Endocrine System

Endocrine vs. Exocrine Glands

The human body contains two main types of glands: endocrine and exocrine. Endocrine glands secrete hormones directly into the bloodstream, while exocrine glands release their products through ducts to external or internal surfaces.

• Endocrine glands: Ductless; release hormones into blood (e.g., thyroid, pituitary).

• Exocrine glands: Have ducts; release substances like sweat, saliva, or digestive enzymes.

Hormones: Definition and Function

Hormones are chemical messengers produced by endocrine glands that regulate physiological processes by acting on specific target cells.

• Definition: Chemical substances secreted into the blood that affect distant organs.

• Function: Regulate growth, metabolism, reproduction, and homeostasis.

Hormone Action and Release

Hormones act on their intended targets by binding to specific receptors, triggering cellular responses. Hormone release is often regulated by feedback mechanisms, neural stimuli, or changes in blood chemistry.

• Target specificity: Only cells with appropriate receptors respond to a hormone.

• Release triggers: Neural signals, other hormones, or changes in blood levels of ions/nutrients.

Classes of Hormones

Hormones are classified into two main types based on their chemical structure:

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

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

Major Endocrine Glands, Hormones, and Functions

The following table summarizes the major endocrine glands, the hormones they release, and their primary functions.

Blood and Hematology

Components of Blood

Blood consists of plasma and formed elements. Plasma is the liquid portion, while formed elements include cells and cell fragments.

• Plasma: Water, proteins, nutrients, hormones, waste products.

• Formed elements: Erythrocytes (red blood cells), leukocytes (white blood cells), thrombocytes (platelets).

Erythrocytes (Red Blood Cells)

Erythrocytes are specialized for oxygen transport via hemoglobin.

• Shape: Biconcave discs for increased surface area.

• Function: Carry oxygen from lungs to tissues; remove carbon dioxide.

Leukocytes (White Blood Cells)

Leukocytes are immune cells classified into several types:

• Neutrophils: Phagocytize bacteria; most abundant.

• Eosinophils: Combat parasites; involved in allergic responses.

• Basophils: Release histamine; involved in inflammation.

• Lymphocytes: Include B cells (antibody production) and T cells (cell-mediated immunity).

• Monocytes: Differentiate into macrophages; phagocytize pathogens and debris.

Thrombocytes (Platelets)

Thrombocytes are cell fragments essential for blood clotting.

• Function: Initiate clot formation by aggregating at injury sites.

Hemostasis

Hemostasis is the process of stopping bleeding through three main steps:

• Vascular spasm

• Platelet plug formation

• Coagulation (blood clotting)

Thrombosis

Thrombosis is the formation of a blood clot inside a blood vessel. Types include:

• Arterial thrombosis: Clot in an artery.

• Venous thrombosis: Clot in a vein.

Cardiovascular System

Major Chambers of the Heart

The heart has four chambers:

• Right atrium

• Right ventricle

• Left atrium

• Left ventricle

Features of the Heart

• Pectinate muscles: Ridges in atrial walls.

• Auricles: Ear-like extensions of atria.

• Trabeculae carneae: Irregular muscle columns in ventricles.

• Papillary muscles: Attach to chordae tendineae; control valve function.

• Chordae tendineae: Tendinous cords connecting valves to papillary muscles.

Types of Cardiac Muscle Fibers

• Contractile fibers: Responsible for heart contraction.

• Autorhythmic fibers: Generate and conduct electrical impulses.

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence:

1. Right atrium → right ventricle → pulmonary arteries → lungs

2. Lungs → pulmonary veins → left atrium → left ventricle → aorta → body

Conduction System Pathway

The heart's conduction system controls heartbeat:

1. Sinoatrial (SA) node

2. Atrioventricular (AV) node

3. AV bundle (Bundle of His)

4. Right and left bundle branches

5. Purkinje fibers

Systemic Circuit

Blood travels through the systemic circuit from the left ventricle to the body and returns to the right atrium.

Cardiac Output

Cardiac output is the volume of blood pumped by the heart per minute.

• Formula:

Lymphatic and Immune Systems

Lymph Propulsion

Lymph is propelled through the body by skeletal muscle contractions, respiratory movements, and smooth muscle in lymphatic vessel walls.

Lymphatic Capillary Adaptations

• Thin walls for easy fluid entry

• Valves to prevent backflow

• Anchoring filaments to maintain structure

Cell Types in the Lymphatic System

• Phagocytes: Engulf and digest pathogens

• Lymphocytes: B and T cells; mediate immune responses

• Macrophages: Derived from monocytes; phagocytize debris

• Dendritic cells: Present antigens to lymphocytes

• Reticular cells: Form structural framework of lymphoid organs

Thymus Function

The thymus is essential for T cell maturation and immune system development.

Main MALT in the Body

MALT (Mucosa-associated lymphoid tissue) includes tonsils, Peyer's patches, and appendix.

Innate vs. Adaptive Immunity

Antibodies and Producing Cells

Antibodies are proteins produced by B lymphocytes that bind to specific antigens.

Fever: Initiation and Benefits

Fever is initiated by pyrogens and helps inhibit pathogen growth and enhance immune responses.

Active vs. Passive Humoral Immunity

• Active immunity: Body produces its own antibodies; can be natural (infection) or artificial (vaccination).

• Passive immunity: Antibodies are received from another source; can be natural (maternal antibodies) or artificial (antibody injection).

Example: Vaccination provides artificial active immunity; receiving antivenom provides artificial passive immunity.

Additional info: Some explanations and table entries were expanded for clarity and completeness.