Endocrine System

Overview of Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate body functions. Major glands include:

• Hypothalamus: Regulates pituitary gland, links nervous and endocrine systems.

• Pituitary Gland (Anterior & Posterior): Master gland controlling other endocrine glands.

• Pineal Gland: Secretes melatonin, regulates sleep cycles.

• Thyroid Gland: Produces hormones regulating metabolism.

• Parathyroid Glands: Regulate calcium levels.

• Thymus: Involved in immune system development (especially in children).

• Adrenal Glands: Produce stress hormones and regulate metabolism.

• Pancreas: Regulates blood glucose.

• Ovaries/Testes: Produce sex hormones.

Anterior Pituitary Hormones

• TSH (Thyroid-Stimulating Hormone): Stimulates thyroid to release T3 and T4.

• ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to release cortisol.

• GH (Growth Hormone): Stimulates growth of bones and muscles.

• FSH (Follicle-Stimulating Hormone): Stimulates gamete (egg/sperm) production.

• LH (Luteinizing Hormone): Triggers ovulation and stimulates testosterone production.

• Prolactin: Stimulates milk production in mammary glands.

Posterior Pituitary Hormones

• ADH (Antidiuretic Hormone): Promotes water reabsorption by kidneys, reducing urine output.

• Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding.

Adrenal Gland Structure and Hormones

• Adrenal Cortex:

  • Zona glomerulosa: Produces aldosterone (regulates sodium and potassium).

  • Zona fasciculata: Produces cortisol (stress response, metabolism).

  • Zona reticularis: Produces androgens (sex hormones).

• Adrenal Medulla: Produces epinephrine and norepinephrine (fight-or-flight response).

Pancreatic Hormones

• Insulin: Lowers blood glucose by promoting cellular uptake.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown.

Common Endocrine Disorders

• Hyperthyroidism/Hypothyroidism: Over/underproduction of thyroid hormones.

• Diabetes Mellitus: Insulin deficiency or resistance, leading to high blood glucose.

• Diabetes Insipidus: ADH deficiency, causing excessive urination.

• Hyperparathyroidism/Hypoparathyroidism: Imbalance in calcium regulation.

• Gigantism/Acromegaly: Excess growth hormone (before/after growth plate closure).

Thyroid Regulation & Blood Composition

Thyroid Hormone Regulation Pathway

Thyroid hormone secretion is controlled by a negative feedback loop:

• Hypothalamus releases TRH (Thyrotropin-Releasing Hormone).

• TRH stimulates anterior pituitary to release TSH.

• TSH stimulates thyroid to release T3 and T4.

• T3/T4 act on target cells and inhibit TRH and TSH release (negative feedback).

Blood Composition

• Plasma (55%): Mostly water, proteins (albumin, globulins, fibrinogen), nutrients, electrolytes.

• Formed Elements (45%): Red blood cells (RBCs), white blood cells (WBCs), platelets.

Blood Cells & Clotting

Red Blood Cells (Erythrocytes)

• Transport oxygen using hemoglobin.

• Biconcave shape increases surface area for gas exchange.

White Blood Cells (Leukocytes)

• Neutrophils: Phagocytize bacteria.

• Eosinophils: Combat parasites, involved in allergic reactions.

• Basophils: Release histamine, mediate inflammation.

• Lymphocytes: B cells (produce antibodies), T cells (cell-mediated immunity).

• Monocytes: Become macrophages, phagocytize pathogens and debris.

Platelets (Thrombocytes)

• Cell fragments involved in blood clotting.

Clotting Sequence

1. Injury to vessel

2. Platelet plug formation

3. Activation of clotting factors

4. Prothrombin converted to thrombin

5. Thrombin converts fibrinogen to fibrin

6. Fibrin forms a stable clot

ABO Blood Types

Heart Anatomy

Chambers and Valves

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

• Valves: Tricuspid (right AV), Pulmonary (semilunar), Mitral (left AV), Aortic (semilunar).

Major Vessels

• Superior/Inferior Vena Cava (SVC/IVC): Bring deoxygenated blood to right atrium.

• Pulmonary trunk/arteries: Carry deoxygenated blood to lungs.

• Pulmonary veins: Return oxygenated blood from lungs to left atrium.

• Aorta: Distributes oxygenated blood to body.

Coronary Circulation

• Supplies blood to the heart muscle itself via coronary arteries and veins.

Blood Flow Through the Heart

1. Body → SVC/IVC → Right atrium

2. Right atrium → Tricuspid valve → Right ventricle

3. Right ventricle → Pulmonary valve → Pulmonary trunk/arteries → Lungs

4. Lungs → Pulmonary veins → Left atrium

5. Left atrium → Mitral valve → Left ventricle

6. Left ventricle → Aortic valve → Aorta → Body

Electrical Conduction of the Heart

Conduction Pathway

1. SA Node (pacemaker)

2. AV Node

3. Bundle of His

4. Right and Left Bundle Branches

5. Purkinje Fibers

Electrocardiogram (ECG) Waves

• P Wave: Atrial depolarization

• QRS Complex: Ventricular depolarization

• T Wave: Ventricular repolarization

• PR Interval: Time from atrial to ventricular depolarization

• ST Segment: Time between ventricular depolarization and repolarization

• QT Interval: Duration of ventricular depolarization and repolarization

Blood Vessels

Vessel Structure

• Tunica intima: Innermost layer, smooth endothelium.

• Tunica media: Middle layer, smooth muscle (thicker in arteries).

• Tunica externa (adventitia): Outermost layer, connective tissue.

Types of Blood Vessels

• Arteries: Thick walls, high pressure, carry blood away from heart.

• Veins: Thin walls, valves to prevent backflow, carry blood to heart.

• Capillaries: One-cell thick, site of exchange between blood and tissues.

Major Arteries and Veins

• Know the main vessels of the head, upper limb, abdomen, pelvis, and lower limb.

• Circle of Willis: Arterial circle at the base of the brain providing collateral blood flow.

Cardiac Cycle & Heart Sounds

Phases of the Cardiac Cycle

1. Ventricular filling

2. Atrial systole

3. Isovolumetric contraction

4. Ventricular ejection

5. Isovolumetric relaxation

Heart Sounds

• S1 (Lub): Closure of AV valves (tricuspid and mitral).

• S2 (Dub): Closure of semilunar valves (aortic and pulmonary).

Blood Pressure Measurement

• Systolic pressure: Top number, pressure during ventricular contraction.

• Diastolic pressure: Bottom number, pressure during ventricular relaxation.

• First Korotkoff sound: Indicates systolic pressure.

• Disappearance of sounds: Indicates diastolic pressure.

Final Lab Checklist

• Identify all endocrine glands and hormones

• Recognize blood cells and clotting pathway

• Understand ABO blood types

• Know heart chambers, valves, vessels, and coronary circulation

• Interpret ECG waves and conduction system

• Identify major arteries and veins, including Circle of Willis

• Describe the cardiac cycle, heart sounds, and blood pressure measurement