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The Endocrine System

The Endocrine System: Overview

Consists of glands and tissues that secrete hormones

Works together with nervous system to regulate organs of the body and thereby maintain homeostasis

Helps regulate: Digestion Fuel Metabolism Blood Pressure and Volume Calcium Balance Response to External Environment

Chemical Signals

• Molecules that affect the behavior of those cells that have receptor proteins to receive them

Chemical Signals Importance

Cells, organs, and even individuals communicate with one another using chemical signals

Many chemical signals act locally (from cell to cell)

Pheromones: chemical signals that act between individuals

Hormones

Chemical signals that affect the behavior of other glands or tissues

Influence the metabolism of cells, the growth and development of body parts, and homeostasis

Though each hormone has a unique composition, can be classified as either: Peptides (proteins, glycoproteins, and modified amino acids) Steroids (all steroid hormones have same four-carbon ring

complex, but differ in side chains)

Hormones

Peptides

Bind to receptor protein in plasma membrane

Operate within a “second messenger” system This hormone stays outside the cell

and directs activities within The peptide hormone is the first

messenger which activates a second messenger This sets an enzyme cascade

(when one enzyme activates several other enzymes, and so on)

Steroids

They are lipids (diffuse across plasma membrane and other cellular membranes)

Bind to receptor proteins after they are inside the cell Hormone-receptor complex

then binds to DNA, activating particular genes

Activation leads to production of cellular enzyme in varying quantities

Hormones

Peptides Steroids

Hormones: Release

The release of hormones is controlled by one or more of the three different mechanisms Negative Feedback

A stimulus cause a body response and the body response, in turn, corrects the initial stimulus

Hormonal Hormone release may also be controlled by specific stimulating or

inhibiting hormones Neural Control

Upon receiving sensory information from the body, the brain can make appropriate adjustments for hormonal secretion, to ensure homeostasis

Endocrine Glands

• Ductless• Secretes hormones

directly into tissue fluid• Major glands include:

o Hypothalamuso Pineal Glando Pituitary Gland (Hypophysis)o Thyroid Glando Parathyroid Glando Thymus Glando Adrenal Glando Pancreaso Gonads (Ovaries and Testes)

Hypothalamus

• Located in the middle of the base of the brain

• Regulates internal environment

• Controls glandular secretions of pituitary gland

Pituitary Gland

• Also known as hypophysis• Small-about 1 cm in

diameter• Connected to hypothalamus

by stalklike structure• Attached to the base of the

brain• Has 2 portions:

• Posterior Pituitary (nuerohypophysis)

• Anterior Pituitary (adrenohypophysis)

Pituitary Gland: Posterior Pituitary

Neurosecretory cells produce hormones the hormones pass through axons into posterior pituitary where they are stored in axon endings

Releases: Antidiuretic Hormone (ADH or vasopressin)

Hormone released from the posterior lobe of the pituitary gland that enhances water conservation by the kidneys

Oxytocin Hormone released by the posterior pituitary that causes

contraction of the uterus and milk letdown when baby is nursing

Pituitary Gland: Anterior Pituitary

Is a portal system Consists of two capillary networks or beds connected by a

vein

Lies between the hypothalamus and anterior pituitary

Controlled by the hypothalamus through Hypothalamic-releasing hormones

Stimulates secretion of an anterior pituitary hormone Hypothalamic-inhibiting hormones

Inhibits secretion of an anterior pituitary hormone

Pituitary Gland: Anterior Pituitary

Produces: Thyroid-stimulating hormone (TSH)

Stimulates thyroid to produce thyroid hormones Adrenocorticotropic hormone (ACTH)

Stimulates the adrenal cortex to produce its hormones Gonadotropic hormones (follicle-stimulating hormone-FSH and

luteinizing hormone-LH) Stimulates the gonads (testes in males and ovaries in females) to produce

gametes and sex hormones Prolactin (PRL)

Causes mammary glands in breasts to develop and produce milk and plays a role in carbohydrate and fat metabolism

Growth hormone (GH or somatotropic hormone) Stimulates protein synthesis within cartilage, bone, and muscle Stimulates rate at which amino acids enter cells and protein synthesis

occurs Promotes fat metabolism as opposed to glucose metabolism

Hypothalamus and Pituitary Gland

Thyroid Gland

• Large gland located in the neck, where it is attached to the trachea just below the larynx

• Composed of a large number of follicles• Each a small spherical

structure made of thyroid cells filled with thyroid hormones

• Produces• Triiodothyronin (T3)

• Thyroxine (T4)

• Calcitonin

Thyroid Gland Hormones: T3 and T4

Production The thyroid gland actively acquires iodine, which is

necessary to produce thyroid hormones

Function The function of the thyroid hormones is to increase the

metabolic rate They do not have a single target organ Thyroid hormones stimulate all the cells of the body to

metabolize at a faster rate More glucose broken down More energy utilized

Thyroid Gland Hormones: Calcitonin• Calcitonin is a hormone

secreted by the thyroid gland when the blood calcium level rises

• Functions:• Plays a significant role in

nervous conduction and muscle contraction

• Necessary for coagulation of blood

• Blood calcium level regulation

Diseases Associated with Thyroid Gland

Simple GoiterLack of iodine in diet

Congenital HypothyroidismFailure of thyroid to develop properly

MyxedemaHypothyroidism in adults

HyperthyroidismThyroid gland is overactive

Parathyroid Gland

• One of the four small endocrine gland embedded in the posterior portion of the thyroid gland

• Produces• Parathyroid Hormone (PTH)

• Causes blood phosphate level to decrease and the ionic blood calcium level to increase

