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THE RESPIRATORY SYSTEM
The function of the respiratory system is to bring about the exchange of
O2 and CO2 . This is called external respiration. Cellular respiration is
the production of ATP from glucose and O2 in the mitochondria of a cell.
CO2 and H2O are released as waste products. External respiration
provides the O2 needed for cellular respiration to occur!!!
I. THE RESPIRATORY PASSAGEWAY
A. Airway to the Lungs
Air normally enters the respiratory system through the mouth and
nasal passages, which begin the process of filtering, warming, and
moistening the air.
From there, the air passes through the pharynx
(upper throat) and the larynx. The larynx contains the vocal cords,
bands of connective tissue that tighten and vibrate to create sound
when air passes through.
Attached to the larynx is a flap of tissue called the epiglottis. The
epiglottis closes when food is swallowed to prevent food from
entering the trachea.
After air passes over the larynx, it enters the trachea, a tube
supported by bands of cartilage to prevent it from collapsing when
air passes in and out.
The trachea divides at its lower end into two tubes called bronchi
(singular = bronchus). The trachea and bronchi are lined with cilia
and cells that secrete mucus. The mucus traps dirt and dust and the
cilia beat upward to push the dirt and dust toward the nasal
passages so that you can either cough or sneeze to get them out of
the respiratory tract.
B. Inside the Lungs
Inside each lung, the bronchi narrow as they branch into smaller
passageways called bronchioles. The bronchioles end in millions
of tiny sacs called alveoli. The alveoli are the site for the exchange
of O2 and CO2. Each alveolus is surrounded by a capillary to allow
O2 to diffuse from the lung to the blood to be delivered to cells for
cellular respiration, and CO2 to diffuse from the blood to the lung to
be exhaled.
II. BREATHING
A. Inhalation & Exhalation
The lungs are surrounded by the pleural membrane and hang
freely in the thoracic cavity. There are no muscles attached to
the lungs. The muscles involved in breathing are the intercostal
muscles, located between the costas and the diaphragm, a
dome-shaped muscle located below (but not attached to!) the lungs.
Breathing occurs as a result of a change in pressure. When the
diaphragm contracts, it flattens which increases the volume of
the chest cavity and decreases the pressure. In addition, the
intercostal muscles contract, further increasing the volume of the
chest cavity. Air rushes into the lungs. As the diaphragm and
intercostal muscles relax, volume of the chest cavity decreases,
and the increased pressure of the lungs help force the air back out.
B. Control of Breathing
Breathing is controlled by the medulla oblongata. Motor neurons
stimulate the skeletal muscle of the diaphragm to contract. The
stimulus for breathing is the concentration of CO2 in the blood,
which is monitored by the hypothalamus.