Respiratory System—Physiology of the Lungs
Lecture 6
We’re back today to talk about the physiology of the respiratory system
and see what really happens now once the air is into the lungs, how it gets
there, and how chemically we’re going to get our gases exchanged.
I
n our continuing study of the respiratory system, we examine the four
areas in which respiration occurs and the consequences of blocking this
respiration (i.e., suffocation). Next, we review the physiology of the
movement of gases into the lungs and of their exchange with waste gases in
the blood at the level of the alveoli. Finally, we turn to the central respiratory
centers that control the pace of respiration, and we examine some common
respiratory disorders and their treatment.
The larynx, the trachea, the bronchi, and their smaller divisions perform
Lecture 6: Respiratory System—Physiology of the Lungs
pulmonary ventilation, which is simply the movement of air. The bronchioles
and alveoli perform pulmonary respiration, which is the exchange of gases.
Four areas of respiration
In pulmonary ventilation, room (or outside) air is taken into the
body. Room air is about 80% nitrogen and 20% oxygen, with
trace gases.
External pulmonary respiration involves the exchange of gases
within the lungs, in the blood of the capillaries of the alveoli.
Internal (tissue) respiration involves the exchange of gases between
cells and blood in the capillaries.
Cellular respiration involves metabolic processes within the cell.
Inspiration is taking air into the lungs; expiration is pushing air out of the
lungs. The term expiration also means “to die.”
32
, Movement of gases
When the mouth and nose are open, the air pressure in the lungs equals the
atmospheric pressure. The pressure of the space between the visceral and
parietal pleura is slightly less than the atmospheric pressure. Inspiration
contracts the diaphragm and increases the diameter of the thoracic cavity,
decreasing the pressure between the lung and the parietal pleura. The
lowered pressure of the pleural space draws air into the lungs. On expiration,
relaxation of the diaphragm increases the pressure in the pleural space and
expels air from the lungs. Expansion and contraction of the exible ribs and
chest aid inspiration and expiration.
The accessory muscles of respiration aid in times of pulmonary stress, such
as during exercise.
The external intercostal muscles connect the ribs and the collarbone,
pulling the ribs up during respiration.
The sternocleidomastoid muscle helps raise the rst rib, sternum,
and collarbone by pulling against the head.
The scalene muscles also pull the ribs up against the collarbone.
The rectus abdominus and internal intercostal muscles pull the ribs
down, aiding expiration.
Alveolar capillary air exchange
This exchange is an example of pulmonary (external) respiration. It involves
the exchange of gases between the air in the alveoli and blood in the
pulmonary capillaries. The movement of this exchange is dictated by gas
laws, which say that gases move from areas of high pressure to areas of low
pressure. Because the partial pressure of oxygen is higher in the lungs than in
the alveoli or capillaries, oxygen pushes into the capillaries and is captured
by hemoglobin in the blood. Because the partial pressure of carbon dioxide
is higher in the capillaries than in the lungs, carbon dioxide pushes through
the alveoli into the lungs. Carbon dioxide also comes from buffer systems in
plasma (not only from red cells) that rid the body of excess acid.
33
Lecture 6
We’re back today to talk about the physiology of the respiratory system
and see what really happens now once the air is into the lungs, how it gets
there, and how chemically we’re going to get our gases exchanged.
I
n our continuing study of the respiratory system, we examine the four
areas in which respiration occurs and the consequences of blocking this
respiration (i.e., suffocation). Next, we review the physiology of the
movement of gases into the lungs and of their exchange with waste gases in
the blood at the level of the alveoli. Finally, we turn to the central respiratory
centers that control the pace of respiration, and we examine some common
respiratory disorders and their treatment.
The larynx, the trachea, the bronchi, and their smaller divisions perform
Lecture 6: Respiratory System—Physiology of the Lungs
pulmonary ventilation, which is simply the movement of air. The bronchioles
and alveoli perform pulmonary respiration, which is the exchange of gases.
Four areas of respiration
In pulmonary ventilation, room (or outside) air is taken into the
body. Room air is about 80% nitrogen and 20% oxygen, with
trace gases.
External pulmonary respiration involves the exchange of gases
within the lungs, in the blood of the capillaries of the alveoli.
Internal (tissue) respiration involves the exchange of gases between
cells and blood in the capillaries.
Cellular respiration involves metabolic processes within the cell.
Inspiration is taking air into the lungs; expiration is pushing air out of the
lungs. The term expiration also means “to die.”
32
, Movement of gases
When the mouth and nose are open, the air pressure in the lungs equals the
atmospheric pressure. The pressure of the space between the visceral and
parietal pleura is slightly less than the atmospheric pressure. Inspiration
contracts the diaphragm and increases the diameter of the thoracic cavity,
decreasing the pressure between the lung and the parietal pleura. The
lowered pressure of the pleural space draws air into the lungs. On expiration,
relaxation of the diaphragm increases the pressure in the pleural space and
expels air from the lungs. Expansion and contraction of the exible ribs and
chest aid inspiration and expiration.
The accessory muscles of respiration aid in times of pulmonary stress, such
as during exercise.
The external intercostal muscles connect the ribs and the collarbone,
pulling the ribs up during respiration.
The sternocleidomastoid muscle helps raise the rst rib, sternum,
and collarbone by pulling against the head.
The scalene muscles also pull the ribs up against the collarbone.
The rectus abdominus and internal intercostal muscles pull the ribs
down, aiding expiration.
Alveolar capillary air exchange
This exchange is an example of pulmonary (external) respiration. It involves
the exchange of gases between the air in the alveoli and blood in the
pulmonary capillaries. The movement of this exchange is dictated by gas
laws, which say that gases move from areas of high pressure to areas of low
pressure. Because the partial pressure of oxygen is higher in the lungs than in
the alveoli or capillaries, oxygen pushes into the capillaries and is captured
by hemoglobin in the blood. Because the partial pressure of carbon dioxide
is higher in the capillaries than in the lungs, carbon dioxide pushes through
the alveoli into the lungs. Carbon dioxide also comes from buffer systems in
plasma (not only from red cells) that rid the body of excess acid.
33
