Lec.4
Medical Physiology
Z.H.Kamil
PHYSIOLOGY OF RESPIRATION
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
Once oxygen has diffused from the alveoli into the pulmonary blood, it is transported to
the peripheral tissue capillaries almost entirely in combination with hemoglobin. The
presence of hemoglobin in the red blood cells allows the blood to transport 30 to 100 times
as much oxygen as could be transported in the form of dissolved oxygen in the water of
the blood.
In the body’s tissue cells, oxygen reacts with various foodstuffs to form large quantities of
carbon dioxide. This carbon dioxide enters the tissue capillaries and is transported back to
the lungs. Carbon dioxide, like oxygen, also combines with chemical substances in the
blood that increase carbon dioxide transport 15- to 20-fold.
Diffusion of Oxygen from the Alveoli to the Pulmonary Capillary Blood
The pulmonary alveolus adjacent to a pulmonary capillary, demonstrating diffusion of
oxygen molecules between the alveolar air and the pulmonary blood. The Po2 of the
gaseous oxygen in the alveolus averages 104 mm Hg, whereas the Po2 of the venous blood
entering the pulmonary capillary at its arterial end averages only 40 mm Hg because a
large amount of oxygen was removed from this blood as it passed through the peripheral
tissues. Therefore, the initial pressure difference that causes oxygen to diffuse into the
pulmonary capillary is 104 – 40, or 64 mm Hg.
Transport of Carbon Dioxide in the Dissolved State.
A small portion of the carbon dioxide is transported in the dissolved state to the lungs.
Recall that the Pco2 of venous blood is 45 mm Hg and that of arterial blood is 40 mm Hg.
The amount of carbon dioxide dissolved in the fluid of the blood at 45 mm Hg is about 2.7
ml/dl (2.7 volumes per cent). The amount dissolved at 40 mm Hg is about 2.4 milliliters,
or a difference of 0.3 milliliter. Therefore, only about 0.3 milliliter of carbon dioxide is
transported in the dissolved form by each 100 milliliters of blood flow. This is about 7 per
cent of all the carbon dioxide normally transported.
Regulation or Control of Respiration
Neural Regulation: Setting the Basic Rhythm
The activity of the respiratory muscles, the diaphragm and external intercostals, is
regulated by nerve impulses transmitted to them from the brain by the phrenic and
intercostals nerves.
The neural centers that control respiratory rhythm and depth are located in medulla and
pons. The medulla, which sets the basic rhythm of breathing, contains a self-exciting
inspiratory center, as well as other respiratory centers. The pons center appear to smooth
out the basic rhythm of inspiration and expiration set by the medulla. Impulses going back
and forth between the pons and medulla centers maintain a rate of 12-15
repirations/minutes. This normal respiratory rate is referred to as eupnea.
In addition, the bronchioles and alveoli have stretch receptors that respond to extreme
overinflation (which might damage the lungs) by initiating protective reflexes. In the case
of overinflation, impulses are sent from the stretch receptors to the medulla by the vagus
nerves; soon thereafter inspiration ends and expiration occurs.
During exercise, we breathe more vigorously and deeply because the brain centers send
more impulses to the respiratory muscles. This respiratory pattern is called hyperpnea.
However, the rate of breathing may not be significantly increased with exercising. After
strenuous exercise, expiration becomes active, and the abdominal muscles and any other
muscles capable of lifting the ribs are used to aid expiration.
If the medullary centers are completely suppressed (as with an overdose of sleeping pills,
morphine, or alcohol), respiration stops completely, and death occurs.
Factors Influencing Respiratory Rate and Depth
Physical Factors
Although brain centers set the basic rhythm of breathing, there is no question that physical
factors such as talking, coughing, and exercising can modify both the rate and depth of
breathing.
Increased body temperature causes an increase in the rate of breathing.
Volition (Conscious Control)
We all have consciously controlled our breathing pattern at one time or another. During
singing and swallowing , breath control is extremely important, and many of us have held
our breath for short periods to swim underwater. However, voluntary control of breathing
is limited, and the respiratory centers will simply ignore messages from the cortex when
the oxygen supply in the blood is getting low or blood pH is falling.
Emotional Factors
Emotional factors also modify the rate and depth of breathing. Like watching a horror
movie with held breath or touching something cold and clammy and gasped. All of these
result from reflexes initiated by emotional stimuli acting through centers in the
hypothalamus.
Chemical Factors
Although many factors can modify respiratory rate and depth, the most important factors
are chemical~the level of carbon dioxide and oxygen in the blood. Increased levels of
carbon dioxide and decreased blood pH are the most important stimuli leading to an
increase in the rate and depth of breathing. Actually, an increase in carbon dioxide levels
and decreased blood pH are the same thing in this case because carbon dioxide retention
leads to increased levels of carbonic acid, which decrease the blood pH. Changes in carbon
dioxide concentrations in the blood seems to act directly on the medulla centers.
Conversely, changes in oxygen concentration in the blood are detected by chemoreceptor
regions in the aorta (aortic arch) and carotid artery (carotid body). These in turn, send
impulses to the medulla when blood oxygen levels are dropping. Although every cell in
the body must have oxygen to live, it is the body’s need to rid itself of carbon dioxide (not
to take in oxygen) that is the most important stimulus for breathing in a healthy person.
Decreases in oxygen levels only become important stimuli when they are dangerously low.
Homeostatic mechanisms of the respiratory system, in healthy individuals, are obvious. As
carbon dioxide or other sources of acids begin to accumulate in blood and blood pH starts
to drop, you begin to breathe more deeply and more rapidly. This breathing pattern is
called hyperventilation.
This blows off more carbon bioxide and decreases the amount of carbonic acid, which
returns blood pH to normal range. On the other hand, when blood starts to become slightly
alkaline, or basic (for whatever reason), breathing slows and become more shallow.
Slower breathing allows carbon dioxide to accumulate in the blood and brings blood pH
back into normal range. Indeed control of breathing during rest is aimed primarily at
regulating the hydrogen ion concentration in the brain.
Hypoventilation (extremely slow or shallow breathing) or hyperventilation can
dramatically change the amount of carbonic acid in the blood. Carbonic acid increases
dramatically during hypoventilation and decreases substantially during hyperventilation.
In both situations, the buffering ability of the blood is likely to be overwhelmed, so that
acidosis or alkalosis occurs.