chapter 7 the endocrine system. two systems that coordinate physiological functions of humans 1.the...
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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/