Page 678 - Fundamentals of anatomy physiology

Chapter 18 The Endocrine System 665

Table 18–5 Hormones Produced by the Pancreatic Islets

Structure/Cells Hormone Primary Targets Hormonal Effect Regulatory Control

Pancreatic Islets Glucagon Liver, adipose tissue Mobilizes lipid reserves; promotes Stimulated by low blood glucose concentrations;
Alpha cells Most cells glucose synthesis and glycogen inhibited by GH–IH from delta cells
breakdown in liver; elevates blood
Beta cells Insulin glucose concentrations Stimulated by high blood glucose concentrations,
Facilitates uptake of glucose by target parasympathetic stimulation, and high levels of
cells; stimulates formation and storage some amino acids; inhibited by GH–IH from delta
of lipids and glycogen cells and by sympathetic activation
Stimulated by protein-rich meal; mechanism
Delta cells GH–IH Other islet cells, Inhibits insulin and glucagon secretion; unclear
F cells (somatostatin) digestive epithelium slows rates of nutrient absorption and Stimulated by protein-rich meal and by
Pancreatic Digestive organs enzyme secretion along digestive tract parasympathetic stimulation
polypeptide Inhibits gallbladder contraction;
(PP) regulates production of pancreatic
enzymes; influences rate of nutrient
absorption by digestive tract

conditions, obesity accelerates the onset and severity of the to the realization that the body’s adipose tissue has important
disease. Diabetes mellitus can also result from other patho- endocrine functions. We include the endocrine functions of
logical conditions, injuries, immune disorders, or hormonal
imbalances. 18adipose tissue in this section, although all the details have yet

The two major types of diabetes mellitus are Type 1 diabetes to be worked out. Table 18–6 provides an overview of some of
(previously known as juvenile diabetes) and Type 2 diabetes. the hormones those organs of other systems produce.
This disorder is described in Spotlight Figure 18–18.
The Intestines
Checkpoint
The intestines process and absorb nutrients. They release a variety
23. Identify the types of cells in the pancreatic islets and of hormones that coordinate the activities of the digestive sys-
the hormones produced by each. tem. Most digestive processes are hormonally controlled locally,
although the autonomic nervous system can affect the pace of
2 4. Why does a person with Type 1 or Type 2 diabetes digestive activities. We describe these hormones in Chapter 24.
urinate frequently and have increased thirst?
The Kidneys
25. What effect would increased levels of glucagon have
on the amount of glycogen stored in the liver? The kidneys release the steroid hormone calcitriol, the peptide
hormone erythropoietin, and the enzyme renin. Calcitriol is im-
See the blue Answers tab at the back of the book. portant for calcium ion homeostasis. Erythropoietin and renin
are involved in the regulation of blood volume and blood
18-9 Many organs have secondary pressure.

endocrine functions Calcitriol

Learning Outcome Describe the functions of the hormones produced Calcitriol is a steroid hormone secreted by the kidneys in re-
by the kidneys, heart, thymus, testes, ovaries, and adipose tissue. sponse to parathyroid hormone (PTH) (Figure 18–19a). Chole-
calciferol (vitamin D3) is a related steroid that is synthesized in
As we noted earlier, many organs of other body systems have the skin or absorbed from the diet. Cholecalciferol is converted
secondary endocrine functions. Examples are the intestines to calcitriol, although not directly. The term vitamin D applies
(digestive system), the kidneys (urinary system), the heart (car- to the entire group of related steroids, including calcitriol, cho-
diovascular system), the thymus (lymphatic system), and the lecalciferol, and various intermediate products.
gonads—the testes in males and the ovaries in females (repro-
ductive system). The best-known function of calcitriol is to stimulate cal-
cium and phosphate ion absorption along the digestive tract.
Several new hormones from these endocrine tissues have The effects of PTH on Ca2+ absorption result primarily from
been identified. In many cases, their structures and modes of stimulation of calcitriol release. Calcitriol’s other effects on
action remain uncertain, and we have not described them in calcium metabolism include (1) stimulating the formation and
this chapter. However, in one instance, researchers traced a sig-
nificant new hormone to an unexpected site of origin, leading