Endocrine System

Endocrinology

• Study of endocrine system

• Endocrine and nervous system work together to maintain a stable internal environment

General functions of endocrine and nervous system

• 1. communication

• 2. integration

• 3. control

Description of endocrine gland:

• Does not have ducts

• Widely scattered throughout body

• Secretes substance called hormones

How hormones work?

• A hormone is substance that signals a response

• They are secreted in blood stream and go to certain tissues called target tissue/cell

• Target cell (tissue)—cell that can only can be signaled by a certain hormone– They get to the tissue by

traveling thru blood

Hormones are classified based on function:

• 1. tropic hormones— they target other endocrine glands and stimulate growth and secretion (only affect other glands)

• 2. sex hormones— target the reproductive system only

• 3. anabolic hormone— stimulate anabolism of their target cells

Classification of hormones based on structure:

• 1.steroid—manufactured by endocrine cells from cholesterol which is an important lipid in the body– Ex. estrogen, testosterone

• 2. nonsteroid—manufactured by cells from amino acids– Ex. peptide hormones

Hormone regulation:

• Most glands secrete hormones continuously• May be 30-40 types of hormones at a given

time in blood• Most are transported in the blood by plasma

proteins• Hormone amounts are regulated by negative

feedback in: • 1. target tissue, • 2. liver inactivation, and • 3.excretion by kidney

Negative feedback regulation

• As one increases, the other decreases • Example: insulin regulates glucose levels• 1. Increase glucose causes insulin to be

secreted• 2. Insulin goes to target tissue causing them to

take up glucose• 3. This decreases glucose level• 4. Decrease glucose, decreases insulin causes

glucose to increase again

How hormones work?

• Uses Lock-and-key mechanism

• Hormone must fit exactly on a receptor on surface of a plasma protein

• Cell that the hormone attaches to is called a target cell

How hormone affects target cell?

• Once hormone has landed on target cell, a hormone-receptor interaction occurs.

• The chemical reaction causes changes to occur in the target cell.

How does change in target cell occur?

Step 1: When hormone attaches to receptor, an enzyme is released called adenyl cyclase (called 1st messenger).

Step 2: Adenyl cyclase moves to the cytoplasm of the cell and triggers ATP to change to AMP (called second messenger). At least 12 hormones use cyclic AMP as their second messenger.

Effects of cyclic AMP:

• 1. it activates specific genes

• 2. it synthesizes specific proteins

• 3. it regulates release of endocrine hormones

Hormones work together or alone

• Synergism—when 2 or more hormones work together to influence a cell better than they can work alone

Hormone disorders:

• Hypersecretion—secretion of too many hormones due to a diseased gland– Example: hyperthyrodism—too many thyroid

hormones

• Hyposecretion– secretion of too few hormones due to diseased gland– Ex. hypothyroidism—too few thyroid

hormones

Types of endocrine glands:

• 1. pituatary gland• 2. thyroid gland• 3. parathyroid gland• 4. adrenal gland• 5. pancreas• 6. testes and ovaries• 7. thymus gland• 8. pineal gland

Pituitary gland:

• Secretes hormones that control:

• 1. growth• 2. ovaries• 3. testes• 4. adrenal• 5. cortex • 6. pregnancy

Pituitary gland cont…

• Called master gland because controls the function of other glands

• It is about the size of a pea

Types of hormones secreted by pituitary:

• 1 releasing hormones— stimulates or inhibits release of other hormones

• 2. growth hormone— stimulates growth of bones, muscles, and other organs by increasing synthesis of protein

• 3. anti-diuretics (ADH)– prevents dehydration 1. by increasing water absorption by the kidneys 2. conserving water by constricting blood vessels

Thyroid gland:

• Consists of 2 lobes that surround trachea

• Secretes thyroid hormones that regulate metabolism

• Hyperthyrodism—too many hormones—slim, nervous

• Hypothyroidism—too few hormones—overweight, sluggish

Parathyroid gland:

• Embedded at the back of thyroid

• Function: secretes parathyroid hormones that regulate calcium levels by increasing absorption by intestines

• Increase parathyroid, increase calcium absorption

Adrenal gland

• Located above kidneys

• Has 2 parts:

• Adrenal medulla—inner

• Adrenal cortex—outer

Adrenal hormones:

• Epinephrine (adrenalin) and norepinephrine– Increases

energy

• Aldosterone• Cortisol• Sex hormones

like estrogen

pancreas

• Hormones secreted are insulin and glycogen

• Both regulate glucose levels

• Beta cells secrete insulin

• Alpha cells secrete glycogen

Testes and ovaries:

• Secrete sex hormones• Male sex hormones

are secreted by testes

• Female sex hormones are secreted by ovaries

• Testosterone—males• Estrogen and

progesterone-female

thymus gland:

• At upper part of thoracic cavity

• Secretes thymosin which causes certain white blood cells (t-cells) so they can fight off infection

Pineal gland:• Small, located

deep in cerebral hemisphere of brain

• Secretes hormone melatonin that responds to light—regulates “biological clock”