Chapter 7The Endocrine System

Two systems that coordinate physiological functions of humans

1. The nervous system

2. The endocrine system

The endocrine system

• A system of ductless glands that secrete hormones into the blood, which carries them to target organs or systems

For more information, visit Endocrine System (Hormones) Topics at www.nlm.nih.gov/medlineplus/endocrinesystemhormones.html

Locations of the major endocrine glands

Hormones

Three major categories of hormones:1. Steroid hormones

2. Protein hormones

3. Phenolic amine hormones

Major endocrine glands, hormones, chemical natures of the hormones, and the general functions elicited in the target tissues

Major endocrine glands, hormones, chemical natures of the hormones, and the general functions elicited in the target tissues (cont.)

The three basic hormone structures

Characteristics of steroid hormones

• Derived from cholesterol

• Four-ring structure

• Enter easily through cell membrane

(a) A steroid hormone passes through a cell membrane and (B) combines with a protein receptor in the cytoplasm. (C) The hormone-receptor complex enters the nucleus and (D) activates the synthesis of messenger RNA. (E) The messenger RNA leaves the nucleus and (F) functions in the manufacture of protein molecules.

Characteristics of protein hormones

• Are not lipid soluble

• Can’t diffuse through cell membrane

(A) The protein hormone reaches its target cell via the blood and (B) combines with a receptor located on the cell membrane. (C) As a result, molecules of an enzyme such as adenyl cyclase are activated and (D) cause the activation of a second messenger such as cyclic AMP, which (E) brings about various cellular changes.

Characteristics of phenolic amine hormones

• Location of the receptor depends on the specific hormone, so

• Some phenolic amine hormones act like steroid hormones and others like protein hormones

Control of hormone secretion

Three types of stimuli initiate secretion

• Neural

• Hormonal

• Concentration of a substance in the blood

The pancreas

• Secretes glucagon and insulin– Glucagon raises blood glucose levels– Insulin lowers blood glucose levels

• Role during exercise:– Decreases body’s capacity for insulin

secretion– Resting insulin levels and insulin

production lower following training– Increase in insulin receptor sensitivity

The adrenal medulla

• Secretes the catecholamines, epinephrine and norepinephrine, which increase the availability of fuel to the active muscles.– Involved in maintaining constant fuel supply to

active tissues– Helps regulate heart rate and blood flow

• Role during exercise– Increases blood levels of both hormones, resulting

in increases in glycogenolysis, lipolysis, heart rate, stroke volume, and the redistribution of blood flow to the working muscles and the skin.

– Training reduces catecholamine response to intensity of exercise, increases response to other stimuli

Thyroid and parathyroid

• Thyroid– Training results in increased levels of T3

and T4. May result in higher metabolic weight in athletes

• Parathyroid– PTH involved in calcium regulation– Relationship to exercise is unclear

Gonads and exercise

• Men– Testosterone levels increase after high-

intensity aerobic and anaerobic exercise– Submaximal and endurance training result in

lower testosterone– Severe aerobic training and weight-lifting result

in increased testosterone• Women

– Estrogen and progesterone rise and LH drops after intense exercise

– Heavy exercise can result in delayed menarche, amenorrhea, and anovulation

Growth hormone

• Is secreted by the pituitary gland

• Regulates growth and metabolism

• Levels increase during strenuous exercise

• Effects of training on growth hormone levels have not been established

Visit The Pituitary Foundation at http://www.pituitary.org.uk/endocrine/index.shtml/