Human Physiology, 14th edition (2016)

Respiratory Physiology 561

100 ml blood ( fig. 16.33 ). Thus, 22%, or 4.5 ml of O 2 out of the

20 ml of O 2 per 100 ml blood, is unloaded to the tissues.

A graphic illustration of the percent oxyhemoglobin satu-

ration at different values of P^ O 2 is called an oxyhemoglobin

dissociation curve ( fig.  16.33 ). The values in this graph are

obtained by subjecting samples of blood in vitro to different

partial oxygen pressures. These percent oxyhemoglobin satu-

rations can then be used to predict what the unloading percent-

ages would be in vivo with a given difference in arterial and

venous P O^2 values.

Figure 16.33 shows the difference between the arterial and

venous P^ O 2 and the percent oxyhemoglobin saturation at rest.

The relatively large amount of oxyhemoglobin remaining in the

venous blood at rest serves as an oxygen reserve. If a person

stops breathing, a sufficient reserve of oxygen in the blood will

keep the brain and heart alive for about 4 to 5 minutes with-

out using cardiopulmonary resuscitation (CPR) techniques. This

reserve supply of oxygen can also be tapped when a tissue’s

requirements for oxygen are raised, as in exercising muscles.

The oxyhemoglobin dissociation curve is S-shaped, or sig-

moidal. The fact that it is relatively flat at high P^ O 2 values indicates

The Loading and Unloading Reactions

Deoxyhemoglobin and oxygen combine to form oxyhemo-
globin; this is called the loading reaction. Oxyhemoglobin,
in turn, dissociates to yield deoxyhemoglobin and free oxygen
molecules; this is the unloading reaction. The loading reac-
tion occurs in the lungs and the unloading reaction occurs in
the systemic capillaries.
Loading and unloading can thus be shown as a reversible
reaction:

(lungs)

Deoxyhemoglobin 1 O 2 Oxyhemoglobin

(tissues)

The extent to which the reaction will go in each direction
depends on two factors: (1) the P^ O 2 of the environment and
(2) the affinity, or bond strength, between hemoglobin and oxy-
gen. High P^ O 2 drives the equation to the right (favors the loading
reaction); at the high P^ O 2 of the pulmonary capillaries, almost
all the deoxyhemoglobin molecules combine with oxygen. Low
P^ O 2 in the systemic capillaries drives the reaction in the opposite
direction to promote unloading. The extent of this unloading
depends on how low the P^ O 2 values are.
The affinity between hemoglobin and oxygen also influ-
ences the loading and unloading reactions. A very strong bond
would favor loading but inhibit unloading; a weak bond would
hinder loading but improve unloading. The bond strength
between hemoglobin and oxygen is normally strong enough
so that 97% of the hemoglobin leaving the lungs is in the form
of oxyhemoglobin, yet the bond is sufficiently weak so that
adequate amounts of oxygen are unloaded to sustain aerobic
respiration in the tissues.

The Oxyhemoglobin

Dissociation Curve

Blood in the systemic arteries, at a P^ O 2 of 100 mmHg, has a per-
cent oxyhemoglobin saturation of 97% (which means that 97% of
the hemoglobin is in the form of oxyhemoglobin). This blood is
delivered to the systemic capillaries, where oxygen diffuses into
the cells and is consumed in aerobic respiration. Blood leaving in
the systemic veins is thus reduced in oxygen; it has a P^ O 2 of about
40 mmHg and a percent oxyhemoglobin saturation of about
75% when a person is at rest ( table 16.7 ). Expressed another
way, blood entering the tissues contains 20 ml O 2 per 100 ml
blood, and blood leaving the tissues contains 15.5 ml O 2 per

Table 16.7 | Relationship Between Percent Oxyhemoglobin Saturation and P^ O 2
(at pH of 7.40 and Temperature of 37 8 C)

P^ O 2 (mmHg) 100 80 61 45 40 36 30 26 23 21 19

Percent Oxyhemoglobin 97 95 90 80 75 70 60 50 40 35 30

Arterial Blood Venous Blood

Figure 16.33 The oxyhemoglobin dissociation

curve. The percentage of oxyhemoglobin saturation and the

blood oxygen content are shown at different values of PO 2. Notice

that the percent oxyhemoglobin decreases by about 25% as the

blood passes through the tissue from arteries to veins, resulting

in the unloading of approximately 5 ml of O 2 per 100 ml of blood

to the tissues.

Oxygen content(ml O

/100 ml blood) 2

Amount of O 2

unloaded to

tissues

0

20

5

10

15

Arteries

Veins

(at rest)

Po 2 (mmHg)

0 20 40 60 80 100

0

20

40

60

80

100

Percent oxyhemoglobin saturation