Introduction to the Endocrine System

Introduction to the Endocrine System

• The endocrine system regulates long-term processes like growth, development, and reproduction
• The endocrine system uses chemical messengers to relay information between cells, similarly to the nervous system
• The key difference between the two systems is how and where these chemical messengers are released

Intercellular Communication

• Direct communication involves the exchange of ions and molecules between adjacent cells through gap junctions and is highly specialized and relatively rare
• Paracrine communication uses chemical signals to transfer information within a single tissue and is the most common form of intercellular communication
• Synaptic communication, used by the nervous system, occurs between neurons across synaptic clefts, and the chemical messenger is called a neurotransmitter
• Endocrine communication happens when endocrine cells release hormones into the bloodstream, affecting many tissues and organs simultaneously
• Target cells are cells with specific receptors needed to bind and interpret hormonal messages

Hormones

• Hormones can be divided into amino acid derivatives (smallest), peptide hormones, and lipid derivatives (largest)
• Hormones circulate either freely or bound to special carrier proteins in the bloodstream
• Free hormones remain functional for less than an hour, diffusing out the bloodstream to bind to receptors on target cells, and are then broken down by the liver, kidneys or enzymes
• Bound hormones remain in circulation much longer because they are attached to carrier proteins, resulting in a substantial reserve of hormones in the bloodstream
• A hormone receptor is a protein molecule that binds strongly to a particular hormone
• Different tissues possess different combinations of hormone receptors

Control of Endocrine Functions

• Hormones can cause three possible changes in target cells: stimulation of enzyme or structural protein synthesis, increasing or decreasing the rate of synthesis of products, or turning a process within the cell up/down or on/off
• Endocrine reflexes allow the endocrine system to respond to changes inside and outside the body, directing long-term activities, and are usually controlled by negative feedback mechanisms
• The hypothalamus controls the endocrine system at the highest level, working closely with the pituitary gland with three mechanisms:
• Production of ADH and oxytocin for posterior pituitary release
• Production of regulatory hormones controlling the anterior pituitary
• Sending commands to the adrenal medulla for epinephrine (E) and norepinephrine (NE) release via the nervous system

Pituitary Gland

• The pituitary gland releases nine important hormones under the direction of the hypothalamus
• The posterior lobe of the pituitary gland along with the hypothalamus produces antidiuretic hormone (ADH) and oxytocin (OXT)
• ADH decreases water lost in urine, returning it to the blood
• OXT stimulates uterine contractions and aids milk ejection during suckling, also affecting social bonding
• The anterior lobe of the pituitary gland produces hormones that activate endocrine glands or support other organs
• The hypothalamus instructs the anterior lobe via regulatory hormones, divided into releasing hormones (RH) which stimulate synthesis and secretion of hormones, and inhibiting hormones (IH) which prevent synthesis and secretion

Thyroid Gland

• The thyroid gland, located in the neck anterior to the thyroid cartilage of the larynx, consists of two lobes containing many thyroid follicles made up of hollow spheres of cuboidal epithelium that secrete thyroid hormones
• Thyroid-stimulating hormone (TSH) is released by the anterior pituitary gland and stimulates the production of thyroid hormones
• The gland produces thyroxine (T4), containing four iodide ions, and triiodothyronine (T3), containing three iodide ions
• Thyroid hormones cause a body-wide calorigenic effect, increasing energy consumption and heat generation
• In children, thyroid hormones are essential for normal skeletal, muscular, and nervous system development
• C (clear) cells, also known as parafollicular cells, produce calcitonin (CT)

Parathyroid Gland

• Four parathyroid glands are embedded in the posterior surface of the thyroid gland, weighing 1.6 g altogether
• Parathyroid hormone (PTH), or parathormone, is produced by parathyroid cells in response to low calcium concentrations and acts as an antagonist for calcitonin:
• It stimulates osteoclasts and inhibits osteoblasts - releasing calcium from bone
• Enhances reabsorption of calcium at the kidneys and reduces urinary losses
• Stimulates formation and secretion of calcitriol by the kidneys - which enhances calcium absorption in the digestive tract

