Endocrine System - Adrenal Glands, Pineal Gland, and Pancreas
Adrenal Glands
Located superior to each kidney, retroperitoneal.
Pyramid shaped and approximately 3 cm in height, 5 cm in length, and up to 1 cm thick.
Heavily vascularized.
Subdivided into:
Outer adrenal cortex:
Makes up 80-90% of the gland's total mass.
Stores lipids (cholesterol and fatty acids) used for hormone synthesis.
Manufactures more than 20 steroid hormones, collectively called corticosteroids, which are vital for regulating metabolic processes.
Inner adrenal medulla:
Makes up 10-20% of the adrenal gland.
Produces catecholamine hormones: epinephrine and norepinephrine.
Secretory activities are controlled by the sympathetic nervous system, preparing the body for "fight or flight".
Metabolic changes persist for several minutes, allowing for sustained response to stress.
Adrenal Cortex
Accounts for the bulk of the adrenal gland.
Produces hormones essential for life; damage to the adrenal cortex leads to deficiency in corticosteroids.
Subdivided into three distinct regions, each producing different corticosteroids:
Zona glomerulosa
Zona fasciculata
Zona reticularis
Zona Glomerulosa
The thin, outer region of the adrenal cortex, directly under the capsule.
Produces mineralocorticoids, primarily aldosterone, which regulates blood pressure and electrolyte balance.
Zona Fasciculata
The thick, middle region of the adrenal cortex that consists of cells arranged in columns.
Produces glucocorticoids, mainly cortisol (hydrocortisone), in response to ACTH from the pituitary gland.
Glucocorticoids influence glucose metabolism and have anti-inflammatory effects.
Have an inhibitory effect on the production of:
Corticotropin-releasing hormone (CRH) in the hypothalamus, providing negative feedback.
ACTH in the anterior pituitary, also contributing to negative feedback.
Glucocorticoids
Affect glucose metabolism, immune response, and stress response.
Adipose tissue breaks down triglycerides (lipid catabolism) to provide energy.
Skeletal muscle breaks down proteins (protein catabolism), releasing amino acids.
Cells utilize fatty acids and amino acids for energy instead of glucose, conserving glucose for the brain.
Accelerates glucose synthesis (Gluconeogenesis) in the liver.
Considered a stress hormone due to its role in the body’s response to stressful situations.
Show anti-inflammatory effects by reducing immune cell activity.
Suppress the immune system, which can be therapeutically useful but also increases susceptibility to infection.
Zona Reticularis
The innermost region of the adrenal cortex, adjacent to the adrenal medulla.
Produces androgens (sex hormones) under stimulation by ACTH.
Androgen production is insignificant in males, as the testes produce much larger amounts.
In females, it promotes muscle mass, blood cell formation, and sex drive but can also contribute to masculinization if overproduced.
Adrenal Medulla
The central part of the adrenal gland, surrounded by the adrenal cortex.
A neuroendocrine structure innervated by sympathetic preganglionic fibers.
Produces catecholamines: Epinephrine (Epi) and Norepinephrine (Norepi), which are involved in the "fight or flight" response.
In muscle and liver, they trigger the breakdown of glycogen into glucose (glycogenolysis), providing quick energy.
In the liver, new glucose molecules are synthesized (gluconeogenesis).
In adipose tissue, triglycerides (TG) are broken down into free fatty acids (FFA).
Fatty acids are released into the bloodstream and used by other tissues for ATP production, sparing glucose for the brain (Glucose Sparing Effect).
In the heart, they increase heart rate and force of contraction to enhance blood flow.
Effects of Epinephrine and Norepinephrine
Increased heart rate, force of contraction, and blood pressure (BP) to enhance oxygen delivery to tissues.
Lipid breakdown and release, providing alternative energy sources.
Glycogen breakdown to increase blood glucose levels.
Increased glucose synthesis to maintain blood glucose levels during stress.
Pineal Gland
A small endocrine gland in the brain.
Attached to the roof of the third ventricle.
Contains pinealocytes that synthesize the hormone melatonin.
Collaterals from visual pathways enter the pineal gland and affect pinealocyte activity, synchronizing melatonin production with light-dark cycles.
Melatonin production is lowest during daylight hours and highest during nighttime hours, influencing sleep patterns.
Functions of Melatonin
Regulates sleep-wake cycles (circadian rhythms).
May be involved in preventing sexual maturation until puberty.
The pineal gland shrinks and melatonin production declines greatly by the end of puberty.
Pineal tumors can cause premature onset of puberty, demonstrating the hormone's role in sexual development.
Influences circadian rhythms (sleep cycles), affecting mood, behavior, and reproductive physiology.
Pancreas
A mixed gland with both exocrine and endocrine functions.
Lies along the inferior border of the stomach, between the first segment of the small intestine and the spleen.
