Endocrine System

Overview of Hormones and Endocrine Glands

The endocrine system is a major control system of the body, working alongside the nervous system to regulate physiological processes through the release of hormones. Hormones are chemical messengers secreted by endocrine glands that travel through the bloodstream to target organs.

• Hormones: Chemical messengers that regulate activities such as growth, metabolism, and reproduction.

• Endocrine glands: Ductless glands that secrete hormones directly into the blood.

• Types of hormones: Amino acid-based (mostly water-soluble) and steroid (lipid-soluble).

• Second messenger systems: Many amino acid-based hormones use cyclic AMP (cAMP) as a second messenger to relay signals inside cells.

Example: Insulin is a peptide hormone produced by the pancreas that lowers blood glucose levels.

Hormone Release Mechanisms

Hormone secretion is regulated by three main types of stimuli:

• Humoral stimuli: Changes in blood levels of ions or nutrients trigger hormone release.

• Neural stimuli: Nerve fibers stimulate hormone release.

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones.

Major Endocrine Glands and Hormones

• Hypothalamus: Located in the brain; regulates the pituitary gland and produces releasing/inhibiting hormones.

• Pituitary gland: Has anterior and posterior lobes, each releasing different hormones.

Anterior Pituitary Hormones

• Growth hormone (GH)

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

• Prolactin

Each hormone has specific target organs and effects. For example, TSH stimulates the thyroid gland to produce thyroid hormones.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions and milk ejection.

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

Thyroid Gland

• Produces thyroxine (T4) and triiodothyronine (T3), collectively called thyroid hormone.

• Regulates metabolism, growth, and development.

• Follicles: Spherical structures that produce thyroid hormone.

• Colloid: Substance in follicles containing thyroglobulin and iodine.

• Iodine: Essential dietary nutrient for thyroid hormone synthesis.

Disorders:

• Hypothyroidism: Deficiency of thyroid hormone; symptoms include fatigue, weight gain, cold intolerance.

• Hyperthyroidism: Excess thyroid hormone; symptoms include weight loss, heat intolerance, increased heart rate.

Parathyroid Glands

• Produce parathyroid hormone (PTH), which regulates blood calcium levels.

Adrenal Glands

• Adrenal cortex: Produces corticosteroids.

• Zona fasciculata: Produces glucocorticoids (e.g., cortisol).

• Zona reticularis: Produces small amounts of sex hormones.

• Cortisol: Increases blood glucose, suppresses immune response.

• Disorders: Excess cortisol (Cushing's syndrome) or deficiency (Addison's disease).

Pineal Gland

• Produces melatonin, which regulates sleep-wake cycles.

Pancreas

• Has both exocrine and endocrine functions.

• Alpha cells: Produce glucagon (raises blood glucose).

• Beta cells: Produce insulin (lowers blood glucose).

• Diabetes mellitus: Disorder of insulin signaling.

• Type 1 DM: Usually childhood onset, insulin deficiency.

• Type 2 DM: Later onset, insulin resistance.

Blood

Functions and Composition

Blood is a connective tissue with multiple functions:

• Transport: Carries oxygen, nutrients, hormones, and waste products.

• Regulation: Maintains pH, temperature, and fluid balance.

• Protection: Defends against infection and prevents blood loss.

• Plasma: The nonliving fluid matrix of blood.

• Hematocrit: Percentage of blood volume occupied by red blood cells (RBCs).

• Buffy coat: Thin layer containing white blood cells (WBCs) and platelets.

• pH range: 7.35–7.45.

• Albumin: Major plasma protein important for osmotic balance.

Formed Elements

• Red blood cells (erythrocytes): Transport oxygen using hemoglobin.

• Hemoglobin: Protein that binds oxygen; each molecule can bind four O2 molecules.

• Hematopoiesis: Formation of blood cells in red bone marrow.

• Erythropoietin: Hormone from kidneys that stimulates RBC production.

• Anemia: Deficiency of RBCs or hemoglobin.

• Polycythemia: Excess RBCs.

• Leukocytes (WBCs): Defend against infection; include granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).

• Leukocytosis: Increased WBC count.

• Leukemia: Cancer of WBCs.

• Platelets: Cell fragments involved in clotting.

Hemostasis

Hemostasis is the process of stopping bleeding, involving:

• Vascular spasm

• Platelet plug formation

• Coagulation (blood clotting)

Blood Typing

Blood types are determined by antigens on RBC membranes. The ABO and Rh systems are most important for transfusions.

Rh+ individuals have the Rh antigen; Rh- individuals do not.

Cardiovascular System: The Heart

Heart Anatomy and Circulation

The heart is a muscular organ that pumps blood through two circuits: pulmonary (to the lungs) and systemic (to the body).

• Pericardium: Double-walled sac enclosing the heart; consists of visceral and parietal layers.

• Epicardium: Outer layer of the heart wall.

• Myocardium: Muscular middle layer; responsible for contraction.

• Endocardium: Inner lining of the heart chambers.

Chambers and Valves

• Atria: Upper chambers; receive blood returning to the heart.

• Ventricles: Lower chambers; pump blood out of the heart.

• Right atrium: Receives deoxygenated blood from the body.

• Left atrium: Receives oxygenated blood from the lungs.

• Right ventricle: Pumps blood to the lungs.

• Left ventricle: Pumps blood to the body.

• Valves: Prevent backflow of blood (AV valves: tricuspid and bicuspid/mitral; semilunar valves: pulmonary and aortic).

Heart Sounds and Blood Flow

• Heart sounds are caused by valve closures.

• Blood flows from body → right atrium → right ventricle → lungs → left atrium → left ventricle → body.

Intrinsic Conduction System

The heart's intrinsic conduction system coordinates contraction:

• Sinoatrial (SA) node: Pacemaker of the heart.

• Atrioventricular (AV) node

• Bundle of His

• Bundle branches

• Purkinje fibers

Defects can cause arrhythmias (irregular heart rhythms).

Electrocardiogram (ECG)

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Cardiac Cycle and Heart Rate

• Systole: Contraction phase

• Diastole: Relaxation phase

• Normal resting heart rate: 60–100 beats per minute

• Tachycardia: Abnormally fast heart rate

• Bradycardia: Abnormally slow heart rate

Additional info: This guide expands on the original notes by providing definitions, examples, and context for each major topic, as well as a summary table for blood types and a brief overview of the cardiac conduction system and ECG interpretation.