Page 202 - 84 human physiolofy part-1

Chapter 11

increases. There are two phases of ventilation; inspiration and expiration. During each phase the body
changes the lung dimensions to produce a flow of air either in or out of the lungs.

The body is able to change the dimensions of the lungs because of the relationship of the lungs to
the thoracic wall. Each lung is completely enclosed in a sac called the pleural sac. 2 structures
contribute to the formation of this sac. The parietal pleura is attached to the thoracic wall where as the
visceral pleura is attached to the lung itself. In-between these two membranes is a thin layer of
intrapleural fluid. The intrapleural fluid completely surrounds the lungs and lubricates the two surfaces
so that they can slide across each other. Changing the pressure of this fluid also allows the lungs and
the thoracic wall to move together during normal breathing. Much the way two glass slides with water
in-between them are difficult to pull apart, such is the relationship of the lungs to the thoracic wall.

The rhythm of ventilation is also controlled by the "Respiratory Center" which is located largely in
the medulla oblongota of the brain stem. This is part of the autonomic system and as such is not
controlled voluntarily (one can increase or decrease breathing rate voluntarily, but that involves a
different part of the brain). While resting, the respiratory center sends out action potentials that travel
along the phrenic nerves into the diaphragm and the external intercostal muscles of the rib cage,
causing inhalation. Relaxed exhalation occurs between impulses when the muscles relax. Normal adults
have a breathing rate of 12-20 respirations per minute.

The Pathway of Air

When one breathes air in at sea level, the inhalation is composed of different gases. These gases
and their quantities are Oxygen which makes up 21%, Nitrogen which is 78%, Carbon Dioxide with
0.04% and others with significantly smaller portions.

In the process of breathing, air enters into the nasal cavity through the nostrils and is filtered by
coarse hairs (vibrissae) and mucous that are found there. The vibrissae filter macroparticles, which are
particles of large size. Dust, pollen, smoke, and fine particles are trapped in the mucous that lines the
nasal cavities (hollow spaces within the bones of the skull that warm, moisten, and filter the air). There
are three bony projections inside the nasal cavity. The superior, middle, and inferior nasal conchae.
Air passes between these chonchae via the nasal meatuses.

Air then travels past the nasopharynx, oropharynx, and laryngopharynx, which are the three
portions that make up the pharynx. The pharynx is a funnel-shaped tube that connects our nasal and
oral cavities to the larynx. The tonsils which are part of the lymphatic system, form a ring at the
connection of the oral cavity and the pharynx. Here, they protect against foreign invasion of antigens.
Therefore the respiratory tract aids the immune system through this protection. Then the air travels
through the larynx. The larynx closes at the epiglottis to prevent the passage of food or drink as a
protection to our trachea and lungs. The larynx is also our voicebox; it contains vocal cords, in which it
produces sound. Sound is produced from the vibration of the vocal cords when air passes through them.

The trachea, which is also known as our windpipe, has ciliated cells and mucous secreting cells
lining it, and is held open by C-shaped cartilage rings. One of its functions is similar to the larynx and
nasal cavity, by way of protection from dust and other particles. The dust will adhere to the sticky
mucous and the cilia helps propel it back up the trachea, to where it is either swallowed or coughed up.
The mucociliary escalator extends from the top of the trachea all the way down to the bronchioles,
which we will discuss later. Through the trachea, the air is now able to pass into the bronchi.

202 | Human Physiology