**1.1 Respiration and Circulation:**

Review of basic respiratory system anatomy

Review of basic cardiovascular system anatomy

Review of basic respiratory physiology (including definitions of
pulmonary function volumes and capacities)

Review of basic cardiovascular physiology

Composition of respired air and pulmonary air

Partial pressure of gases at altitude (physiological implications of gas
laws)

Respiratory system and cardiovascular system physiology at altitude

RESPIRATION AND CIRCULATION

Section II, 1.1

The respiratory system consists basically of the trachea, bronchi,
bronchioles, and alveoli or air sacs. Its primary function is to
exchange O~2~ and CO~2~ between inhaled air and the pulmonary
vasculature. Through the respiratory tract, inspired air is brought to
the terminal alveoli or air sacs where oxygen diffuses into the blood
stream for circulation to the organs of the body. At the same time,
carbon dioxide brought to the pulmonary vasculature from the body
tissues diffuses into the air sacs to be exhaled. The entire respiratory
process involves pulmonary ventilation, gas diffusion(lungs),
transportation of gases, gas diffusion (tissues), and O~2~ utilization.

The first step in the respiratory process is ventilation, i.e.,
inhalation and exhalation, which is controlled by chemoreceptors, the
medullary respiratory center (MRC), and the pulmonary stretch receptors.
Chemoreceptors (aortic and carotid bodies) located on the arch of the
aorta and at the carotid bifurcations are sensitive to the PaO~2~.
Consequently, with increased altitude, the aortic and carotid bodies
will detect a lower PAO2 and transmit a signal via the
vagal-glossopharyngeal nerves to the MRC, stimulating an increased
respiratory rate and tidal volume.

This is one physiological compensatory mechanism that is activated when
in an hypoxic environment.

The MRC itself is most sensitive to pH and PaCO~2~ (and minimally
sensitive to PaO~2~). With a lowered pH and/or an increased PaCO~2~, the
MRC will stimulate respiration in an effort to rid the body of excess
CO~2~. Another mechanism of ventilation control is the stretch receptors
within the lung which stimulate the expiratory drive at peak inhalation
and the inspiratory drive at peak exhalation. This vagal-mediated
response is known as the Hering-Breuer Reflex.

The second step in the respiratory process is gas diffusion whereby
oxygen crosses from the air sacs into the pulmonary capillaries and
carbon dioxide diffuses in the opposite direction from the pulmonary
capillaries into the air sacs to be exhaled. This exchange is basically
driven by a pressure differential of the gases (O~2~ and CO~2~) between
the air sacs and pulmonary capillaries

Fig 1

GAS DIFFUSION

[Air Sacs (Alveoli) Pulmonary Capillaries (mm Hg)]{.underline}

O~2~ pressure: 102 mm Hg → 40 mm Hg

CO~2~ pressure: 40 mm Hg ← 47 mm Hg

If the pulmonary system were abnormal because of pneumonia, asthma, or
emphysema, for example, the efficiency of this diffusion process could
be compromised. Hence, in questionable cases, pulmonary function tests
(PFTs) including FEV~1~ and FVC and other lung measurements may be
helpful to determine the extent, if any, of abnormal lung function.

LUNG MEASUREMENTS

DEFINITIONS:

Inspiratory Reserve Volume---amount of air inhaled with maximum
inhalation at the end of tidal inspiration.

Tidal Volume---amount of inspired and expired air with normal breathing.

Expiratory Reserve Volume---amount of air expired with maximum
exhalation at the end of tidal expiration.

Residual Volume---amount of air left in lungs after maximum exhalation.

Functional Residual Capacity---amount of air in lungs after tidal
expiration (expiratory reserve volume +residual volume).

Inspiratory Capacity---tidal volume +inspiratory reserve volume.

Vital Capacity---sum of inspiratory reserve, tidal and expiratory
reserve volume.

Total Lung Capacity---total capacity of lungs to hold air.

As described in the previous section, the barometric pressure at sea
level is 760 mm Hg

comprised mainly of O~2~ (20 % or 152 mm Hg) and N~2~ (80% or 608 mm
Hg). In accordance with Henry's Law, the total pressure in the lungs
must also equal 760 mm Hg. However, the gas composition in the lungs
differs from that in the atmosphere in that there is also significant
CO~2~ and water vapor in the lungs which is not present in the
atmosphere (Table 4). Therefore, the partial pressure of each gas in the
lungs will differ from those in the atmosphere even though the

total pressure is the same in each (760 mm Hg).

Because of the gas mixture in the lungs, the pO~2~ is reduced from an
ambient pressure of 152 mm Hg to approximately 102 mm Hg. O~2~ diffuses
through the air sacs to the pulmonary capillaries and CO~2~ in the
pulmonary capillaries diffuses outward to the alveoli because of the
pressure differential shown in Fig. 1.

TABLE IV

PARTIAL PRESSURES

[Sea Level(mm Hg)]{.underline} [Alveoli(mm Hg)]{.underline}

N~2~ 608 571

O~2~ 152 102

CO~2~ 40

H~2~O Vapor\* 47

Total=760 Total=760

\*H~2~O vapor pressure in the lungs is constant at 47 mm Hg.

