Human Physiology, 14th edition (2016)

110 Chapter 5

Red blood cells lack mitochondria and so can metabolize

only anaerobically; they cannot use the oxygen they carry. This

spares the oxygen within red blood cells for delivery to the

other cells of the body. Except for red blood cells, anaerobic

metabolism occurs for only a limited period of time in tissues

that have energy requirements in excess of their aerobic ability.

Anaerobic metabolism occurs in the skeletal muscles and heart

when the ratio of oxygen supply to oxygen need (related to the

concentration of NADH) falls below a critical level. Anaerobic

metabolism is, in a sense, an emergency procedure that provides

some ATP until the emergency (oxygen deficiency) has passed.

It should be noted, though, that there is no real “emergency”

in the case of skeletal muscles, where lactic acid fermentation

is a normal, daily occurrence that does not harm muscle tis-

sue or the individual. Excessive lactic acid production by mus-

cles, however, is associated with pain and muscle fatigue. (The

metabolism of skeletal muscles is discussed in chapter 12, sec-

tion 12.4.) In contrast to skeletal muscles, the heart normally

respires only aerobically. If anaerobic conditions do occur in

the heart, a potentially dangerous situation may be present.

Figure 5.3 Glycolysis. In glycolysis, 1 glucose is
converted into 2 pyruvic acids in nine separate steps. In addition
to 2 pyruvic acids, the products of glycolysis include 2 NADH
and 4 ATP. Because 2 ATP were used at the beginning, however,
the net gain is 2 ATP per glucose. Dashed arrows indicate
reverse reactions that may occur under other conditions.

Glucose (C 6 H 12 O 6 )

Pyruvic acid (C 3 H 4 O 3 )

ADP

AT P

Glucose 6-phosphate

Fructose 6-phosphate

ADP

AT P

Fructose 1,6-biphosphate

3–Phosphoglyceraldehyde

5

1,3–Biphosphoglyceric acid

Pyruvic acid (C 3 H 4 O 3 )

6

3–Phosphoglyceric acid

7

2–Phosphoglyceric acid

8

Phosphoenolpyruvic acid

9

ADP

AT P

ADP

AT P

ADP

AT P

ADP

AT P

NADH

NAD

2H

NADH

NAD

2H

Pi

3–Phosphoglyceraldehyde

5

1,3–Biphosphoglyceric acid

6

3–Phosphoglyceric acid

7

2–Phosphoglyceric acid

8

Phosphoenolpyruvic acid

9

Pi

Dihydroxy-

acetone

phosphate

1

2

3

4

Figure 5.4 The formation of lactic acid. The addition

of 2 hydrogen atoms (colored boxes) from reduced NAD to

pyruvic acid produces lactic acid and oxidized NAD. This

reaction is catalyzed by lactic acid dehydrogenase (LDH) and is

reversible under the proper conditions.

H C C

HO

H

C

O

OH

O

OH

NADH + H+ NAD

H C C

H

H

C

OH

H

Pyruvic acid Lactic acid

LDH

CLINICAL APPLICATION

Myocardia ischemia refers to inadequate blood flow to the

heart muscle, a condition of ischemic heart disease. It may

also be called coronary heart disease, because of inad-

equate blood flow in the coronary arteries. This is usually

the result of the development of plaques in the artery wall, a

process termed atherosclerosis. Thus, the condition is also

called atherosclerotic heart disease. When the myocar-

dial cells do not obtain sufficient oxygen from the blood for

aerobic respiration, they obtain energy from the less-efficient

lactic acid pathway. Prolonged ischemia can cause death of

these cells, producing myocardial infarction (heart attack).

Lactic acid within the myocardium produces a chest pain

called angina pectoris. Angina is usually treated with vasodi-

lator drugs such as nitroglycerin, which improves blood flow

to the heart and reduces the work of the heart by dilating

peripheral blood vessels.