Endocrine System

Overview of the Endocrine System

The endocrine system is composed of glands and tissues that secrete hormones into the bloodstream, regulating distant target organs. Unlike the nervous system, which uses neurotransmitters and acts locally and rapidly, the endocrine system acts more slowly but has widespread and long-lasting effects.

• Hormones: Chemical messengers secreted into the blood, affecting distant targets.

• Types of hormone signaling:

  • Endocrine: Distant targets via bloodstream.

  • Paracrine: Affect nearby cells.

  • Autocrine: Affect the secreting cell itself.

• Major endocrine organs: Anterior pituitary, thyroid, parathyroid, adrenal cortices, thymus, endocrine pancreas, testes/ovaries.

• Neuroendocrine organs: Hypothalamus, adrenal medulla, pineal gland (secrete neurohormones).

• Hormone types:

  • Amino acid–based hormones: Usually hydrophilic, bind to plasma membrane receptors, use second messengers (e.g., cAMP).

  • Steroid hormones: Hydrophobic, bind to intracellular receptors, directly affect gene transcription.

• Hormone transport: Free (unbound) or bound to plasma proteins.

• Hormone effects: Alter gene transcription, enzyme activity, cell division, membrane permeability, and secretion of other hormones.

• Regulation of secretion:

  • Hormonal stimuli: Hormones stimulate release of other hormones.

  • Humoral stimuli: Changes in blood levels of ions/nutrients.

  • Neural stimuli: Nerve fibers stimulate hormone release.

• Half-life: Time for hormone concentration to decrease by half.

• Feedback: Most hormones regulated by negative feedback loops.

The Hypothalamus and Pituitary Gland

The hypothalamus links the nervous and endocrine systems, controlling the pituitary gland, which is divided into anterior and posterior lobes.

• Posterior pituitary (neurohypophysis): Stores and releases neurohormones produced by the hypothalamus.

  • Antidiuretic hormone (ADH): Promotes water retention in kidneys.

  • Oxytocin: Stimulates uterine contractions and milk ejection.

• Anterior pituitary (adenohypophysis): True endocrine gland, controlled by hypothalamic releasing/inhibiting hormones via the hypothalamic-hypophyseal portal system.

  • Thyroid-stimulating hormone (TSH): Stimulates thyroid gland.

  • Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex.

  • Prolactin (PRL): Stimulates milk production.

  • Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH): Gonadotropins; regulate reproductive functions.

  • Growth hormone (GH): Stimulates growth via IGF; disorders include pituitary dwarfism and gigantism.

The Thyroid and Parathyroid Glands

The thyroid gland regulates metabolism, growth, and development, while the parathyroid glands regulate blood calcium levels.

• Thyroid structure: Follicles filled with colloid, lined by follicle cells (produce thyroid hormones), and parafollicular cells (produce calcitonin).

• Thyroid hormones:

  • Triiodothyronine (T3): 3 iodine atoms.

  • Thyroxine (T4): 4 iodine atoms.

  • Regulate metabolic rate, thermoregulation, growth, and synergize with sympathetic nervous system.

  • Production regulated by TRH (hypothalamus) and TSH (anterior pituitary).

• Disorders:

  • Hyperthyroidism: Weight loss, heat intolerance, goiter, exophthalmos.

  • Hypothyroidism: Weight gain, cold intolerance, goiter, bradycardia.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and renal retention.

• Calcitonin: Inhibits osteoclasts (high concentrations), regulates bone remodeling.

The Adrenal Glands

Each adrenal gland consists of an outer cortex and inner medulla, producing steroid hormones and catecholamines, respectively.

• Adrenal cortex:

  • Zona glomerulosa: Mineralocorticoids (e.g., aldosterone) – regulate sodium, potassium, and acid-base balance.

  • Zona fasciculata & reticularis: Glucocorticoids (e.g., cortisol) – stress response, gluconeogenesis, anti-inflammatory.

  • Androgenic steroids: Affect reproductive organs and other tissues.

• Adrenal medulla: Chromaffin cells secrete catecholamines (mainly epinephrine) in response to sympathetic stimulation.

The Endocrine Pancreas

The pancreas has both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Pancreatic islets:

  • Alpha (α) cells: Secrete glucagon – increases blood glucose via glycogenolysis, gluconeogenesis, and fat/protein breakdown.

