Endocrine System and Hormones

Hormones: Types and Mechanisms

Hormones are chemical messengers secreted by endocrine glands that regulate physiological processes in target cells throughout the body.

• Definition: Hormones are substances produced by glands and transported in the bloodstream to stimulate specific cells or tissues into action.

• Types of Hormones:

  • Peptide/Protein Hormones: Composed of amino acids (e.g., insulin, growth hormone).

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

• Hormone Action: Hormones bind to specific receptors on or in target cells, triggering cellular responses.

Hormone Regulation: Feedback Mechanisms

• Negative Feedback: A process where the output of a system inhibits its own production. Example: Regulation of blood glucose by insulin.

• Positive Feedback: A process where the output enhances the original stimulus. Example: Release of oxytocin during childbirth.

• Examples:

  • Negative feedback:

  • Positive feedback:

Major Endocrine Glands and Their Hormones

• Pituitary Gland: Produces growth hormone, prolactin, ACTH, TSH, LH, FSH.

• Thyroid Gland: Produces thyroxine (T4), triiodothyronine (T3), calcitonin.

• Adrenal Glands: Produce cortisol, aldosterone, adrenaline.

• Pancreas: Produces insulin, glucagon.

• Gonads (Ovaries/Testes): Produce estrogen, progesterone, testosterone.

Additional info: Each hormone has specific target organs and physiological effects, such as metabolism regulation, stress response, and reproductive functions.

Blood: Composition and Functions

Plasma and Formed Elements

Blood is a connective tissue composed of plasma and formed elements (cells and cell fragments).

• Plasma: The liquid component of blood, containing water, proteins (albumin, globulins, fibrinogen), nutrients, hormones, and waste products.

• Formed Elements:

  • Red Blood Cells (Erythrocytes): Transport oxygen using hemoglobin.

  • White Blood Cells (Leukocytes): Defend against infection.

  • Platelets (Thrombocytes): Involved in blood clotting.

Functions of Blood Cells and Platelets

• Red Blood Cells: Carry oxygen from lungs to tissues and carbon dioxide from tissues to lungs.

• White Blood Cells: Provide immune defense against pathogens.

• Platelets: Initiate blood clotting to prevent blood loss.

Hematopoiesis and Erythropoiesis

• Hematopoiesis: The process of blood cell formation, primarily in the bone marrow.

• Erythropoiesis: The production of red blood cells, stimulated by the hormone erythropoietin (produced by the kidneys).

Hemostasis: Stages and Molecules Involved

Hemostasis is the process that stops bleeding at the site of injury.

• Stages:

  1. Vascular spasm (vasoconstriction)

  2. Platelet plug formation

  3. Coagulation (blood clotting)

• Key Molecule: Fibrin forms the mesh that stabilizes the blood clot.

Blood Types and Antigens

• Blood Groups: Determined by the presence of specific antigens (A, B, AB, O) on red blood cells.

• Rh Factor: Presence (+) or absence (−) of Rh antigen.

• Antibodies: Present in plasma; react against foreign antigens.

Cardiovascular System: The Heart

Chambers and Valves of the Heart

The heart is a muscular organ with four chambers and valves that ensure unidirectional blood flow.

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

• Valves: Tricuspid, pulmonary, mitral (bicuspid), aortic.

• Function of Valves: Prevent backflow of blood.

Pathway of Blood Through the Heart

• Deoxygenated blood enters the right atrium → right ventricle → pulmonary artery → lungs (oxygenation).

• Oxygenated blood returns via pulmonary veins → left atrium → left ventricle → aorta → systemic circulation.

Cardiac Cycle and Conduction System

• Cardiac Cycle: Sequence of events in one heartbeat, including systole (contraction) and diastole (relaxation).

• Conduction System: Specialized cardiac muscle cells generate and transmit electrical impulses.

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

  • Atrioventricular (AV) node, bundle of His, Purkinje fibers: Coordinate contraction.

Cardiac Output and Factors Affecting It

• Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.

Formula:

• Factors Affecting Cardiac Output: Heart rate, stroke volume, contractility, preload, afterload.

Blood Pressure and Its Regulation

• Blood Pressure (BP): The force exerted by circulating blood on vessel walls.

• Regulation: Influenced by cardiac output, blood volume, resistance of blood vessels, and neural/hormonal controls.

• Role of the Nervous System: Baroreceptors detect changes in BP and adjust heart rate and vessel diameter via autonomic pathways.

Lymphatic System and Immunity

Lymph and Lymphatic Organs

The lymphatic system returns excess tissue fluid to the bloodstream and plays a key role in immune defense.

• Lymph: Clear fluid derived from interstitial fluid, transported through lymphatic vessels.

• Major Organs:

  • Lymph nodes: Filter lymph and house lymphocytes.

  • Spleen: Filters blood, recycles old red blood cells, and supports immune function.

  • Thymus: Site of T cell maturation.

  • Tonsils: Protect against pathogens entering via the mouth and throat.

Immune System: Innate and Adaptive Immunity

• Innate Immunity: Non-specific, immediate defense mechanisms (e.g., skin, phagocytes, inflammation).

• Adaptive Immunity: Specific, acquired defense involving lymphocytes (B cells and T cells).

Humoral and Cell-Mediated Immunity

• Humoral Immunity: Mediated by B cells producing antibodies that neutralize pathogens in body fluids.

• Cell-Mediated Immunity: Mediated by T cells that destroy infected or abnormal cells.

• Helper T Cells: Activate B cells and cytotoxic T cells.

• Cytotoxic T Cells: Destroy virus-infected and cancerous cells.

Additional info: The immune response involves both innate and adaptive components working together to protect the body from disease.