Human Physiology, 14th edition (2016)

626 Chapter 18

Cl–

Cl–

K+

K+

H+

H+

H+

Primary active transport

(ATPase carrier)

Facilitative

diffusion Parietal cell

Lumen of stomach

ClClClC–––

CA

HCO 3 –

HCO 3 –

H 2 CO 3

CO 2 + H 2 O

Secondary

active transport Blood capillaries

Cl–

out through K^1 channels to prevent depletion of K^1 in the gas-

tric lumen ( fig. 18.7 ). The apical surface of each parietal cell has

numerous microvilli with a high surface area to allow the inser-

tion of a large number of H^1 /K^1 pumps.

The parietal cells’ basolateral membranes (facing the

blood in capillaries of the lamina propria) take in Cl^2 against

its electrochemical gradient by coupling its transport to the

downhill movement of bicarbonate (HCO^23 ). The bicarbonate

ion is produced within the parietal cells by the dissociation of

carbonic acid, formed from CO 2 and H 2 O by the enzyme car-

bonic anhydrase. Therefore, the parietal cells can secrete Cl^2

(by facilitative diffusion) as well as H^1 into the gastric juice

while they secrete bicarbonate into the blood ( fig.  18.7 ). The

secretion of Cl^2 , as well as K^1 recycling, is required for con-

tinued activity of the H^1 /K^1 pumps.

Secretion of hydrochloric acid (HCl) by the parietal cells is

stimulated by the hormone gastrin, the paracrine regulator his-

tamine, and the neurotransmitter ACh. Gastrin is secreted by G

cells of the gastric mucosa, enters the general circulation, and

can stimulate parietal cells directly by binding to receptors on the

parietal cell basolateral membrane. However, gastrin stimulation

of HCl secretion is mostly indirect; gastrin stimulates ECL cells

to secrete histamine, and histamine acts as a paracrine regulator

to stimulate the parietal cells to secrete HCl (see fig. 18.30 ). His-

tamine stimulation of parietal cells is mediated by the H 2 type of

histamine receptor, which is different from the H 1 type of hista-

mine receptor involved in allergic reactions (chapter 15).

signal from the stomach to the brain that helps regulate hunger
(chapter 19, section 19.2).
The exocrine secretions of the gastric cells, together with a
large amount of water (2 to 3 L/day), form a highly acidic solu-
tion known as gastric juice.

Pepsin and Hydrochloric Acid

Secretion

The parietal cells secrete H^1 (protons), at a pH as low as 0.8, into
the gastric lumen by primary active transport (involving carriers
that function as an ATPase). These carriers, known as  H 1  /K 1
ATPase pumps, transport H^1 uphill against a million-to-one
concentration gradient into the lumen of the stomach while
they transport K^1 in the opposite direction. In a process termed
potassium recycling, the K^1 inside the parietal cell then leaks

Clinical Investigation CLUES

George had a vertical sleeve gastrectomy.

• What kind of surgery is this, and what can it
  accomplish?


• How can George still digest and absorb food after
  this procedure?

Figure 18.7 Secretion of gastric acid by parietal cells. The apical membrane (facing the lumen) secretes H^1 in exchange
for K^1 using a primary active transport carrier that is powered by the hydrolysis of ATP. The basolateral membrane (facing the blood)
secretes bicarbonate (HCO^23 ) in exchange for Cl^2. The Cl^2 moves into the cell against its electrochemical gradient, powered by the
downhill movement of HCO^23 out of the cell. This HCO^23 is produced by the dissociation of carbonic acid (H 2 CO 3 ), which is formed
from CO 2 and H 2 O by the action of the enzyme carbonic anhydrase (abbreviated CA). The Cl^2 then leaves the apical portion of the
membrane by diffusion through a membrane channel. The parietal cells thus secrete HCl into the stomach lumen as they secrete HCO^23
into the blood.