Cracking the AP Biology Exam

11 Animal Structure and Function

VIII. THE ENDOCRINE SYSTEM

Chemical messengers can be produced in one region of the body to act on target cells in another part. These chemicals, known as hormones, are produced in specialized organs called endocrine glands. Hormones have a number of functions including regulating growth, behavior, development, and reproduction. For example, the hormone ecdysone promotes molting and the metamorphosis of a larva to a butterfly. Ecdysone is stimulated to release when targeted by another insect hormone, brain hormone, in the prothoracic glands. Another hormone, juvenile hormone, causes larvae to retain their characteristics. Other chemical messengers are used for communication. For example, pheromones help animals to communicate with members of their species and attract the opposite sex.

An endocrine gland releases hormones directly into the bloodstream, which carries them throughout the body. Take a look at the endocrine glands in the human body:

Before we launch into a review of the different hormones in the body, let’s talk about how hormones work. Although hormones flow in your blood, they affect only specific cells. The cells that a hormone affects are known as the target cells. Suppose, for example, that gland X makes hormone Y. Hormone Y, in turn, has some effect on organ Z. We would then say that organ Z is the target organ of hormone Y.

Hormones also operate by a negative feedback system. That is, an excess of the hormone will signal the endocrine gland to temporarily shut down production. For example, when hormone Y reaches a peak level in the bloodstream, the organ secreting the hormone, gland X, will get a signal to stop producing hormone Y. Once the levels of hormone Y decline, the gland can resume production of the hormone.

THE PITUITARY GLAND

The pituitary is called the master gland because it releases many hormones that reach other glands and stimulate them to secrete their own hormones. The anterior pituitary therefore has many target organs. The pituitary has two parts: the anterior pituitary and the posterior pituitary. Each part secretes its own set of hormones.

The anterior pituitary secretes six hormones, three of which regulate growth and other organs. The other three are involved in regulating the reproductive systems. The hormones of the pituitary are:

Growth hormone (GH)—stimulates growth throughout the body, targets bones and muscles
Adrenocorticotropic hormone (ACTH)—stimulates the adrenal cortex to secrete glucocorticoids and mineralocorticoids
Thyroid-stimulating hormone (TSH)—stimulates the thyroid to secrete thyroxine
Follicle-stimulating hormone (FSH)—stimulates the follicle to grow in females, and spermatogenesis in males
Luteinizing hormone (LH)—causes the release of the ovum during the menstrual cycle in females, and testosterone production in males
Prolactin—stimulates the mammary glands to produce milk

The pituitary works in tandem with a part of the brain called the hypothalamus. The pituitary sits just below the hypothalamus:

The hypothalamus regulates the anterior pituitary by secreting neurohormones that can stimulate or inhibit the actions of the anterior pituitary. The other part of the pituitary, the posterior pituitary, secretes two hormones:

Antidiuretic hormone (or vasopressin)—regulates water intake by nephrons
Oxytocin—stimulates contraction of uterus and ducts of mammary glands

These hormones are actually made in the hypothalamus but are stored in the posterior pituitary. How about a mnemonic? When you think of the pituitary, think of the GATOR pit:

Growth hormone

ACTH

Thyroid-stimulating hormone

Oxytocin

R, for antidiuRetic hormone (or vasopressin)

(All of which come from the pituitary gland.)

And, to make things easier, the first three come first, or are “anterior” to the last two, which are “posterior.” For us, that means that G, A, and T come from the anterior pituitary, while O and R come from the posterior pituitary.

As for the other three hormones, think of “FLAP.” FSH, LH, and prolactin are all hormones that have to do with the reproductive system. Together, these two mnemonics should help you keep the different hormones of the pituitary gland straight. Now let’s move on to the target organs.

THE PANCREAS

We already know that the pancreas produces enzymes that it releases into the small intestine via the pancreatic duct. The pancreas also secretes two hormones, glucagon (alpha cell) and insulin (beta cell), both of which are produced in clusters of cells called the islets of Langerhans. The target organs for these hormones are the liver and muscle cells. Glucagon, produced by α cells, stimulates the liver to convert glycogen into glucose and to release that glucose into the blood. Glucagon therefore increases the levels of glucose in the blood. Insulin has precisely the opposite effect that glucagon does.

When the blood has too much glucose floating around, insulin, produced by β cells, allows body cells to remove glucose from the blood. Consequently, insulin decreases the level of glucose in the blood. Insulin is particularly effective on muscle and liver cells. In short:

Insulin lowers the blood sugar level.
Glucagon raises the blood sugar level.

THE ADRENAL GLANDS

The adrenal glands contain two separate endocrine glands. One is called the adrenal cortex, and the other is called the adrenal medulla. Although they are part of the same organ, these two endocrine glands have very different effects on the body. Let’s start by discussing the adrenal cortex.

The Adrenal Cortex

Earlier we mentioned that adrenocorticotropic hormone, or simply ACTH, targets the adrenal cortex. When ACTH is released from the pituitary, it stimulates the adrenal cortex to produce and secrete its different hormones.

