Endocrine System

General Characteristics of the Endocrine System

The endocrine system consists of glands that secrete hormones directly into the bloodstream, regulating various physiological processes. It is distinct from the exocrine system, which releases substances through ducts to external or internal surfaces.

• Exocrine vs. Endocrine Glands: Exocrine glands secrete substances (e.g., sweat, saliva) via ducts; endocrine glands release hormones into the blood.

• Hormone: A chemical messenger that alters the activity of target cells by binding to specific receptors.

• Target Cell: A cell with receptors specific to a particular hormone.

• Up-Regulation/Down-Regulation: Target cell sensitivity changes by increasing (up-regulation) or decreasing (down-regulation) receptor numbers.

• Chemical Classes of Hormones:

  • Steroid Hormones: Derived from cholesterol; e.g., cortisol, estrogen.

  • Amine Hormones: Derived from amino acids; e.g., epinephrine, thyroxine.

  • Peptide/Protein Hormones: Chains of amino acids; e.g., insulin, growth hormone.

• Hydrophobic Hormones: Steroid and thyroid hormones; receptors are intracellular. They alter gene expression and protein synthesis.

• Hydrophilic Hormones: Peptide/protein and most amine hormones; receptors are on the cell membrane. They activate second messenger pathways.

• Lipophilic: Fat-soluble; can cross cell membranes.

• Lipophobic: Water-soluble; cannot cross cell membranes easily.

• Hormone Release Control Mechanisms:

  1. Humoral (changes in blood composition)

  2. Neural (nervous system stimulation)

  3. Hormonal (other hormones stimulate release)

Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland are central regulators of endocrine function, controlling many other glands through hormone release.

• Pituitary Gland Location: Base of the brain, connected to the hypothalamus by the infundibulum.

• Histology: Anterior pituitary (glandular tissue); posterior pituitary (neural tissue).

• Anterior Pituitary Hormones: Growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH).

• Posterior Pituitary Hormones: Antidiuretic hormone (ADH), oxytocin.

• Hormonal Imbalances:

  • Pituitary Dwarfism: GH deficiency in childhood.

  • Pituitary Giantism: GH excess in childhood.

  • Acromegaly: GH excess in adulthood.

  • Diabetes Insipidus: ADH deficiency.

Thyroid and Parathyroid Glands

The thyroid and parathyroid glands regulate metabolism and calcium homeostasis.

• Thyroid Follicle Hormones: Thyroxine (T4), triiodothyronine (T3) – increase metabolic rate.

• Hyperthyroidism Symptoms: Weight loss, increased heart rate, anxiety.

• Hypothyroidism Symptoms: Weight gain, fatigue, cold intolerance.

• Graves Disease: Autoimmune hyperthyroidism.

• Simple Goiter: Thyroid enlargement due to iodine deficiency.

• Extrafollicular Cell Hormone: Calcitonin – lowers blood calcium.

• Parathyroid Hormone: Parathyroid hormone (PTH) – increases blood calcium.

Adrenal Glands

The adrenal glands produce hormones involved in stress response and metabolic regulation.

• Adrenal Medulla Hormones: Epinephrine, norepinephrine – fight or flight response.

• Adrenal Cortex Hormones: Cortisol (metabolism), aldosterone (sodium retention), androgens.

• Cushing’s Syndrome: Excess cortisol; symptoms include obesity, hypertension, and muscle weakness.

Pancreas, Pineal Gland, and Thymus

These glands regulate blood glucose, circadian rhythms, and immune function.

• Pancreas: Both exocrine (digestive enzymes) and endocrine (hormones).

• Hormone-Releasing Cells: Islets of Langerhans – alpha (glucagon), beta (insulin).

• Diabetes Mellitus:

  • Type 1: Autoimmune destruction of beta cells.

  • Type 2: Insulin resistance.

  • Symptoms: Polyuria, polydipsia, polyphagia, hyperglycemia.

• Pineal Gland Hormone: Melatonin – regulates sleep-wake cycle.

• Thymus Hormones: Thymosins – stimulate T cell development.