Contains exocrine and endocrine cells arranged in distinct regions.
Exocrine portion:
Takes up 99% of the pancreas.
Consists of acinar cells that secrete digestive enzymes.
Secretes an alkaline, enzyme-rich fluid into the digestive tract to aid digestion.
Endocrine Pancreas
Consists of cells that form clusters known as pancreatic islets, or islets of Langerhans.
Alpha cells produce glucagon in response to low blood glucose, raising blood glucose levels.
Beta cells produce insulin in response to high blood glucose, lowering blood glucose levels.
Delta cells produce somatostatin, a peptide hormone identical to GH–IH (Growth Hormone–Inhibiting Hormone), inhibiting both insulin and glucagon secretion.
F cells / PP Cells secrete pancreatic polypeptide (PP), which regulates pancreatic exocrine and endocrine secretions.
Pancreas: Blood Glucose Control
Maintains glucose homeostasis by controlling blood glucose levels via the secretion of insulin and glucagon.
When glucose levels rise (hyperglycemia):
Beta cells secrete insulin, stimulating:
Transport of glucose into cells to lower blood glucose levels.
Increased glucose use (to make ATP) for energy production.
Increased conversion of glucose to glycogen for storage in the liver and muscles.
Increased amino acid uptake and utilization (to make proteins) for growth and repair.
Increased synthesis of triglycerides in adipose tissue for long-term energy storage.
When glucose levels decline (hypoglycemia):
Alpha cells release glucagon, stimulating:
Breakdown of glycogen into glucose in skeletal muscle and liver to raise blood glucose levels.
Breakdown of triglycerides into fatty acids in adipose tissue to provide alternative fuel sources.
Gluconeogenesis in the liver, synthesizing glucose from amino acids and other precursors.
Synthesis of glucose from amino acids when glycogen stores are depleted.
Effects of Insulin
Increased rate of glucose transport into target cells, especially muscle and adipose tissue.
Increased rate of glucose utilization and ATP generation, promoting energy production.
Increased conversion of glucose to glycogen, storing glucose for later use.
Increased amino acid absorption and protein synthesis, supporting growth and repair processes.
Increased triglyceride synthesis in adipose tissue, storing excess energy as fat.
Effects of Glucagon
Increased breakdown of glycogen to glucose (in liver, skeletal muscle), raising blood glucose levels.
Increased breakdown of fat to fatty acids (in adipose tissue), providing alternative fuel sources.
Increased synthesis and release of glucose (by the liver) through gluconeogenesis, maintaining blood glucose levels.
Diabetes Mellitus
A group of metabolic disorders characterized by hyperglycemia over a prolonged period.
Characterized by glucose concentrations high enough to overwhelm the reabsorption capabilities of the kidneys, leading to glucosuria.
Hyperglycemia = abnormally high glucose levels in the blood due to defects in insulin secretion, insulin action, or both.
In DM, glucose appears in the urine (glucosuria), and urine volume generally becomes excessive (polyuria) due to the osmotic effect of glucose.
Type 1 Diabetes Mellitus
(Insulin dependent), formerly known as juvenile diabetes.
Characterized by inadequate insulin production by beta cells, leading to absolute insulin deficiency.
Patients require exogenous insulin to survive and usually require multiple injections daily or continuous infusion through an insulin pump or other device.
This form of diabetes accounts for only around 5%–10% of cases.
Often develops in childhood or adolescence but can occur at any age.
May be associated with autoimmune destruction of beta cells, triggered by genetic or environmental factors.
Obesity is usually not present at diagnosis.
Type 2 Diabetes Mellitus
(Non-insulin dependent), formerly known as adult-onset diabetes.
Is the most common form of diabetes mellitus, accounting for 90-95% of cases.
Most people with this form of diabetes produce normal or even elevated amounts of insulin, at least initially, but their tissues do not respond properly, a condition known as insulin resistance.
Insulin resistance is often associated with obesity, physical inactivity, and genetic factors.
Type 2 diabetes is strongly associated with obesity and is becoming increasingly prevalent in younger populations.
Weight loss through diet and exercise can improve insulin sensitivity and be an effective treatment.
Complications of Untreated or Poorly Managed Diabetes Mellitus
Chronic hyperglycemia leads to various microvascular and macrovascular complications.
Kidney damage (diabetic nephropathy) leading to renal failure and the need for dialysis or kidney transplant.
Retinal damage (diabetic retinopathy) leading to blindness or significant vision impairment.
Early heart attacks and stroke (cardiovascular disease) due to accelerated atherosclerosis.
Peripheral nerve damage (diabetic neuropathy) leading to pain, numbness, and increased risk of foot ulcers.
Poor circulation combined with slow wound healing and numbness lead to frequent amputations due to diabetic foot ulcers and infections.