Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. It is essential for maintaining homeostasis and coordinating long-term processes such as growth, metabolism, and reproduction.

• Exocrine vs. Endocrine Glands: Exocrine glands secrete substances through ducts to an epithelial surface, while endocrine glands release hormones directly into the blood.

• Major Endocrine Glands: Includes the pituitary, thyroid, parathyroid, adrenal, pineal, and pancreas (islets of Langerhans).

• Hormone Classes: Hormones are classified as amino acid-based (peptides, proteins, amines) or steroid hormones (derived from cholesterol).

• Hormone Transport: Hormones travel in the blood and bind to specific receptors on target cells, either on the cell membrane (for peptide hormones) or inside the cell (for steroid hormones).

• Cellular Response: Hormones affect target cells by altering gene expression, enzyme activity, or membrane permeability.

• Hormone Regulation: Hormone secretion is regulated by negative feedback (most common), positive feedback, and neural or hormonal stimuli.

• Hypothalamic-Pituitary Axis: The hypothalamus controls the pituitary gland, which in turn regulates other endocrine glands.

• Hormone Effects: Hormones can have humoral (blood-borne), hormonal (other hormones), or neural (nerve signals) triggers.

Example: The pancreas releases insulin in response to high blood glucose (humoral stimulus).

Respiratory System

Structure and Function

The respiratory system enables gas exchange between the body and the environment, supplying oxygen and removing carbon dioxide.

• Airway Anatomy: Includes the nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.

• Respiratory Physiology: Involves ventilation (movement of air), external respiration (gas exchange in lungs), transport of gases, and internal respiration (gas exchange in tissues).

• Boyle's Law: Pressure and volume of a gas are inversely related (), explaining inspiration and expiration.

• Factors Affecting Ventilation: Airway resistance, lung compliance, and surface tension.

• Respiratory Control: Regulated by the brainstem (medulla and pons) and chemoreceptors sensitive to CO2, O2, and pH.

• Oxygen and Carbon Dioxide Transport: O2 is mostly carried by hemoglobin; CO2 is transported as bicarbonate, dissolved in plasma, or bound to hemoglobin.

Example: During exercise, increased CO2 production stimulates faster breathing to expel excess CO2.

Blood

Composition and Function

Blood is a connective tissue composed of plasma and formed elements (red blood cells, white blood cells, and platelets). It transports gases, nutrients, hormones, and waste products.

• Types of Formed Elements: Erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets).

• Blood Typing: Based on the presence of A, B, and Rh antigens on RBCs. Important for safe transfusions.

• Hematopoiesis: The process of blood cell formation, primarily in the red bone marrow. Erythropoiesis is the production of RBCs, stimulated by erythropoietin.

• Plasma Proteins: Albumin (maintains osmotic pressure), globulins (immune function), and fibrinogen (clotting).

• Blood Transfusions: Compatibility depends on ABO and Rh blood groups.

Additional info: Rh factor (+ or -) must also be considered for compatibility.

Circulatory System: Heart

Anatomy and Physiology of the Heart

The heart is a muscular organ that pumps blood through the pulmonary and systemic circuits. Its structure is closely related to its function.

• Chambers: Right and left atria, right and left ventricles.

• Blood Flow Pathway: Blood enters the right atrium, passes to the right ventricle, is pumped to the lungs, returns to the left atrium, then to the left ventricle, and out to the body.

• Electrical Conduction System: Includes the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers. Coordinates heartbeats.

• Cardiac Cycle: Systole (contraction) and diastole (relaxation) phases.

• Stroke Volume and Heart Rate: Cardiac output () is the volume of blood pumped per minute.

• Electrocardiogram (ECG): Records the electrical activity of the heart; P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

Example: An abnormal ECG can indicate arrhythmias or myocardial infarction.

Circulatory System: Blood Vessels

Types and Functions of Blood Vessels

Blood vessels form a closed circuit to transport blood throughout the body. They include arteries, veins, and capillaries, each with distinct structure and function.

• Arteries: Carry blood away from the heart; thick, elastic walls.

• Veins: Carry blood toward the heart; thinner walls, valves prevent backflow.

• Capillaries: Microscopic vessels for exchange of gases, nutrients, and wastes.

• Blood Pressure: The force exerted by blood on vessel walls; highest in arteries, lowest in veins.

• Resistance: Opposition to blood flow, mainly in arterioles.

• Factors Affecting Blood Flow: Vessel diameter, blood viscosity, and total vessel length.

• Capillary Exchange: Movement of substances between blood and tissues via diffusion, filtration, and osmosis.

Example: Edema can result from increased capillary hydrostatic pressure or decreased plasma proteins.

Lymphatic System

Structure and Function

The lymphatic system returns excess interstitial fluid to the bloodstream, absorbs dietary fats, and provides immune defense.

• Lymph Flow: Lymphatic capillaries → lymphatic vessels → lymph nodes → lymphatic trunks → ducts → subclavian veins.

• Lymph Nodes: Filter lymph and house lymphocytes for immune surveillance.

• Other Lymphatic Organs: Spleen (filters blood), thymus (T cell maturation), tonsils (protect against inhaled/ingested pathogens).

Example: Swollen lymph nodes may indicate infection.

Immunity

Types of Immunity and Immune Responses

The immune system protects the body from pathogens through nonspecific (innate) and specific (adaptive) defenses.

• Nonspecific Defenses: Physical barriers (skin, mucous membranes), phagocytes, inflammation, fever, and antimicrobial proteins.

• Specific Defenses: Involve lymphocytes (B cells and T cells) and the production of antibodies.

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

• Antigen-Antibody Interactions: Antibodies bind to specific antigens to neutralize or mark them for destruction.

Example: Vaccination stimulates specific immunity by exposing the body to harmless antigens.