  • Beta (β) cells: Secrete insulin – lowers blood glucose by promoting cellular uptake and storage as glycogen/fat.

• Disorders:

  • Hypoglycemia: Excess insulin; symptoms include confusion, weakness, coma.

  • Hyperglycemia: Insufficient/ineffective insulin; causes include type 1 and type 2 diabetes mellitus.

Other Endocrine Glands and Hormone-Secreting Tissues

• Pineal gland: Secretes melatonin (regulates sleep/wake cycle).

• Thymus: Produces thymosin and thymopoietin (T cell maturation).

• Gonads:

  • Testes: Produce testosterone (androgenic and anabolic effects).

  • Ovaries: Produce estrogens (female sex characteristics, menstrual cycle) and progesterone (pregnancy support).

• Adipose tissue: Produces leptin (induces satiety).

• Cardiac muscle: Produces atrial natriuretic peptide (ANP; promotes vasodilation and water loss).

• Kidneys: Produce erythropoietin (stimulates RBC production) and renin (activates aldosterone system).

Examples of Endocrine Control of Physiological Variables

• Fluid balance: Regulated by ADH, aldosterone (increase retention), and ANP (promotes loss).

• Metabolic homeostasis: Maintained by thyroid hormones, GH, glucagon, insulin, catecholamines.

• Stress response: Involves HPA axis hormones, GH, catecholamines, angiotensin-II, glucagon.

The Heart

Overview of the Heart

The heart is a muscular organ that pumps blood through the pulmonary and systemic circuits, ensuring oxygen and nutrient delivery throughout the body.

• Two atria (receive blood) and two ventricles (pump blood).

• Right side: Pulmonary pump (to lungs, deoxygenated blood).

• Left side: Systemic pump (to body, oxygenated blood).

• Secretes atrial natriuretic peptide (ANP).

Heart Anatomy and Blood Flow Pathway

• Heart wall layers:

  • Pericardium: Fibrous (outer) and serous (parietal and visceral) layers, with pericardial cavity.

  • Myocardium: Cardiac muscle tissue.

  • Endocardium: Inner lining.

• Coronary circulation: Supplies myocardium via right and left coronary arteries; drained by coronary veins into coronary sinus.

• Great vessels: Venae cavae, pulmonary trunk, pulmonary veins, aorta.

• Valves:

  • Atrioventricular (AV) valves: Tricuspid (right), mitral (left); prevent backflow into atria.

  • Semilunar (SL) valves: Pulmonary and aortic; prevent backflow into ventricles.

• Septae: Interatrial and interventricular septa separate chambers.

Cardiac Muscle Tissue Anatomy and Electrophysiology

• Cardiac muscle cells: Short, branched, striated, joined by intercalated discs.

• Pacemaker cells: Autorhythmic, generate action potentials spontaneously.

• Action potential phases (contractile cells):

  1. Rapid depolarization (Na+ influx)

  2. Initial repolarization (K+ efflux)

  3. Plateau (Ca2+ influx, K+ efflux)

  4. Repolarization (continued K+ efflux)

• Cardiac conduction system:

  1. Sinoatrial (SA) node: Main pacemaker

  2. Atrioventricular (AV) node: Backup pacemaker

  3. AV bundle → bundle branches → Purkinje fibers

• Electrocardiogram (ECG): Measures electrical activity of the heart.

Mechanical Physiology of the Heart: The Cardiac Cycle

• Cardiac cycle: Sequence of events in one heartbeat; includes diastole (relaxation) and systole (contraction).

• Phases:

  1. Ventricular filling

  2. Isovolumetric contraction

  3. Ventricular ejection

  4. Isovolumetric relaxation

• Heart sounds:

  • S1: AV valves close (start of isovolumetric contraction)

  • S2: SL valves close (start of isovolumetric relaxation)

• Volumes:

  • End-diastolic volume (EDV): Volume after filling

  • End-systolic volume (ESV): Volume after ejection

Cardiac Output and Regulation

• Cardiac output (CO): Volume of blood pumped by each ventricle per minute.

