Page 451 - Anatomy and Physiology of Farm Animals, 8th Edition

436 / Anatomy and Physiology of Farm Animals

potassium balance are maintained. Most Intercalated cell Plasma
potassium in excreted urine reaches the
VetBooks.ir tubular fluid by tubular secretion in the Urine H O + CO 2 CO 2
2
collecting duct.
Unregulated secretion of aldosterone H + H + HCO 3 – HCO 3 –
by adrenal tumors can cause significant
reductions in plasma potassium concen- Carbonic
trations, and these can threaten life. anhydrase
Such individuals may also have moder-
ate degrees of sodium retention and Figure 23-12. Formation and secretion of
increases in ECF volume, but this is usu- hydrogen and bicarbonate ions by intercalated
ally not as severe as the changes in cells. Formation of hydrogen and bicarbonate from
plasma potassium. The more moderate water and carbon dioxide in the cell is promoted by
changes in sodium balance are because the action of intracellular carbonic anhydrase.
sodium transport by other nephron seg-
ments is regulated by other factors that Some of the secreted hydrogen ions com-
can counterbalance the sodium‐retain- bine with HPO ions in the tubular fluid to
−2
ing effects of aldosterone. 4
−
−2
form H PO . In this manner, HPO acts as
4
2
4
an intratubular buffer to reduce the concen-
Urine Acidification tration of free hydrogen ions and prevent the
urine pH from dropping too low. The phos-
Intercalated cells in the walls of collect- phate ions originally entered the tubular fluid
ing ducts are capable of the active trans- as part of the glomerular filtrate. Ammonia
port of hydrogen ions into the tubular may also serve as an intratubular buffer
lumen to acidify the urine. This system (forming ammonium ions), and it is secreted
can generate a difference between blood into the tubular fluid by the collecting duct.
and urine of about 3 pH units, so if blood
pH is 7.4, urine with a pH of 4.4 can be Regulation of Acid‐Base Balance
formed. The hydrogen ions to be secreted
are generated in intercalated cells by the
hydration of carbon dioxide under the The pH of blood plasma and other extracel-
influence of carbonic anhydrase. A bicar- lular fluids is maintained relatively constant
bonate ion is also generated during this within a narrow range (about 7.35 to 7.45).
process, and these are secreted into the It is important to maintain this constancy
extracellular fluids at the base of the cell, because enzyme activity and metabolic
from where the bicarbonate can diffuse processes require close control of pH for
into the plasma (Fig. 23‐12). optimal function. Three major mechanisms
The secretion of hydrogen ions by function together to prevent the develop-
intercalated cells is regulated in part by ment of an abnormal pH. These are: (1)
the concentrations of carbon dioxide and extracellular and intracellular chemical
bicarbonate ions in the plasma and other buffers; (2) ventilatory control of plasma
extracellular fluids. If carbon dioxide con- carbon dioxide; and (3) urinary excretion of
centration increases or if bicarbonate ion bicarbonate or an acid urine as needed.
concentration decreases, the rate of hydro-
gen ion secretion accelerates, and urine Extracellular and Intracellular Buffers
becomes more acidic. The importance and
relevance of this regulation by carbon A chemical buffer system acts to maintain
dioxide or bicarbonate to the regulation of a constant pH by either donating or remov-
overall acid‐base balance is discussed in a ing free hydrogen ions in a solution (see
later section of this chapter. Chapter 2).