P - bei Swiss-Cave-Diving

Physiology Workshop
Respiratory Issues in Technical Diving
Air
PO 2 = 160
PCO 2 = “0”
Mouth /
Nose
Alveolus
Venous
blood
PO 2
= 40
PCO 2
= 45
PO 2
= 100
PCO 2
= 40
CO 2
O 2
PO 2
= 100
PCO 2
= 40
Arterial
blood
Figure 2. Partial pressures of O2 and CO2 at the mouth, in the
alveolus, and in blood entering and leaving the alveolar capillaries.
contrast, the body extracts about 50 ml of oxygen from every liter of blood on each
pass, so there would be a dramatic shortfall in oxygen delivery to tissues in the absence
of Hb. Hemoglobin has multiple binding sites for oxygen and can carry about 1.34 ml
per gram of Hb when all those sites are occupied (or “saturated” as is the usual term).
Not surprisingly, we have evolved in such a way that the oxygen binding sites on Hb
are fully saturated when exposed to the normal PO 2
in our alveoli during air breathing
(i.e., 100 mmHg as shown in Figure 2). Assuming a “normal” Hb level in the blood
of 150 g/L (this is variable and less in women) than the oxygen carrying capacity on
Hb is given by 150 g/L x 1.34 ml/g = 200 ml/L of oxygen (see Figure 3). This is
markedly greater than the 3 ml/L carried as dissolved oxygen, and exceeds the usual
extraction of oxygen (50 ml/L) by 4 times.
When arterial blood enters the capillaries of various tissues, the pressure of dissolved
oxygen (at a PO 2
of approximately 100 mmHg) is much greater than that in the
surrounding tissue, so oxygen diffuses out of the blood. As the dissolved oxygen
diffuses outward the PO 2
in the blood falls, and this “stimulates” the Hb to release
more oxygen, which itself diffuses out into the tissues. In this way, the tissues are
delivered the oxygen they rely on. The tendency for Hb to lose oxygen as the PO 2
falls
is described by the so-called oxygen-hemoglobin dissociation curve (see Figure 3).
With the usual oxygen extraction by body tissues, the PO 2
in the venous blood falls to
40 mmHg and you will note from the curve that the Hb is still about 75% saturated.
Technical Diving Conference Proceedings 17