Blood

Characteristics of Blood

Blood is a connective tissue composed of plasma and formed elements, essential for transport, regulation, and protection.

• Blood Plasma: Liquid component; contains water, proteins, nutrients, hormones.

• Formed Elements: Erythrocytes (RBCs), leukocytes (WBCs), platelets.

• Hematocrit: Percentage of RBCs in blood.

• Buffy Coat: Layer of WBCs and platelets between plasma and RBCs after centrifugation.

• Plasma Proteins:

  • Albumins: Maintain osmotic pressure; made in liver.

  • Globulins: Transport and immune functions; made in liver and lymphoid tissue.

  • Fibrinogen: Clotting; made in liver.

Red Blood Cells

Red blood cells (erythrocytes) transport oxygen and carbon dioxide. Their production and destruction are tightly regulated.

• Hematopoiesis: Formation of blood cells; occurs in bone marrow after birth.

• Hemocytoblast: Stem cell for all blood cells.

• Physical Characteristics: Biconcave, anucleate, flexible.

• Hemoglobin: Four globin chains, each with a heme group; binds oxygen and carbon dioxide.

• Oxygen Binding: Each hemoglobin molecule can bind four oxygen molecules.

• Reticulocytes: Immature RBCs.

• Normal RBC Count: 4.5–6 million/µL.

• Erythropoietin: Hormone from kidneys stimulates RBC production.

• RBC Lifespan: ~120 days.

• Dietary Factors: Iron, vitamin B12, folic acid.

• Anemia Types: Iron-deficiency, pernicious, sickle cell, aplastic.

• Sickle Cell Disease: Abnormal hemoglobin causes RBCs to sickle, leading to blockages and pain.

• RBC Removal: Spleen and liver; hemoglobin broken down to heme (bilirubin) and globin (amino acids).

• Jaundice: Excess bilirubin causes yellowing of skin.

White Blood Cells

White blood cells (leukocytes) are essential for immune defense and are classified as granulocytes or agranulocytes.

• Granulocytes: Neutrophils, eosinophils, basophils.

• Agranulocytes: Lymphocytes, monocytes.

• Normal WBC Count: 4,000–11,000/µL.

• Differential WBC Count: Percentage of each type of WBC.

• Leukemia: Cancer of WBCs; acute or chronic types.

Platelets and Hemostasis

Platelets (thrombocytes) are cell fragments involved in blood clotting. Hemostasis is the process of stopping bleeding.

• Platelet Structure: Small, anucleate cell fragments.

• Origin: Megakaryocytes in bone marrow.

• Normal Platelet Count: 150,000–400,000/µL.

• Hemostasis Phases:

  1. Vascular spasm

  2. Platelet plug formation

  3. Coagulation

• Intrinsic vs. Extrinsic Pathways: Both lead to conversion of fibrinogen to fibrin.

• Clotting Substances: Calcium, vitamin K, clotting factors.

• Positive Feedback: Clot formation amplifies itself.

• Clot Dissolution: Plasmin enzyme dissolves clots.

• Clotting Disorders: Hemophilia (factor deficiency), thrombus (stationary clot), embolus (traveling clot), embolism (clot blocks vessel).

Blood Types and Blood Transfusions

Blood types are determined by antigens and antibodies. Compatibility is crucial for safe transfusions.

• ABO Blood Group:

  Blood Type	Antigens	Antibodies
  A	A	Anti-B
  B	B	Anti-A
  AB	A, B	None
  O	None	Anti-A, Anti-B

• Universal Recipient: AB

• Universal Donor: O

• Rh Factor: Rh+ has D antigen; Rh- lacks it.

• Hemolytic Disease of the Newborn: Rh incompatibility between mother and fetus.

Cardiovascular System

General Characteristics, Heart Anatomy, and Blood Flow

The cardiovascular system transports blood throughout the body, delivering oxygen and nutrients and removing waste products.

• Pulmonary Circuit: Carries blood from heart to lungs and back.

• Systemic Circuit: Carries blood from heart to body tissues and back.

