Page 269 - Anatomy and Physiology of Farm Animals, 8th Edition

254 / Anatomy and Physiology of Farm Animals

s with the nervous system, the basic will be given in later chapters on the spe­
cific systems that the agents affect.
A functions of the endocrine system are
VetBooks.ir communication and regulation. The clas­
sic endocrine system (Fig. 13‐1) consists of
a group of ductless glands that secrete Hormones and Their Receptors
hormones (chemical messengers that
function in extremely small concentra­ Chemical Classes of Hormones
tions). The hormones circulate through­
out the body to bring about physiologic The classic hormones can be grouped
responses. However, this classic descrip­ according to their chemical structure and
tion does not account for other types of include peptides, steroids, and amines.
chemical messengers involved in other The amines are biochemical modifications
types of cell‐to‐cell communication and of a single amino acid, tyrosine, and can be
regulation. For example, normal endothe­ either hydrophobic or hydrophilic depend­
lial cells of blood vessels release prostacy­ ing on their modifications. The synthesis
clin, a prostaglandin (discussed later) that of steroid hormones begins with choles­
acts locally to inhibit the adhesion of terol as a substrate. Cholesterol has carbon
blood platelets. This action helps prevent atoms arranged in adjoining four rings,
platelets from forming inappropriate and this ring structure is common to all
blood clots in normal vessels. When pros­ steroid hormones (Fig. 13‐2). Within the
tacyclin diffuses away from its site of mitochondria of a cell, the enzyme
production or is washed away by blood cytochrome P450 catalyzes the conversion
flow, it rapidly degrades, so it has no sys­ of cholesterol to pregnenolone via a pro­
temic effect. A local effect such as this is cess called side‐chain cleavage. While
termed a paracrine effect. Many types of pregnenolone is the starting point for all
compounds, including proteins, small steroid hormone production, the conver­
peptides, amines (derivatives of amino sion of pregnenolone to other steroid
acids), derivatives of fatty acids, and even hormones will depend upon tissue specific
a gas (nitric oxide), have been found to enzymatic pathways for hormone synthe­
function as paracrine agents. sis. Tissue specificity for hormone synthe­
The objective of this chapter is to intro­ sis and receptor binding are a common
duce the basic concepts of endocrinology, theme in endocrinology. All steroid hor­
including the relationships between the mones are hydrophobic and often require a
nervous and endocrine systems. More carrier protein for transport in the extra­
details on specific hormones and paracrine cellular fluid (blood). Protein or peptide
agents, their regulation, and their actions hormones can vary in size and undergo

Hypothalamus Adrenal gland Ovary
Pineal gland

Pituitary gland
Pancreas
Thyroid
gland

Parathyroid gland

Figure 13-1. Approximate location of classic endocrine glands. Note that in the female, the endocrine
gonad is the ovary.