voice box, which is made up largely
of cartilage. The larynx is located at the upper
end of the trachea, which is the air tube
leading to the lungs. The vocal cords are two
pairs of membranes that are stretched across
the interior of the larynx. As air is exhaled,

vibrations of the vocal cords can be controlled to make sounds.
Another function of the larynx is to prevent choking. The
elongated space between the vocal cords is called the glottis.

Trachea

The larynx Is connected to the r?z c he a
or windpipe. The trachea is a tube about
12 centimeters long and 1 5 cer.::me:ers
wide. The trachea is kept open by rings
of horseshoe-shaped cartilage embedded
in its walls. Like the nasal passages, the
trachea is also lined with cilia and mucous
membrane. Normally, the cilia trap foreign
matters and the mucus secreted by the
mucous membrane binds them together. All
together, the cilia and mucous membrane
filter and purify the air that we breathe

Lungs

The lungs occupy a large portion of the chest cavity. The lungs, together with the
heart, are enclosed and protected within the ribcage. The chest cavity is separated
from the abdominal cavity by a sheet of smooth muscle called the diaphragm. Thus,
the diaphragm forms the floor of the chest cavity. Each lung is enclosed by a two-
layered membrane called the pleura. One layer of the pleura closely covers each
lung, while the other layer lines the entire wall of the chest cavity. A lubricating
fluid, called pleural fluid, between these layers allows the lungs to expand freely in

the chest cavity during breathing.

Near the center of the chest, the trachea divides into two smaller cartilage- ringed tubes called bronchi. The bronchi enter the lungs and then subdivide into smaller tubes forming a tree-like structure called the bronchial tree. The bronchi allow the lungs to expand and contract during the breathing process.

As the bronchial tubes divide and subdivide, their walls become thinner, and they gradually lose their cartilage support, becoming a network of microscopic tubes called bronchioles.

At the end of each bronchiole is a cluster of tiny sacs called alveoli. A cluster of alveoli forms the air sac, the functional unit of the lungs.

An air sac resembles a cluster of grapes. Each air sac contains several cupshaped cavities called alveoli. The walls of the alveoli, which are only one cell thick, are the respiratory surface of the lungs. They are thin and moist and are surrounded by a rich network of capillaries. It is through these walls that the exchange of oxygen and carbon dioxide between blood and air occurs.

Diaphragm

The diaphragm is a large sheet of smooth muscle below the ribs. It controls the process of breathing. The diaphragm moves up and down during the breathing process. The diaphragm is a smooth muscle, which means its contraction is not controlled by will, or is involuntary.

Breathing refers to the mechanical process of inhaling and exhaling air.

Inhaling is the taking in of air. During inhalation, the diaphragm contracts and becomes smaller so the chest cavity expands. As the chest expands, there is less air pressure in the lungs so air rushes in, filling and enlarging the lungs.

Exhaling is expelling used air from the lungs. During exhalation, the diaphragm moves upward while the rib cage moves downward so it goes back to its dome-shaped position. The chest cavity shrinks. As the chest shrinks, there is high air pressure in the lungs, which pushes air out of the lungs.

Exchange of Gases in Your Lungs

Each of your lungs is made up of millions of tiny air sacs. These air sacs look like bunches of grapes. The wall of each air sac is very thin and has a net of tiny blood vessels around it.

When you inhale, oxygen in the air enters the air sacs and then diffuses through the walls of the blood vessels. The oxygen is now in your blood. At the very same time, carbon dioxide, the “waste” gas carried by the blood, diffuses through the blood vessel walls and enters the air sacs. The carbon dioxide in your blood is removed and exchanged for the oxygen in your air sacs.

Your blood then takes the oxygen to the different cells composing all parts of your body. After supplying the cells with the needed oxygen, the blood then collects the carbon dioxide that is created by your cells into the air sacs. The carbon dioxide is expelled from your body when you exhale.