• Pericardium Layers: Fibrous, parietal, visceral (epicardium).

• Heart Layers: Epicardium, myocardium, endocardium.

• Right Side Blood: Deoxygenated.

• Left Side Blood: Oxygenated.

• Blood Vessels to Right Atrium: Superior vena cava, inferior vena cava, coronary sinus.

• Blood Flow Path: Right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → bicuspid valve → left ventricle → aortic valve → aorta.

• Fetal Circulation Remnants: Foramen ovale, ductus arteriosus.

Coronary Circulation

Coronary arteries supply the heart muscle with oxygenated blood; coronary veins remove deoxygenated blood.

• Angina Pectoris: Chest pain from reduced blood flow.

• Myocardial Infarction: Heart attack; death of heart tissue.

Cardiac Conduction System and ECGs

The cardiac conduction system coordinates heart contractions, and ECGs record electrical activity.

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Cardiac Cycle: Complete heartbeat.

• Cardiac Muscle Cells: Intercalated discs allow functional syncytium.

• Conduction System Components: SA node, AV node, AV bundle, bundle branches, Purkinje fibers.

• Normal Pacemaker: SA node.

• ECG Deflections: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

• Arrhythmia Terms: Fibrillation, tachycardia, bradycardia, flutter, ectopic pacemaker.

The Cardiac Cycle

Pressure and volume changes drive blood flow and valve operation during the cardiac cycle.

• Valve Operation: Open/close based on pressure differences.

• Heart Sounds: S1 (lubb) – AV valves close; S2 (dupp) – semilunar valves close.

• Heart Murmur: Abnormal sound; caused by valve defects.

• Sympathetic/Parasympathetic Effects: Sympathetic increases heart rate; parasympathetic decreases it.

• Parasympathetic Nerve: Vagus nerve.

• Baroreceptors: Pressure sensors in carotid sinus and aortic arch.

• Cardiac Center Location: Medulla oblongata.

• Temperature Effects: Higher temperature increases heart rate.

Blood Vessels

Blood vessels transport blood and facilitate exchange of substances between blood and tissues.

• Systemic Circuit Sequence: Arteries → arterioles → capillaries → venules → veins.

• Blood Vessel Layers: Tunica intima, tunica media, tunica externa.

• Arteries: Thick, elastic walls; carry blood away from heart.

• Capillaries: Single layer of endothelium; site of exchange.

• Exchange Methods: Diffusion, filtration, osmosis, vesicular transport.

• Filtration/Reabsorption: Driven by hydrostatic and osmotic pressures.

• Disruptions: Heart failure, liver failure, immune reactions.

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

• Blood Movement Factors: Skeletal muscle contraction, respiratory movements, valves.

• Vascular Disorders: Atherosclerosis, arteriosclerosis, aneurysm, varicose veins.

Blood Pressure

Blood pressure is the force exerted by blood on vessel walls, primarily measured in arteries.

• Blood Pressure Reading: Systolic/diastolic; pulse pressure = systolic - diastolic; mean arterial pressure (MAP) = (systolic) + (diastolic).

• Cardiac Output: Volume pumped per minute; .

• Stroke Volume: Volume per beat.

• Blood Volume: Directly affects blood pressure.

• Hormones Influencing Blood Volume: ADH, aldosterone, ANP.

• Peripheral Resistance: Resistance to flow; affected by vessel diameter, blood viscosity.

• Radius Effect: ; small changes in radius greatly affect resistance.

• Sympathetic Effects: Vasoconstriction increases resistance.

• Hormonal Effects: Epinephrine (vasoconstriction), angiotensin II (vasoconstriction), ANP (vasodilation), histamine (vasodilation).

• Blood Pressure Regulation: Baroreceptor reflex, hormonal control.

• Hypertension: Chronic high blood pressure.

Systemic Arteries and Veins

Major systemic arteries and veins are essential for distributing blood throughout the body. Identification is required for laboratory exams.

• Systemic Arteries: Aorta, carotid, subclavian, femoral, etc.

• Systemic Veins: Superior/inferior vena cava, jugular, femoral, etc.