Page 51 - 86 human physiology part-2

Chapter 14

been repaired, or negated, corrective actions decrease or discontinue. For example, the amount of
glucose in the blood controls the secretion of insulin and glucagons via negative feedback.

The production of some hormones is controlled by positive feedback. In such a system, hormones
cause a condition to intensify, rather than decrease. As the condition intensifies, hormone production
increases. Such positive feedback is uncommon, but does occur during childbirth, where hormone
levels build with increasingly intense labor contractions. Also in lactation, hormone levels increase in
response to nursing, which causes milk production to increase. The hormone produced by the
hypothalamas causing the milk let down and uterine contraction is oxytocin.

Endocrine Glands

Pituitary gland

The hypothalamus makes up the lower region of the diencephalons and lies just above the brain
stem. The pituitary gland (hypophysis) is attached to the bottom of the hypothalamus by a slender stalk
called the infundibulum. The pituitary gland consists of two major regions, the anterior pituitary gland
(anterior lobe or adenohypophysis) and the posterior pituitary gland (posterior lobe or
neurohypophysis). The hypothalamas also controls the glandular secretion of the pituitary gland.

The hypothalamus oversees many internal body conditions. It receives nervous stimuli from
receptors throughout the body and monitors chemical and physical characteristics of the blood,
including temperature, blood pressure, and nutrient, hormone, and water content. When deviations
from homeostasis occur or when certain developmental changes are required, the hypothalamus
stimulates cellular activity in various parts of the body by directing the release of hormones from the
anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by
one of the following two pathways:

The Pituitary gland is found in the inferior part of the brain and is connected by the Pituitary Stalk.
It can be referred to as the master gland because it is the main place for everything that happens within
the endocrine system. It is divided into two sections: the anterior lobe (adenohypophysis) and the
posterior lobe (neurohypophysis). The Posterior pituitary is involved in sending hormones that control
all other hormones of the body.

Posterior pituitary

Communication between the hypothalamus and the posterior pituitary occurs through
neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary.
Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and
transported through the axon and stored in the axon terminals that lie in the posterior pituitary. When
the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored
hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones,
oxytocin and antidiuretic hormone (ADH), are produced and released this way. If one's not able to
produce ADH it can cause Diabetes Insipidus which means a person is producing large amounts of
urine, resulting in loss of ions from the blood.

266 | Human Physiology