Adrenal Glands

• Located on superior portion of kidneys

• Stress of all types prompts hypothalamus to stimulate adrenal glands

• Consists of• Adrenal Medulla (inner

portion)

• Adrenal Cortex (outer portion)

Adrenal Medulla

Under nervous control Hypothalamus initiates nerve impulses to travel to adrenal

medulla

Secretes hormones that bring about body changes that occur when individual reacts to emergency situations

Secretes Epinephrine (adrenaline) and Norepinephrine (noradrenaline)

Achieves same results as sympathetic stimulation Fight-or-Flight Response

Assist sympathetic nerves in providing short-term response to stress

Adrenal Cortex

Provide long-term response to stress

Under control by ACTH (corticotropin-anterior pituitary hormone)

Secretes Mineralocorticoids Glucocorticoids Small amounts of both male and female sex

hormones

Adrenal Cortex Hormones

Mineralocorticoids Regulates salt and water balance, leading to increases in blood volume and pressure Aldosterone is the most important of the mineralocorticoids

Targets the kidneys where it promotes renal absorption of sodium and water, and renal excretion of potassium

Glucocorticoids Regulates carbohydrate, protein, and fat metabolism, leading to an increase in blood

glucose level Cortisol is a major glucocorticoid

Raises blood glucose level Counteracts inflammatory response that leads to pain and swelling of joints in arthritis and

bursitis

Small amounts of both male and female sex hormones Promotes skeletal growth in adolescents Male sex hormones

Stimulates growth of axillary and pubic hair at puberty Helps sustain sex drive (libido) in both men and women

Adrenal Glands

Adrenal Cortex Malfunctions

Addison Disease Cushing Syndrome

Level of adrenal cortex hormone is low, due to

hyposecretion

Level of adrenal cortex hormone is high, due to

hypersecretion

Pancreas

• Long organ that lies transversely in the abdomen between the kidneys and near the duodenum of the small intestines

• Composed of two types of tissues• Exocrine

• Endocrine (Pancreatic Islets)

Pancreas Tissues

Exocrine Tissues Produces and secretes digestive juices that go by the way of the

ducts to the small intestine

Endocrine Tissues (Pancreatic Islets) Produces and secretes the hormones insulin and glucagon

directly into the blood These two hormones help maintain the normal level of glucose in

blood Insulin secreted when blood glucose is high and stimulates the

uptake of glucose by most body cells Glucagon secreted when blood glucose low and stimulates liver

to break down glycogen to glucose and to use fat and protein in preference to glucose as an energy source

Gonads

• Testes (Males)• Ovaries (Females)• Hormonal secretions of

these organs controlled by the hypothalamus and the pituitary gland

Testes

Located in the scrotum

Produces androgens Stimulates growth of penis and testes Brings about and maintains male secondary sex

characteristics that develop during puberty Growth of beard, axillary hair, and pubic hair Prompts larynx and vocal cords to enlarge

Partially responsible for muscular strength in males Stimulates oil and sweat glands in skin

Ovaries

Located in pelvic cavity

Produces Estrogen

Stimulates growth of uterus and vagina Necessary for ovum maturation Responsible for secondary sex characteristics in females

Progesterone With estrogen, both are required for breast development

and for regulation of uterine cycle (monthly menstruation)

Thymus Gland

• Lobular gland that lies in the neck and chest area

• Necessary for development of immunity

• Reaches largest size and is most active during childhood

• When lymphocytes pass complete development in the thymus, they are transformed into thymus-derives lymphocytes

• Lobules of thymus are lines by epithelial cells that secrete thymosins

Pineal Gland

• Located in the brain• Produces melatonin

• Involved in circadian rhythm (pattern of repeated behavior associated with cycles of night and day)• Controlled by internal

timing mechanism called biological clock

Hormones From Other Tissues• Several of the other

endocrine cells and tissues are sometimes referred to as the diffuse endocrine system to reflect the concept that many organs house clusters of cells that secrete hormones

• These organs have no significant function other than to produce hormones

Adipose Tissue

Produces leptin A protein hormone Acts on hypothalamus,

where it signals satiety (that the individual has had enough to eat)

Growth Factors

A number of different types of organs and cells produce peptide growth factors, which stimulate cell division and mitosis

Some are released into the blood and others are diffuse to nearby cells

Some major ones are: Granulocyte and macrophage colony-stimulating factor (GM-CSF) Platelet-derived growth factor Epidermal growth factor and nerve growth factor Tumor angiogenesis factor

Growth Factors

Granulocyte and macrophage colony-stimulating factor (GM-CSF) Secreted by many different tissues Causes bone marrow stem cells to form either granulocyte or

macrophage cells, depending on whether concentration is low or high

Platelet-derived growth factor Released from platelets and from many other cell types Helps wound healing and causes an increase in number of

fibroblasts, smooth muscle cells, and certain cells of nervous system

Growth Factors

• Epidermal growth factor and nerve growth factor• Stimulate the cells

indicated by their name and many others

• Important in wound healing

• Tumor angiogenesis factor• Stimulates formation of

capillary networks

• Released by tumor cells

Prostaglandins

Potent chemical signals produced within cells from arachidonate (a fatty acid)

Acts locally (close to where it is produced)

Effects: In uterus, causes muscle to contract

and may be involved in pain and discomfort during menstruation

Mediates effects of pyrogens (chemicals that are believed to reset temperature regulatory center in the brain)

Some reduces gastric secretion and have been used to treat ulcers

Can be used to lower blood pressure and treat hypertension

Can be used to inhibit platelet aggregation and prevent thrombosis

The Endocrine System• The endocrine system

particularly influences the digestive, cardiovascular, and urinary systems in a way that maintains homeostasis