One group of hormones released by the adrenal cortex is the glucocorticoids. They increase the blood’s concentration of glucose and help the body adapt to stress. In fact, glucocorticoids accomplish the same thing as glucagon, but in a slightly different way. Glucocorticoids promote the conversion of amino acids and fatty acids to glucose.

Another set of hormones is the mineralocorticoids. They help the body retain Na⁺ and water in the kidneys. They accomplish this by promoting the reabsorption of sodium (Na⁺) and chlorine (Cl^–), which get together to form common salt (NaCl). When salt is retained, water soon follows.

So then, just remember that the adrenal cortex releases two types of hormones.

Glucocorticoids target the liver and promote the release of glucose.
Mineralocorticoids target the kidney and promote the retention of water.

The Adrenal Medulla

The other adrenal gland, the adrenal medulla, is often referred to as the “emergency gland.” It secretes two hormones: epinephrine and norepinephrine. These are the hormones involved in the fight-or-flight response. Both epinephrine and norepinephrine “kick in” under extreme stress. They increase your heart rate, metabolic rate, blood pressure, and give you a quick boost of energy.

THE THYROID

The thyroid gland, which is located in the neck, is the target organ of the thyroid-stimulating hormone (TSH):

When the thyroid is stimulated by TSH, it releases the hormone thyroxine. Thyroxine, which contains iodine, is responsible for regulating the metabolic rate in your body tissues. Two conditions are associated with thyroid hormones. Hyperthyroidism occurs in individuals who regularly release too much thyroxine. They have a fast metabolic rate and tend to be irritable and nervous. On the other hand, individuals who suffer from hypothyroidism have too little thyroxine circulating in their bloodstream. They exhibit a slow metabolic rate and tend to be sluggish and overweight. To summarize:

An individual with hypothyroidism has a slow metabolic rate, while an individual with hyperthyroidism has a fast metabolic rate.

The thyroid also secretes another hormone called calcitonin. This hormone decreases your blood’s concentration of calcium by concentrating free-floating calcium in the bones. You’ll recall from our discussion of bone cells that bones contain collagen and calcium salts. Calcitonin is responsible for depositing these calcium molecules in the bones.

THE PARATHYROIDS

The parathyroids are four little pea-shaped organs that rest on the thyroid. They secrete parathyroid hormone. Parathyroid hormone increases your blood calcium levels. Consequently, parathyroid hormone has the opposite effect that calcitonin does. If your blood needs more calcium, parathyroid hormone releases calcium ions stored in the bones. This process of building or breaking down bones to store and release calcium is called bone remodeling.

THE SEX HORMONES

Three hormones that we’ll discuss in detail in the chapter on reproduction are estrogen, progesterone, and testosterone. Estrogen and progesterone are hormones released by the ovaries and they regulate the menstrual cycle. Testosterone is the male hormone responsible for promoting spermatogenesis, the production of sperm. In addition, these hormones maintain secondary sex characteristics.

HOW HORMONES WORK

How do hormones trigger the activities of their target cells? That all depends on whether the hormone is a steroid (lipid soluble) or a protein, peptide, or amine (not lipid soluble). If the hormone is a steroid, then the hormone can diffuse across the membrane of the target cell. It then binds to a receptor protein in the nucleus and activates specific genes contained in the DNA, which in turn make proteins.

However, if the hormone is a protein, peptide, or amine, it can’t get into the target cell by means of simple diffusion. Remember: “Like dissolves like.” The hormone binds to a receptor protein on the cell membrane of the target cell. This protein in turn stimulates the production of a second messenger called cyclic AMP (cAMP). The cAMP molecule then triggers various enzymes, leading to specific cellular changes. Here’s a summary of the hormones and their effects on the body.

While the nervous system and the endocrine system work in close coordination, there are significant differences between the two:

The nervous system sends nerve impulses using neurons, whereas the endocrine system secretes hormones.
Nerve impulses control rapidly changing activities, such as muscle contractions, whereas hormones deal with long-term adjustments.

ENDOCRINE SYSTEM QUIZ

Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the answer that is best in each case. Answers can be found here.

1. Consider an experiment in which normal chimpanzees are injected with extract from the anterior pituitary gland. Which of the following probably would not be observed?

(A) Increased production of ACTH

(B) Increased blood levels of glucocorticoids

(D) Increased secretion of vasopressin

(E) Increased secretion of growth hormone

2. High levels of hypothalamic and pituitary secretions would NOT be found in

(A) a man running on a treadmill

(B) a doctor delivering a baby

(D) a student taking an examination

(E) a person speaking at a conference

3. The concentration of which of the following hormones in the bloodstream stimulates the contraction of uterine muscles in pregnant women?

(A) FSH

(B) LH

(D) Oxytocin

(E) Epinephrine

4. Which of the following is true regarding the hypothalamus?

(A) It secretes thyroid-stimulating hormone.

(B) It secretes luteinizing hormone.

(D) It produces neurosecretory hormones.

(E) It secretes neurotransmitters into synapses.

5. Diabetes mellitus is characterized by high blood glucose levels and can be caused by a shortage or absence of which of the following hormones?

(A) Glucagon

(B) Insulin

(D) Calcitonin

(E) Norepinephrine