may precipitate life-threatening hypertension, cardiac arrhythmias, anxiety, and fear of impending death. Most  10% are found incidentally during unrelated imaging pheochromocytomas contain predominantly chromaffin studies.
supplied with blood that has passed through the cortical sinusoids and thus contains secreted glucocorticoids. In some species, the norepinephrine-containing cells are more numerous in those regions of the medulla supplied by capillaries derived from the cortical arterioles.
The catecholamines, in concert with the glucocorticoids, prepare the body for the “fight-or-flight” response.
The sudden release of catecholamines establishes conditions for maximum use of energy and thus maximum physical effort. Both epinephrine and norepinephrine stimulate glycogenolysis (release glucose into the bloodstream) and mobilization of free fatty acids from adipose tissue. The  release of catecholamines also causes an increase in blood pressure, dilation of the coronary blood vessels, vasodilation of vessels supplying skeletal muscle, vasoconstriction of vessels conveying blood to the skin and gut, an increase in heart rate and output, and an increase in the rate and depth of breathing.
Zonation of the Adrenal Cortex
The adrenal cortex is divided into three zones on the basis of the arrangement of its cells (Fig. 21.23):  Zona glomerulosa, the narrow outer zone that consti tutes up to 15% of the cortical volume  Zona fasciculata, the thick middle zone that constitutes nearly 80% of the cortical volume  Zona reticularis, the inner zone that constitutes only 5% to 7% of the cortical volume but is thicker than the glomerulosa because of its more central location
The cells of the zona glomerulosa are arranged in closely packed ovoid clusters and curved columns that are continuous with the cellular cords in the zona fasciculata. Cells of the zona glomerulosa are relatively small and columnar or pyramidal. Their spherical nuclei appear closely packed and stain densely. In humans, some areas of the cortex may lack a recognizable zona glomerulosa. A rich network of fenestrated sinusoidal capillaries surrounds each cell cluster. The cells have abundant smooth-surfaced endoplasmic reticulum (sER), multiple Golgi complexes, large mitochondria with shelf-like cristae, free ribosomes, and some rER. Lipid droplets are sparse.
The zona glomerulosa secretes aldosterone, which functions in the control of blood pressure.
The cells of the zona glomerulosa secrete mineralocorticoids, compounds that function in the regulation of sodium and potassium homeostasis and water balance. The principal secretion, aldosterone, acts on the distal tubules of the nephron in the kidney, the gastric mucosa, and the salivary and sweat glands to stimulate resorption of sodium at these sites, as well as to stimulate excretion of potassium by the kidney.
FIGURE 21.22 • Electron micrograph of medullary cells. Two types of medullary cells are present. The norepinephrinesecreting cells (NE) are identified by their vesicles, which contain a very dense core. The epinephrine-secreting cells (E) possess vesicles with less intensely staining granules. 15,000.
The renin–angiotensin–aldosterone system provides feedback control of the zona glomerulosa.