Page 903 - Fundamentals of anatomy physiology

890 Unit 5 Environmental Exchange

This reaction is completely reversible. In peripheral capil- Figure 23–22 Carbon Dioxide Transport in Blood.

laries, it proceeds vigorously, tying up large numbers of CO2 CO2 diffuses 7% remains
molecules. The reaction continues as carbon dioxide diffuses into the dissolved in
bloodstream plasma (as CO2)
out of the interstitial fluids.
93% diffuses
The hydrogen ions and bicarbonate ions have different into RBCs

fates. Most of the hydrogen ions bind to hemoglobin mol- 23% binds to Hb, 70% converted to
ecules, forming HbH1. The Hb molecules function as pH buf- forming H2CO3 by carbonic
fers, tying up the released hydrogen ions before the ions can carbaminohemoglobin, anhydrase
Hb•CO2
leave the RBCs and lower the pH of the plasma. The bicarbon-
RBC
ate ions move into the plasma with the aid of a countertrans-
H+ removed
port mechanism that exchanges intracellular bicarbonate ions by buffers,
(HCO3−) for extracellular chloride ions (Cl2). This exchange especially Hb
trades one anion for another and does not require ATP. The

result is a mass movement of chloride ions into the RBCs, an
event known as the chloride shift.

CO2 Binding to Hemoglobin Hin2toCOH3+ dainsdsoHcCiaOte3s_ Cl–
H+
About 23 percent of the carbon dioxide carried by blood is
HCO3– moves
bound to the protein portions of Hb molecules inside RBCs. out of RBC in
exchange for
These CO2 molecules are attached to exposed amino groups CI– (chloride
(2NH2) of the Hb molecules. The resulting compound is shift)
called carbaminohemoglobin (kar-BAM-i-no. -he. -mo. -glo. -

bin), HbCO2. We can summarize the reversible reaction as
follows:

CO2 + HbNH2 ÷ HbNHCOOH PLASMA
We can abbreviate this reaction without the amino groups as

CO2 + Hb ÷ HbCO2

23 Transport in Plasma respiratory systems. We consider those levels of control and
regulation next.
Plasma becomes saturated with carbon dioxide quite rapidly.
Only about 7 percent of the carbon dioxide absorbed at periph- Checkpoint
eral capillaries is transported as dissolved gas molecules. RBCs 25. Identify the three ways that carbon dioxide is
absorb the rest and convert it using carbonic anhydrase or store
it as carbaminohemoglobin. transported in the bloodstream.
26. As you exercise, hemoglobin releases more oxygen to
Figure 23–22 summarizes carbon dioxide transport.
active skeletal muscles than it does when those muscles
Summary: Gas Transport are at rest. Why?
27. How would blockage of the trachea affect blood pH?
Figure 23–23 summarizes the transport of oxygen and carbon
dioxide in the respiratory and cardiovascular systems. Note See the blue Answers tab at the back of the book.
that the bottom portion of the figure shows the carbon dioxide
being delivered to the alveoli. The reactions we have just dis- 23-10 Neurons in the medulla
cussed then proceed in the reverse direction.
oblongata and pons, along with
Gas transport is a dynamic process. It is capable of vary- respiratory reflexes, control respiration
ing its responses to meet changing circumstances. Some of the
responses are automatic and result from the basic chemistry Learning Outcome List the factors that influence respiration rate, and
of the transport mechanisms. Other responses require coordi- discuss reflex respiratory activity and the brain centers involved in the
nated adjustments in the activities of the cardiovascular and control of respiration.