Chapter 16 – The Endocrine System

Endocrine system

• Along with the nervous system, is responsible for homeostasis – Nervous system – fast-acting; short-term effects– Endocrine system – slow-acting; long-term effects

• Utilizes chemical messengers called hormones• Gland types – Exocrine – have ducts – Endocrine – ductless when mature

• Secrete into surrounding fluid and then enters bloodstream • Carried body-wide; affects distant tissues

Endocrine System

Structural classification of hormones

• Amino acid derivatives– Hydrophilic

• Peptide – Hydrophilic

• Lipid derivatives– Hydrophobic– Steroids; sex hormones – Synthesized from cholesterol

Hydrophilic hormone receptors • Water soluble hormones can no

diffuse through plasma membrane; receptors are on cell surface – Transmembrane proteins – span

entire membrane

• Hormone binds to receptor; receptor changes shape

• Shape change activates G protein – Converts to an active form – Starts chain reaction that ultimately

activates kinases • Kinases attach phosphate groups to

molecules – This either activates the molecule or

inactivates it, depending on the specific molecule

Hydrophobic hormone receptors • Fat soluble

hormones can diffuse through plasma membrane – Receptors are

located in cell’s interior (the cytoplasm)

• Hormone-receptor complex enters nucleus and binds to DNA to affect gene expression

Control of hormone release

• Endocrine reflexes – Humoral • Detects and responds to change in concentration of

certain molecules

– Neural • Signal from nervous system stimulates secretion

– Hormonal • Some hormones target other endocrine glands

– Tropic hormones (sometimes called trophic hormones)

• All can be modified by nervous system

Pituitary gland (hypophysis)

• Located in the sella turcica of the sphenoid bone

• Attached to the hypothalamus by a stalk called the infundibulum

• 2 lobes – Posterior

(neurohypophysis)– Anterior

(adenohypophysis)

Pituitary gland hormones

Posterior pituitary (neurohypophysis)

• Composed of neural tissue; stores and releases hormones produced by the hypothalamus

• Antidiuretic hormone (ADH)– Increases water reabsorption

• Aquaporins placed in the collecting ducts of the kidneys • Increases blood volume/pressure

• Oxytocin (OXT)– Stimulates smooth muscle contractions

• Uterine contractions during childbirth• Milk letdown in lactating females • Durng sexual arousal in males and females

Anterior pituitary (adenohypophysis)

• Release or inhibition is controlled by hypothalamus

• Growth hormone (GH)– Bones and skeletal muscles the major target

tissues • Melanocyte-stimulating hormone (MSH)– Stimulates melanocytes in skin; contributes to skin

pigmentation

Anterior pituitary (adenohypophysis)

• Tropic hormones – Thyroid-stimulating hormone (TSH)• Thyroid gland

– Adrenocorticotropic hormone (ACTH)• Adrenal cortex

– Follicle-stimulating hormone (FSH)• Gonads to produce gametes

– Leutinizing hormone (LH)• Stimulation of gonadal hormones

Thyroid gland

• On trachea, inferior to larynx

• Butterfly-shaped – 2 lobes

connected by isthmus

Thyroid gland microscopic anatomy

• Follicle cells produce thyroglobin (a glycoprotein) – Simple cuboidal

epithelium – Central cavity of follicles is

filled with colloid – thyroglobulin with iodine atoms • Forms T3 and T3 hormones

• Parathyroid cells (C cells) produce calcitonin

Thyroid hormones

• Thyroxine (T4) and Triiodothyronine (T3)– #4 and #3 refer to number of iodine atoms attached – Affects most cell in body

• Increases basal metabolic rate and heat production • Aids in maintenance of blood pressure • Helps regulate growth and development

• Calcitonin – Lowers blood calcium levels

• Inhibits osteoclasts

Parathyroid glands • Usually 4 on posterior

thryoid • Parathyroid hormone– Produced by chief cells – Increases blood calcium levels

• Antagonistic to calcitonin • Stimulates osteoclasts• Increases reabsorption of

calcium by kidneys – Kidney also converts vitamin D

to its active form, which aids in calcium absorption in digestive system

Adrenal glands

• Also called suprarenal glands due to their location

• Adrenal cortex (superficial) and adrenal medulla (deep)

Adrenal cortex microscopic anatomy

• 3 layers/zones (superficial to deep)– Zona glomerulosa

• Produces mineralcorticoids

– Zona fasciculata • Produces mainly

glucocorticoids, some gonadocorticoids

– Zona reticularis • Produces mainly

gonadocortcoids, some glucocorticoids

Adrenal cortex hormones • Mineralcorticoids

– Regulation of electrolytes in extracellular fluid– Aldosterone

• Stimulates sodium reabsorption in distal convoluted tubules of kidneys

• Glucocorticoids – Influence energy metabolism – Regulates blood glucose and blood pressure levels – Cortisol (hydrocortisone)

• Gluconeogenesis – formation of glucose from non-carbohydrate sources

– Excess of glucocorticoids results in oversuppression of inflammatory and anti-immune responses

• Gonadocorticoids– Androgens

• Male sex hormones • Converted to testosterone in males; estrogen in females

– Estrogens

Adrenal medulla hormones

• Autonomic nervous system• Epinephrine and norepinephrine – “Fight or flight” response – Blood glucose levels rise, increases heart rate,

blood directed to cardiac and skeletal muscles – Epinephrine serves as a blood vessel contrictor

and a bronchiole dilator

Pineal gland

• Located at roof of third ventricle in brain

• Melatonin – Concentration rises

and falls within a 24 hour time period • Peaks at night –

causes sleepiness

– Affects biological clock/circadian rhythms

Pancreas• Posterior to stomach • Microscopic anatomy– Acinar cells

• Produce pancreatic secretions for digestive system

– Islets of Langerhans• Alpha cells – produce

glucagon • Beta cells – produce

insulin

Pancreatic hormones• Glucagon – Increases blood glucose levels

• Glycogenolysis – Breaks down glycogen to glucose

• Gluconeogenesis – Synthesizes glucose from non-carbohydrate sources

• Releases glucose from the liver

• Insulin – Decreases blood glucose levels (antagonistic to glucagon)

• Increases glucose transport into cells (primarily adipose and muscle)– Uses glucose for ATP production – Stores as glycogen – Excess stored as fat

Thymus

• Overlays heart • Produces several

peptide hormones – Thymosin, thymopoetin,

thymic factor – Development/maturation

of T lymphocytes

Gonadal hormones • Hormones are steroids (fat-soluble)• Ovaries (female)– Estrogen

• Secondary sex characteristics • Maturation of reproductive organs

– Progesterone • Along with estrogen:

– Breast development and menstrual cycle regulation

• Testes (male)– Testosterone

• Secondary sex characteristics • Maturation of reproductive organs • Production of sperm