Adrenal Glands

• Adrenal glands lie along the superior border of each kidney and are subdivided into the adrenal cortex and adrenal medulla
• The adrenal cortex manufactures steroid hormones called corticosteroids, while the inner adrenal medulla's secretory activities are controlled by the sympathetic division of the ANS, producing epinephrine and norepinephrine
• The adrenal cortex produces aldosterone, cortisol, and androgens (male hormones)
• Aldosterone stimulates the conservation of sodium ions and elimination of potassium ions and increases the sensitivity of salt receptors in taste buds
• Cortisol increases resistance to stress and blood glucose levels while showing anti-inflammatory effects
• Androgens, produced in both males and females, contribute to the sex drive in females
• The adrenal medulla contains two types of secretory cells:
• One produces epinephrine (adrenaline) - 75% to 80% of medullary secretions
• The other produces norepinephrine (noradrenaline) - 20% to 25% of medullary secretions
• Activation of the adrenal medullae triggers mobilization of energy reserves in skeletal muscles for ATP production and promotes the fight-or-flight response of the sympathetic division

Pineal Gland

• The Pineal Gland lies in the posterior portion of the roof of the third ventricle
• It synthesizes melatonin which influences circadian rhythms, the day/night and sleep/wake cycles

Pancreas

• The pancreas lies between the inferior border of the stomach and the small intestine, contains both exocrine and endocrine cells
• Exocrine cells secrete products into ducts that lead to the inner or outer surface of an organ, while endocrine cells secrete products into the bloodstream
• The Exocrine Pancreas consists of clusters of gland cells called pancreatic acini and their attached ducts, taking up roughly 99% of pancreatic volume
• Gland and duct cells secrete an alkaline, enzyme-rich fluid called digestive juice which reaches the digestive tract through a network of secretory ducts
• The Endocrine Pancreas consists of cells that form clusters known as pancreatic islets:
• Alpha cells produce glucagon
• Beta cells produce insulin
• Pancreatic islets are associated with capillaries
• Beta cells secrete insulin stimulates transport of glucose across plasma membranes, into the cells of the body and out of the bloodstream which maintains normal blood glucose for normal activities
• Alpha cells release glucagon stimulating glucose release by liver which increases blood glucose

Blood Glucose Levels and Homeostasis

• When blood glucose levels rise due to food consumption, beta cells release insulin, prompting glucose transport across plasma membranes into body cells, lowering the level back to normal
• Conversely, when blood glucose levels drop, alpha cells release glucagon, leading to glucose release by the liver, thus increasing levels back to normal
• Insulin accelerates glucose uptake into cells, enhances ATP production, and stimulates triglyceride formation in adipose tissue
• Glucagon mobilizes energy reserves, affects target cells by stimulating glycogen breakdown in skeletal muscle and liver cells, stimulates breakdown of triglycerides in adipose tissue, and stimulates glucose production in the liver (gluconeogenesis)

Diabetes

• Hyperglycemia is abnormally high glucose levels in the blood
• Diabetes is a condition characterized by glucose concentrations high enough to overwhelm the reabsorption capabilities of the kidneys resulting in glucose appearance in the urine and excessive urine volume (polyuria)
• There are two types of diabetes:
• Type I (insulin-dependent) diabetes is categorized by inadequate insulin production by pancreatic beta cells and accounts for only around 5%-10% of cases that require insulin and is developed in childhood
• Type II (non-insulin-dependent) diabetes is the most common form where tissues do not respond properly to insulin, a condition known as insulin resistance and is associated with obesity
• The kidneys are responsible for filtering and cleaning up" the blood with untreated, or poorly managed, diabetes overwhelming the filtration abilities of the kidneys due to excess glucose
• This damages the kidneys and leads to body-wide complications due to poor blood quality, including:
• Diabetic nephropathy -Retinal Damage: Diabetic retinopathy
• Early Heart Attacks
• Peripheral problems

Endocrine Tissues of Other Systems

• Many organs of other body systems also have secondary endocrine functions
• Intestines produce hormones important to coordination of digestive activities
• Kidneys produce calcitriol (promoting calcium absorption) and erythropoietin (stimulating red blood cell production) and also produce the enzyme renin - which creates a cascade of activity leading to increased blood volume and pressure
• The heart produces hormones that result in a reduction in blood volume and pressure
• The thymus produces thymosins ( a blend of thymic hormones) that helps to develop and maintain normal immune defenses
• Testes produce androgens in interstitial cells with testosterone being the most important male hormone
• Ovaries produce estrogens and progesterone

Hormone Interactions and Aging

• Aging and Hormone Production causes few functional changes
• Decline in concentration of:
• Growth hormone
• Reproductive hormones