Page 297 - Color Atlas of Biochemistry
P. 297
288 Tissues and organs
+
Acid–base balance the urine, the H ions are buffered by NH 3 and
phosphate.
A. Hydrogen ion concentration in the blood
plasma C. Buffer systems in the plasma
+
The H concentration in the blood and ex- The buffering capacity of a buffer system de-
tracellular space is approximately 40 nM pends on its concentration and its pK a value.
–1
(4 10 –8 mol L ). This corresponds to a pH The strongest effect is achieved if the pH
of 7.40. The body triesto keepthisvalue con- value corresponds to the buffer system’s pK a
stant, as large shifts in pH are incompatible value(seep.30).For this reason,weakacids
with life. with pK a values of around 7 are best suited for
The pH value is kept constant by buffer buffering purposes in the blood.
systems that cushion minor disturbances in The most important buffer in the blood is
the acid–base balance (C). In the longer term, the CO 2 /bicarbonate buffer. Thisconsistsof
the decisive aspect is maintaining a balanced water, carbon dioxide (CO 2 ,the anhydrideof
+
equilibrium between H production and up- carbonic acid H 2 CO 3 ), and hydrogen carbo-
+
–
take and H release. If the blood’s buffering nate (HCO 3 ,bicarbonate). Theadjustment
–
capacity is not suf cient, or if the acid–base of the balance between CO 2 and HCO 3 is
balance is not in equilibrium—e. g., in kidney accelerated by the zinc-containing enzyme
disease or during hypoventilation or hyper- carbonate dehydratase (carbonic anhydrase
ventilation—shifts in the plasma pH value [1]; seealso p. 282). At thepH value of the
–
can occur. A reduction by more than 0.03 plasma, HCO 3 and CO 2 are present in a ratio
unitsisknown as acidosis, and an increase is of about 20 : 1. However, the CO 2 in solution
called alkalosis. in the blood is in equilibrium with the gaseous
–
CO 2 in the pulmonary alveoli. The CO 2 /HCO 3
system is therefore a powerful open buffer
B. Acid–base balance
system, despite having a not entirely optimal
Protons are mainly derived from two sour- pK a valueof 6.1.Fasterorslowerrespiration
ces—free acids in the diet and sulfur–contain- increases or reduces CO 2 release in the lungs.
–
ing amino acids. Acids takenup withfood— This shifts the CO 2 /HCO 3 ratio and thus the
e. g., citric acid, ascorbic acid, and phosphoric plasma pH value (respiratory acidosis or alka-
acid—already release protons in the alkaline losis). In this way, respiration can compensate
pH of the intestinal tract. More important for to a certain extent for changes in plasma pH
proton balance, however, are the amino acids values. However, it does not lead to the ex-
methionine and cysteine,which arisefrom cretion of protons.
protein degradation in the cells. Their S atoms Due to their high concentration, plasma
are oxidized in the liver to form sulfuric acid, proteins—and hemoglobin in the erythro-
which supplies protons by dissociation into cytes in particular—provide about one-quar-
sulfate. ter of the blood’s buffering capacity. The buf-
During anaerobic glycolysis in the muscles fering effect of proteins involves contribu-
and erythrocytes, glucose is converted into tions from all of the ionizable side chains. At
lactate, releasing protons in the process (see the pH value of blood, the acidic amino acids
p. 338). The synthesis of the ketone bodies (Asp, Glu) and histidine are particularly effec-
acetoacetic acid and 3–hydroxybutyric acid tive.
in the liver (see p. 312) also releases protons. The second dissociation step in phosphate
2–
Normally, the amounts formed are small and (H 2 PO 4 /HPO 4 ) also contributes to the buf-
of little influence on the proton balance. If fering capacity of the blood plasma. Although
acids are formed in large amounts, however the pK a value of thissystemisnearly optimal,
(e. g., during starvation or in diabetes mellitus; its contribution remains small due to the low
see p. 160), they strain the buffer systems and total concentration of phosphate in the blood
can lead to a reduction in pH (metabolic (around 1 mM).
acidoses; lactacidosis or ketoacidosis).
Only the kidney is capable of excreting pro-
+
tons in exchange for Na ions (see p. 326). In
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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