  • Formula:

  • Where = heart rate, = stroke volume

• Stroke volume (SV):

• Determinants of SV:

  • Preload: Degree of stretch of cardiac muscle

  • Contractility: Strength of contraction

  • Afterload: Resistance ventricles must overcome

• Regulation:

  • Chronotropic agents (affect HR): ANS, endocrine system

  • Inotropic agents (affect contractility): Epinephrine, norepinephrine (positive); acetylcholine (negative chronotrope)

Blood Vessels and Circulation

Overview of Arteries and Veins

The vascular system consists of arteries, capillaries, and veins, each with distinct structure and function.

• Arteries: Carry blood away from the heart; thick walls, prominent tunica media, elastic laminae.

• Veins: Return blood to the heart; thinner walls, larger lumens, contain valves.

• Capillaries: Exchange vessels; single layer of endothelium.

• Blood vessel layers:

  • Tunica intima: Innermost, endothelium.

  • Tunica media: Smooth muscle, elastic fibers.

  • Tunica externa: Connective tissue.

• Types of arteries:

  • Elastic (conducting) arteries

  • Muscular (distributing) arteries

  • Arterioles (resistance vessels)

• Venules: Smallest veins, drain capillaries.

• Anastomoses: Connections between vessels.

Physiology of Blood Flow (Hemodynamics)

• Blood pressure (BP): Force exerted by blood on vessel walls.

• Blood flow: Directly proportional to pressure gradient, inversely proportional to resistance.

  • Formula:

  • Where = flow, = pressure gradient, = resistance

• Factors affecting resistance:

  • Vessel radius (decreased radius increases resistance)

  • Vessel length (longer = more resistance)

  • Blood viscosity (thicker = more resistance)

  • Obstructions

• Mean arterial pressure (MAP):

  • Formula:

• Venous return: Aided by valves, skeletal muscle pump, respiratory pump, and venoconstriction.

Maintenance of Blood Pressure

• Short-term regulation:

  • Sympathetic NS: Increases BP (vasoconstriction, increased CO)

  • Parasympathetic NS: Decreases BP (decreased CO)

  • Baroreceptor and chemoreceptor reflexes

  • Hormones: Epinephrine, norepinephrine, thyroid hormone (increase CO); angiotensin-II (increase resistance)

• Long-term regulation:

  • Urinary system: Adjusts blood volume via water excretion/retention

  • Hormones: Aldosterone, ADH, angiotensin-II (increase volume); ANP (decreases volume)

• Hypertension: Systolic > 140 mmHg and/or diastolic > 90 mmHg

• Hypotension: Systolic < 90 mmHg and/or diastolic < 60 mmHg

Capillaries and Tissue Perfusion

• Capillary exchange: Substances move by diffusion, osmosis, and transcytosis.

• Types of capillaries:

  • Continuous: Least leaky

  • Fenestrated: More permeable

  • Sinusoidal: Most leaky

• Autoregulation: Local control of blood flow to match tissue needs.

• Myogenic mechanism: Vessel constriction/dilation in response to stretch.

• Metabolic regulation: Arterioles dilate with decreased O2, increased CO2 and H+.

Capillary Pressures and Water Movement

• Filtration: Movement of water out of capillaries due to hydrostatic pressure.

• Osmotic pressure: Draws water into capillaries; higher in blood due to plasma proteins (colloid osmotic pressure).

• Net filtration pressure (NFP):

  • Formula:

  • Where = hydrostatic pressure, = osmotic pressure, = capillary, = interstitial fluid

• Edema: Excess interstitial fluid due to imbalance in filtration/reabsorption.

Anatomy of the Systemic Arteries and Veins

• Major arteries:

  • Aorta (ascending, arch, descending)

  • Brachiocephalic, left common carotid, left subclavian (aortic arch branches)

  • Common iliac arteries (terminal branches of abdominal aorta)

  • Subclavian → axillary → brachial → radial/ulnar (upper limb)

  • External iliac → femoral → popliteal → tibial (lower limb)

• Major veins:

  • Brachiocephalic veins → superior vena cava (drain head, neck, upper limbs)

  • Common iliac veins → inferior vena cava (drain lower limbs, pelvis)

  • Jugular veins (drain head/brain)

  • Hepatic portal vein (drains digestive organs to liver)

  • Superficial veins: cephalic, basilic, median antebrachial (upper limb); great/small saphenous (lower limb)

  • Deep veins: brachial, axillary, femoral

Summary Table: Major Hormones and Their Functions

Additional info: Some details, such as the exact values for blood pressures and the full list of all arteries/veins, were inferred or summarized for clarity and completeness.