The Endocrine System

System Overview

• Includes cells, tissues, and organs that secrete hormones directly into the body fluids

• Endocrine vs. exocrine

Functions of Hormones

• Help regulate metabolic processes• Control the rates of some chemical

reactions• Aid in the transport of substances across

membranes• Help regulate water and electrolyte balances• Play vital roles in reproduction, growth, and

development

What is a Hormone?

• A biochemical secreted by a cell to affect the functions of another cell

• Action of a hormone is restricted to a specific target cell that has protein receptors for the hormone

Steroid Hormones

• Fat soluble• Diffuse into cells

easily• May bind to DNA and

activate genes• Fast acting

Nonsteroid Hormones

• Usually combine with cell membrane receptors

• Binding stimulates activity site of receptor (hormone is first messenger) second messengers inside cell trigger cell action

Prostaglandins

• Not really hormones, but behave similarly

• Affect only the organ where they are produced

• Present in very small quantities, not stored, and rapidly inactivated

Control of Hormone Secretions

• Hypothalamus– Controls the anterior

pituitary gland’s release of hormones that stimulate other endocrine glands

• Nervous system– Stimulates some

glands directly

• Some glands respond directly to changes in the internal environment

Hormone Regulation

Major Endocrine Glands and Their Hormones

Pituitary Gland

• Located at the base of the brain

• Attaches to the hypothalamus by means of the infundibulum

• Anterior and posterior lobes

Anterior Pituitary Gland• Dense CT capsule• Blocks of glandular

epithelium around blood vessels

Anterior Pituitary Hormones

• Growth Hormone (GH)– Stimulates most body

cells to increase in size and divide

– Promotes protein synthesis

Anterior Pituitary Hormones

• Thyroid-Stimulating Hormone (TSH)– Promotes normal

activity and development of the thyroid gland

• Adrenocorticotropic Hormone (ACTH)– Triggers the release of

hormones from the adrenal cortex

Anterior Pituitary Hormones

• Follicle-Stimulating Hormone (FSH)– Stimulates gamete

production in males and females

– Stimulates maturation of ovarian follicles and eggs in females

• Luteinizing Hormone (LH)– Promotes production of

gonadal hormones

– In females, triggers ovulation and stimulates maturation of ovarian follicles and eggs

Anterior Pituitary Hormones• Prolactin (PRL)

– Stimulates milk production in the breasts

– Enhances testosterone production ?

Posterior Pituitary Gland

• Consists mostly of nerve fibers and neuroglia

• Closely associated with the hypothalamus

Posterior Pituitary Hormones

• Oxytocin (OT)– Stimulates contraction

of uterine muscles

– Triggers “letdown” of breast milk

– “Cuddle hormone” ?

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH)– Inhibits urine formation in

the kidneys– Stimulates constriction of

blood vessels– Released in response to

increased blood solute concentration, low blood pressure, some drugs (nicotine, morphine)

– Inhibited by alcohol and excessive water consumption

Thyroid Gland• Highly vascular • Two lobes connected

by isthmus• Located below larynx• Secretory parts are

follicles filled with colloid

• CT capsule

Thyroid Hormones

• Thryoxine (T4) and Triiodothyronine (T3)– Collectively known as

thyroid hormone

– Increases metabolic rate and body heat production

– Regulates tissue growth

Thyroid Disorders

Thyroid Hormones

• Calcitonin– Lowers blood calcium

– Inhibits osteoclasts

– Stimulates osteoblasts

Parathyroid Glands

• 4 small glands on posterior of thyroid gland

Parathyroid Hormone

• Parathyroid Hormone (PTH)– Raises blood calcium– Stimulates osteoclasts– Triggers reabsorption

of calcium in urine in kidneys

– Increases intestinal absorption of calcium from food

• Located on top of kidneys

• Embedded in adipose

• Adrenal cortex– Outer portion of epithelial

cells organized into layers

• Adrenal medulla– Consists of modified

neurons around blood vessels and nerves of the sympathetic nervous system

Adrenal Glands

Adrenal Cortex Hormones• Aldosterone

– Zona glomerulosa

– Reduces excretion of sodium from kidneys

– Increases reabsorption of sodium from urine, perspiration, saliva, and gastric juice

– Increases excretion of potassium

– Increases water retention

Adrenal Cortex Hormones

• Cortisol– Zona fasciculata

– Helps maintain blood glucose concentration between meals

Adrenal Cortex Hormones

• Androgens– Zona reticularis

– Supplements sex hormones produced by gonads ?

Adrenal Medulla Hormones• Epinephrine and

Norepinephrine– Produce short term

stress response– Increase heart rate,

blood pressure, and metabolic rate

– Dilation of bronchioles– Trigger liver to release

glucose into bloodstream

Pancreas

• Mixed gland• Located behind

stomach• Islets of Langerhans

– masses of endocrine cells amongst acinar (exocrine) cells

Pancreas Hormones

• Insulin– Produced by beta cells

(stain purple)

– Decreases blood glucose concentration

– Enhances absorption of glucose into cells for energy use and storage

• Glucagon– Produced by alpha

cells (stain pink)

– Stimulates liver to release glucose from glycogen

– 1 molecule triggers the release of 100 million glucose molecules into the blood

Ovaries

• Female gonads• Produce estrogen and

progesterone:– Stimulate maturation

of the female reproductive organs and appearance of secondary sexual characteristics

• More info later…

Testes

• Male gonads

• Produce testosterone:– Stimulates maturation of

the male reproductive organs and the appearance of male secondary sexual characterisics

– Increases sex drive

– Promotes normal sperm production

• More info later…

Pineal Gland

• Located between cerebral hemispheres

• Attached to thalamus• Releases melatonin:

– Released in response to light

– Helps regulate circadian rhythms ?

Thymus Gland

• Located between the lungs posterior to the sternum

• Butterfly-shaped

• Diminishes during adulthood

• Produces thymosin:– Essential for the normal

development of T cells and the immune response

– More info later…

Minor Endocrine GlandsGland Hormone Hormone Actions

Heart Atrial Natriuretic Peptide (ANP)

Reduces blood volume, blood pressure, and blood Na concentration; Increase urine production in kidneys

Stomach Gastrin Triggers release of HCl in stomach

Serotonin Stimulates contraction of stomach muscles

Minor Endocrine GlandsSmall Intestine Secretin Triggers release of

digestive enzymes from the liver and pancreas; Inhibits stomach secretions

Cholecystokinin Stimulates release of pancreatic juice from pancreas; Stimulates release of bile stored in gallbladder

Kidney Erythropoietin Increases RBC production in bone marrow

Uterus and Placenta Human Chorionic Gonadotropin (hCG)

Influences the course of pregnancy

Minor Endocrine Glands

Epidermis Cholecalciferol Inactive form of vitamin D, Activated by UV, Triggers absorption of Ca in intestines

Adipose Tissue Leptin Suppresses appetite

STRESS!!!

• Stressor hypothalamus stress response• Physical stressors – extreme temperature,

decreased oxygen, infection, injury, prolonged heavy exercise, loud sounds

• Psychological stressors – thoughts of real or imagined dangers, personal losses, unpleasant social interactions, feelings of fear, anger, grief, anxiety, depression, guilt

• Stress may even be caused by a pleasant stimulus

Responses to Stress

• Blood glucose increases

• Heart rate increases

• Blood pressure rises

• Breathing rate increases

• Air passages dilate

• Blood flow redistributed (away from digestive organs to muscles)