The Endocrine System

I. The Endocrine system contains glands and tissues that secrete hormones. Hormones are chemical signals that affect the behavior of other glands or tissues. Hormones are important because they influence the metabolism of cells, the growth and development of body parts, and homeostasis.

A. Endocrine glands: Do not have ducts. They secrete their hormones into tissue fluid. Once in the tissue fluid they then go into the bloodstream to be distributed throughout the body.

B. Exocrine glands: Do have ducts. They secrete their products into these ducts

C. Hormones can be categorized as either peptides or steroids.

1. Peptides: These include proteins, glycoproteins, and modified amino acids. These hormones cannot be taken orally because digestive enzymes would act on them.

2. Steroids: Can be taken orally because they require no prior digestion before passing through the plasma membrane.

D. The effects of hormones can usually be controlled in two ways

1. Negative feedback: Opposes their release

a. The activity of the hormone is maintained within normal limits

b. The negative feedback system can be sensitive to either a resulting condition or to the blood level of a hormone

2. Antagonistic hormones: Oppose each other’s actions

a. Hormones work opposite to each other, and as a result bring about the regulation of a substance in the blood.

E. Hypothalamus and Pituitary Gland

1. Hypothalamus- regulates the internal environment and controls the glandular secretions of the pituitary gland.

2. Pituitary Gland- Has two portion the posterior pituitary (neurohypophysis) and the anterior pituitary (adrenohypophysis).

a. Posterior Pituitary- Neurons in the hypothalamus called neurosecretory cells produce the hormones oxytocin and anti-diuretic hormone. These hormones then pass through the posterior pituitary where they are stored. The anti-diuretic hormone causes more water to be reabsorbed into kidney capillaries which in turn dilutes the blood. Oxytocin causes uterine contractions during childbirth and milk letdown when a baby is nursing.

b. Anterior Pituitary- The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing hormones and hypothalamic-inhibiting hormones. The anterior pituitary produces three hormones that have an effect on other glands, they are as follows: Thyroid-stimulating hormone which stimulates the thyroid to produce the thyroid hormones, adrenocorticotropic hormone which stimulates the adrenal cortex to produce its hormones, and gonadotropic hormones which stimulate the gonads to produce gametes and sex hormones.

F. Thyroid and Parathyroid Glands

1. Thyroid Gland- Contains large number of follicles, each being a small spherical structure made of thyroid cells filled with triiodothyronine and thyroxine.

a. To produce triiodothyronine and thyroxine the thyroid gland acquires iodine. When iodine is lacking in the diet, the thyroid gland is unable to produce the thyroid hormones, which increase the metabolic rate.

2. Parathyroid Gland- Parathyroid hormone (produced by the parathyroid gland) causes the blood phosphate level to decrease and the blood calcium level to increase.

3. Adrenal Glands-Adrenal glands are located on top of the kidneys. Each adrenal gland has an inner portion known as the adrenal medulla and outer portion known as the adrenal cortex. All types of stress, from emotional to physical trauma, promt the hypothalamus to stimulate the adrenal glands.

a. Adrenal medulla- Under nervous control. Norepinephrine and epinephrine are produced by the adrenal medulla. These secreted hormones ring about all the body changes that happen when an individual reacts to an emergency situation. They provide a short term response to stress.

b. Adrenal cortex-Under the control of ACTH, which is an anterior pituitary hormone. The adrenal cortex secretes two major types of hormones: mineralocorticoids and the glucocorticoids. They provide a long-term response to stress.

1. mineralocorticoids- regulate salt and water balance, which increases in blood volume and pressure.

2. glucocorticoids- regulate carbohydrate, protein, and fat metabolism, which leads to an increase in blood glucose level.

G. Pancreas

1. Lies transversely in the abdomen between the kidneys and near the duodenum of the small intestine. It is made up of two types of tissue: Exocrine tissue and endocrine tissue.

a. Exocrine tissue produces and secretes digestive juices that go by way of ducts to the small intestine.

b. Endocrine tissue (also known as pancreatic islets) produces and secretes the hormones insulin and glucagons direcly into the blood. These two antagonist hormones produced by the pancreas help maintain the normal level of glucose in the blood.

H. Other Endocrine Glands

1. Gonads- The secretion of these hormones are controlled by the hypothalamus and pituitary gland.

a. Testes- located in th scrotum, they produce androgens, which are the male sex hormones.

b. Ovaries- Located in the pelvic cavity, they produce estrogens and progesterone, which are the female sex hormones.

2. Thymus Gland- Located just beneath the sternum. It is most active and at its largest size during childhood.

3. Pineal Gland- Located in the brain. It produces the hormone melatonin, mainly at night. It regulates our sleep cycle. It is also known to regulate sexual development.

4. Hormones from other tissues

a. Leptin- A protein hormone produced by adipose tissue. It acts on the hypothalamus, telling a person they have had enough to eat.

b. Growth factors- stimulate cell division and mitosis.

c. Prostaglandis- chemical signals produced within cells from arachidonate (fatty acid).

I. Chemical Signals- Molecules that affect the behavior of those cells that have receptor proteins to receive them. There are two basic chemical classes that hormones fall under.

1. peptide hormones- Includes peptides, proteins, glycoproteins, or modified amino acids.

2. Steroid hormones- Each have the same four-carbon ring complex, but with different side chains.

Warnings

There are several diseases that can result from an insufficient endocrine system. The following are not necessarily fatal.

· Simple goiter: Lack of iodine in the diet doesn’t allow the thytoi dgland to produce the thyroid hormones, and as a result the thyroid enlarges.

· Cretinism: Individuals with this disease are short and stocky and have hypothyroidism.

· Graves disease: The thyroid gland is overactive and a goiter forms.

· Cushing syndrome: The level of adrenal cortex hormones is high due to hypersecretion.

Dangers

· Addison disease: Occurs when the level of adrenal cortex hormones is low due to hyposecretion. The presence of ineffective ACTH causes a bronzing of the skin. If left untreated it can be fatal.

· Diabetes Mellitus: It is a fairly common hormonal disease in which liver cells are unable to take up or metabolize glucose. There are two forms of diabetes mellitus: type I (insulin-dependent diabetes mellitus) and type II (noninsulin-dependent diabetes). If left untreated it can be fatal.