Page 207 - 84 human physiolofy part-1

The Respiratory System

It is in the mitochondria of the cells where oxygen is actually consumed and carbon dioxide
produced. Oxygen is produced as it combines with hydrogen ions to form water at the end of the
electron transport chain (see chapter on cells). As cells take apart the carbon molecules from glucose,
these get released as carbon dioxide. Each body cell releases carbon dioxide into nearby capillaries by
diffusion, because the level of carbon dioxide is higher in the body cells than in the blood. In the
capillaries, some of the carbon dioxide is dissolved in plasma and some is taken by the hemoglobin, but
most enters the red blood cells where it binds with water to form carbonic acid. It travels to the
capillaries surrounding the lung where a water molecule leaves, causing it to turn back into carbon
dioxide. It then enters the lungs where it is exhaled into the atmosphere.

Lung Capacity

The normal volume moved in or out of the lungs during quiet breathing is called tidal volume.
When we are in a relaxed state, only a small amount of air is brought in and out, about 500 mL. You
can increase both the amount you inhale, and the amount you exhale, by breathing deeply. Breathing in
very deeply is Inspiratory Reserve Volume and can increase lung volume by 2900 mL, which is quite
a bit more than the tidal volume of 500 mL. We can also increase expiration by contracting our thoracic
and abdominal muscles. This is called expiratory reserve volume and is about 1400 ml of air. Vital
capacity is the total of tidal, inspiratory reserve and expiratory reserve volumes; it is called vital
capacity because it is vital for life, and the more air you can move, the better off you are. There are a
number of illnesses that we will discuss later in the chapter that decrease vital capacity. Vital Capacity
can vary a little depending on how much we can increase inspiration by expanding our chest and lungs.
Some air that we breathe never even reaches the lungs! Instead it fills our nasal cavities, trachea,
bronchi, and bronchioles. These passages aren't used in gas exchange so they are considered to be dead
air space. To make sure that the inhaled air gets to the lungs, we need to breathe slowly and deeply.
Even when we exhale deeply some air is still in the lungs,(about 1000 ml) and is called residual
volume. This air isn't useful for gas exchange. There are certain types of diseases of the lung where
residual volume builds up because the person cannot fully empty the lungs. This means that the vital
capacity is also reduced because their lungs are filled with useless air.

Stimulation of Breathing

There are two pathways of motor neuron stimulation of the respiratory muscles. The first is the
control of voluntary breathing by the cerebral cortex. The second is involuntary breathing controlled by
the medulla oblongata.

There are chemoreceptors in the aorta, the carotid arteries, and in the medulla oblongata of the
brainstem that are sensitive to pH. As carbon dioxide levels increase there is a buildup of carbonic acid,
which releases hydrogen ions and lowers pH. Thus, the chemoreceptors do not respond to changes in
oxygen levels (which actually change much more slowly), but to pH, which is an indirect measure of
carbon dioxide levels. In other words, CO2 is the driving force for breathing. The receptors in the
aorta and the carotid arteries stimulate an immediate increase in breathing rate and the receptors in the
medulla stimulate a sustained increase in breathing until blood pH returns to normal.

This response can be experienced by running a 100 meter dash. During this exertion (or any other
sustained exercise) your muscle cells must metabolize ATP at a much faster rate than usual, and thus
will produce much higher quantities of CO2. The blood pH drops as CO2 levels increase, and you will

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