The third step in the respiratory process is gas (O~2~ and CO~2~)
transportation. In order tounderstand the transportation of O~2~ and
CO~2~, it is necessary to examine the oxyhemoglobin dissociation curve
(Fig. 3). Note that the curve shifts left with an increase in pH and
right with a decrease in pH.

In the lungs where CO~2~ is being offloaded to be exhaled, the blood
PaCO~2~ will decrease with a concomitant increase in pH. This will shift
the oxyhemoglobin dissociation curve to the left which (at any given
PaO~2~) facilitates the onloading of O~2~ by hemoglobin (which would be
physiologically desirable). Consequently, well saturated hemoglobin
leaves the lungs with a rich supply of O~2~ for transport to the tissues
of the body. Because of the unique characteristics of Hb, its affinity
for O~2~, and the S-shape of the oxyhemoglobin dissociation curve, the
hemoglobin saturation remains over 90% even at altitudes of 8,000 ft.
Hence, a normal individual should be adequately oxygenated even at high
cabin altitudes, however, this may not be true for individuals with
significant cardiopulmonary disease.

It should be noted that beyond 10,000 ft., the curve turns abruptly
downward (Fig 3 and Table V).

With even small decrements of PaO~2~, the hemoglobin saturation is
significantly reduced above this altitude. For this reason, aircrew (and
passengers) will develop signs and symptoms of hypoxia very quickly as
an altitude of 10,000 ft. is exceeded.

The oxyhemoglobin dissociation curve can be affected not only by the
PaO~2~, but also by changes in temperature, pH, and 2,3
diphosphoglycerate (2,3 DPG). For example, an increase or decrease of
temperature will shift the curve to the right and left, respectively.
Likewise, an increased blood pH (as occurs in the lungs) shifts the
curve to the left and a decreased blood pH (as occurs at the tissues)
shifts it to the right.

The kidneys release 2,3 DPG when the PaO~2~ is reduced, which also
shifts the curve to the right to facilitate O~2~ off-loading in the
tissues.

If the curve shifts to the left, at a given PaO~2~, the Hb saturation
will be increased (facilitating Hb onloading of O~2~) while with a
rightward shift, the Hb saturation will be decreased (facilitating Hb
offloading of O~2~).

Once in the blood, most of the O~2~ is carried as oxyhemoglobin (98.5%)
with a small amount

(1.5%) physically dissolved in the blood. (See Table VI for normal
values of O~2~ and CO~2~ in the

blood.)

Diffusion of gases at the tissue level is the 4^th^ step of the
respiratory process. Here the opposite

occurs from that of the alveoli. CO~2~ is off-loaded from the tissues to
the blood and O~2~ leaves the

blood for the tissues. The increased PaCO~2~ decreases the blood pH
shifting the oxyhemoglobin

dissociation curve to the right thereby facilitating oxygen off-loading
for utilization by the

tissues. Once in the blood, carbon dioxide is carried back to the lungs
mainly as bicarbonate

(90%) with smaller amounts either dissolved in the blood (5%) or
combined with hemoglobin

(carbaminohemoglobin) (5%).

The final step is oxygen utilization by the tissues for vital functions
such as muscle

contraction, glandular secretions, body heat, urine formation, and nerve
impulses.

TABLE V

PAO~2~ AND % Hb SATURATION

[Altitude (ft.)]{.underline} [PAO~2~ (mm Hg)]{.underline} [% Hb
Saturation]{.underline}

Sea Level 100 98

10,000 60 87

18,000 38 72

22,000 30 60

TABLE VI

NORMAL VALUES

[Arterial Venous]{.underline}

pO~2~ (mm Hg) 100 40

%Hb Sat. 98 75

plasma pH 7.4 7.37

pCO~2~ (mm Hg) 40 46

The primary purpose of the cardiovascular system (CVS) consisting of the
heart and its four chambers, arteries, veins, and capillaries, its
primary purpose is to serve as a conduit for blood to deliver O~2~ from
the lungs to the tissues of the body and to transport CO~2~ from the
tissues of the body to the lungs.

In addition, important compensatory mechanisms will activate the CVS if
an aviator is in

an hypoxic environment. There are nerve connections between the
carotid/aortic bodies and the

central nervous system (CNS) which will cause an increased heart rate
and blood pressure if the

PaO~2~ falls below normal levels. Hence, when an aviator becomes
hypoxic, protection is offered not only by the respiratory system as
described above, but also by compensatory mechanisms of

the CVS.

In summary, as cabin altitude increases, the crew and passengers will
become increasingly

hypoxic. This goes unnoticed by most relatively healthy individuals to
altitude as high as 10,000

ft. because of cardiopulmonary compensatory mechanisms. These
compensatory mechanisms

will stimulate the respiratory system with an increased respiratory rate
and tidal volume.

Likewise, stimulation of the CVS will induce an increase in heart rate,
cardiac output, and

systolic blood pressure. All of these compensatory mechanisms serve to
protect the body when in

an hypoxic environment.