Hypothalamus and Pituitary Gland
The hypothalamus regulates the internal environment. For example, through the autonomic system, it helps control heartbeat, body temperature, and water balance (by creating thirst). The hypothalamus also controls the glandular secretions of the pituitary gland (hypophysis). The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalklike structure. The pituitary has two portions: the posterior pituitary (neurohypophysis) and the anterior pituitary (adrenohypophysis).
Neurons in the hypothalamus called neurosecretory cells produce the hormones antidiuretic hormone (ADH) and oxytocin (Fig. 10.2, left). These hormones pass through axons into the posterior pituitary where they are stored in axon endings.
Antidiuretic Hormone and Oxytocin
Certain neurons in the hypothalamus are sensitive to the water-salt balance of the blood. When these cells determine that the blood is too concentrated, antidiuretic hormone (ADH) is released from the posterior pituitary.
Upon reaching the kidneys, ADH causes more water to be reabsorbed into kidney capillaries. As the blood becomes dilute, ADH is no longer released. This is an example of control by negative feedback because the effect of the hormone (to dilute blood) acts to shut down the release of the hormone. Negative feedback maintains stable conditions and homeostasis.
Inability to produce ADH causes diabetes insipidus (watery urine), in which a person produces copious amounts of urine with a resultant loss of ions from the blood. The condition can be corrected by the administration of ADH.
Oxytocin, the other hormone made in the hypothalamus, causes uterine contraction during childbirth and milk letdown when a baby is nursing. The more the uterus contracts during labor, the more nerve impulses reach the hypothalamus, causing oxytocin to be released. Similarly, the more a baby suckles, the more oxytocin is released. In both instances, the release of oxytocin from the posterior pituitary is controlled by positive feedback-that is, the stimulus continues to bring about an effect that ever increases in intensity. Positive feedback is not a way to maintain stable conditions and homeostasis.
Figure 10.2 The hypothalamus and the pituitary. Left: The hypothalamus produces two hormones, ADH and oxytocin, which are stored and secreted by the posterior pituitary. Right: The hypothalamus controls the secretions of the anterior pituitary, and the anterior pituitary controls the secretions of the thyroid, adrenal cortex, and gonads, which are also endocrine glands. It also secretes growth hormone and prolactin.
A portal system, consisting of two capillary systems connected by a vein, lies between the hypothalamus and the anterior pituitary (Fig. 10.2, right). The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing hormones and hypothalamic-inhibiting hormones. For example, there is a thyrotropin-releasing hormone (TRH) and a prolactin-inhibiting hormone (PIH). TRH stimulates the anterior pituitary to secrete thyroid-stimulating hormone, and PIH inhibits the pituitary from secreting prolactin.
Hormones That Affect Other Glands
Three of the hormones produced by the anterior pituitary have an effect on other glands: Thyroid-stimulating hormone (TSH) stimulates the thyroid to produce the thyroid hormones; adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to produce its hormones; and gonadotropic hormones stimulate the gonads-the testes in males and the ovaries in females-to produce gametes and sex hormones. The hypothalamus, the anterior pituitary, and other glands controlled by the anterior pituitary are all involved in self-regulating negative feedback mechanisms that maintain stable conditions. In each instance, the blood level of the last hormone in the sequence exerts negative feedback control over the secretion of the first two hormones:
Effects of Other Hormones
Other hormones produced by the anterior pituitary do not affect other endocrine glands. Prolactin (PRL) is produced in quantity after childbirth. It causes the mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism.
Growth hormone (GH), or somatotropic hormone, stimulates protein synthesis within cartilage, bone, and muscle. It stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism as opposed to glucose metabolism.
Effects of Growth Hormone
The amount of GH produced by the anterior pituitary affects the height of the individual. The quantity of GH produced is greatest during childhood and adolescence, when most body growth is occurring (Fig. 10.3a). If too little GH is produced during childhood, the individual has pituitary dwarfism, characterized by perfect proportions but small stature. If too much GH is secreted, a person can become a giant (Fig. 10.3b). Giants usually have poor health, primarily because GH has a secondary effect on the blood sugar level, promoting an illness called diabetes mellitus.
On occasion, GH is overproduced in the adult, and a condition called acromegaly results. Because long bone growth is no longer possible in adults, only the feet, hands, and face (particularly the chin, nose, and eyebrow ridges) can respond, and these portions of the body become overly large (Fig. 10.4).
Figure 10.3 Effect of growth hormone. a. The amount of growth hormone produced by the anterior pituitary during childhood affects the height of an individual. Plentiful growth hormone produces very tall basketball players. b. Too much growth hormone can lead to giantism, while an insufficient amount results in limited stature and even pituitary dwarfism.
Figure 10.4 Acromegaly. Acromegaly is caused by overproduction of GH in the adult. It is characterized by enlargement of the bones in the face, the fingers, and the toes as a person ages.