Page 452 - Anatomy and Physiology of Farm Animals, 8th Edition

The Urinary System / 437

The bicarbonate buffer system is quan- bicarbonate ions can be excreted in the
titatively the most important chemical
urine, while carbon dioxide is excreted via
VetBooks.ir buffer in blood plasma and other extracel- the respiratory system. Because carbon
lular fluids. Bicarbonate is the base in this
dioxide is a potential acid (it is in equilib-
system, and the acid is carbonic acid. rium with carbonic acid) and can be
However, because carbonic acid is difficult excreted in expired air, it is termed a vola-
to measure and in body fluids it is in equi- tile acid. The respiratory system is respon-
librium with carbon dioxide, the levels of sible for excreting this potential volatile
carbon dioxide are routinely used as indi- acid produced by cellular metabolism.
cators of carbonic acid levels. Other acids in body fluids, such as lactic
The bicarbonate buffer system is the acid, are not volatile, and the kidneys are
most important body buffer system from a responsible for excreting these nonvola-
physiologic standpoint, because the con- tile acids. The A shown in Figure 23‐13
−
centrations of the components of the represents the anion of nonvolatile acids
system can be rapidly adjusted by changes (e.g., lactate) that may be produced by cel-
in either renal excretion of bicarbonate or lular metabolism or absorbed from the
pulmonary ventilation to remove carbon gastrointestinal tract.
dioxide. Bicarbonate is avidly reabsorbed Both intracellular and extracellular
by renal tubules, but a renal threshold proteins function as buffers; that is, these
value permits rapid excretion of excess proteins are capable of accepting excess
bicarbonate. Also, plasma carbon dioxide hydrogen ions or donating free hydrogen
levels are normally the primary regulator ions to assist in the maintenance of a sta-
of ventilation, so these levels are also under ble pH. Because of the large quantity of
constant control. intracellular proteins in organs such as
Figure 23‐13 summarizes the roles of skeletal muscle, intracellular proteins
the urinary and respiratory systems in account for a large percentage of the total
maintaining the concentrations of the buffering capacity in the body. However,
components of the bicarbonate buffer sys- intracellular buffers cannot be as easily
tem. Note that free hydrogen ions and regulated as the bicarbonate buffer system

Extracellular fluid
GI absorption + Urinary excretion
+
H , HCO – 3 H , – H , HCO , A –
–
+
HCO
A – A – 3 3
Respiratory excretion
CO 2 CO 2

Cellular metabolism H ,
+
–
+
H , HCO , HCO 3 –
3
CO , A – CO 2
2
A –
Intracellular fluid
Figure 23-13. Primary factors affecting extracellular fluid pH. The A refers to a variety of anions such
−
as lactate. The arrow between intracellular and the extracellular fluids indicates that exchanges can occur
in either direction, but this exchange is limited by the abilities of the different ions to cross cell
membranes.