Lecture 12

Human Anatomy and Physiology - BIO 209 (Winter 2026)

Lecture 12 Overview

  • Systems Covered:

    • Skeletal System

    • Digestive System

    • Muscle System

    • Respiratory System

    • Nervous System

    • Circulatory System

    • Reproductive System

Capillary Exchange and Homeostasis

  • Capillary Exchange Mechanisms:

    • Capillary pressure (CP) changes along the capillary length from arterial to venous end:

    • This leads to variations in filtration and reabsorption rates as blood passes through.

  • Key Factors:

    • Capillary Hydrostatic Pressure (CHP):

    • Definition: Pushes water and solutes out of capillaries into interstitial fluid.

    • Blood Colloid Osmotic Pressure (BCOP):

    • Definition: Pulls water and solutes into capillaries from the interstitial fluid.

  • Lymphatic System Role:

    • Facilitates movement of water out of capillaries through peripheral tissue and returning it to the bloodstream.

Impact of Hemorrhaging and Dehydration on Capillary Exchange

  • Hemorrhaging Effects:

    • Decrease in blood volume and blood pressure leads to:

    • Lowering of CHP, resulting in reduced Net Filtration Pressure (NFP).

    • Increased capillary reabsorption due to lowered NFP.

    • Reduced interstitial fluid volume and increased circulating plasma volume:

      • This is termed a recall of fluids.

  • Dehydration Effects:

    • Plasma volume decreases due to water loss:

    • Increases plasma protein concentration.

    • Heightened BCOP accelerates reabsorption and fluid recall, delaying clinical issues from low blood volume and pressure.

  • Edema Development:

    • Occurs when:

    • CHP rises or BCOP decreases, causing fluid to move out of blood in capillaries and accumulate in peripheral tissues.

    • Results in an abnormal condition known as edema.

Overview of the Lymphatic System

  • Components:

    • Lymphocytes: Primary cells of the lymphatic system.

    • Lymph: Interstitial fluid that has entered a lymphatic vessel.

    • Lymphatic vessels (Lymphatics): Begin in peripheral tissues and end at connections to veins.

    • Lymphoid Organs: Include lymph nodes, thymus, tonsils, and spleen.

Lymphatic Capillaries

  • Characteristics:

    • Begin with lymphatic capillaries:

    • Closed at one end.

    • Larger in diameter, thinner walls compared to blood capillaries.

    • Basement membrane is incomplete or absent.

    • Overlapping endothelial cells act as a one-way valve:

      • Permits entry of fluids and solutes inclusive of proteins, pathogens, and debris while preventing return to the intercellular space.

Lymphatic Vessels

  • Right Lymphatic Duct:

    • Drains lymph from the right upper extremity, head, neck, and upper torso.

  • Thoracic Duct:

    • The largest lymphatic vessel, drains lymph from approximately three-fourths of the body.

    • Notable feature: Enlarged pouch along its course called the cisterna chyli.

Lymphedema

  • Definition:

    • Condition caused by blocked lymphatic drainage, leading to:

    • Accumulation of interstitial fluids.

    • Affected area becoming swollen and distended, often in limbs.

    • Potential for permanent swelling, loss of connective tissue elasticity, and accumulation of toxins or pathogens due to stagnant fluids.

Lymphocytes Overview

  • Composition:

    • Represent 20-40% of circulating leukocytes; mostly in lymphatic tissues.

  • Classes of Lymphocytes:

    • T cells: Responsible for cell-mediated immunity.

    • B cells: Account for 10-15% of circulating lymphocytes, responsible for antibody-mediated immunity.

    • Natural Killer (NK) cells: Comprise 5-10% of circulating lymphocytes, conduct immune surveillance.

    • Attack foreign cells, virally infected cells, and cancer cells.

Functions of Lymphocytes

  • T Cells:

    • Include cytotoxic, helper, regulatory cells, and memory T cells, involved in targeting infected or abnormal cells.

  • B Cells:

    • Differentiate into plasma cells when stimulated, producing antibodies.

  • NK Cells:

    • Provide continuous monitoring and attack infected or tumorous cells.

Anatomy of Lymph Nodes

  • Physical Characteristics:

    • Composed of afferent and efferent lymphatic vessels.

    • Clusters of lymphocytes and lymphoid tissues inside lymphoid organs play a critical role in defensive responses and formation of white blood cells (WBCs).

Thymus

  • Function:

    • Plays a key role in the development of T lymphocytes.

  • Location:

    • Situated in the mediastinum, posterior to the sternum, divided into lobes covered by a capsule and fibrous partitions.

  • Histology:

    • Contains a dark outer cortex where lymphocytes are dividing, regulated by epithelial reticular cells (ERCs), which maintain the blood-thymus barrier and support the development of T cells.

Tonsils

  • Definition:

    • Composed of lymphoid tissue around openings of the mouth and throat; includes palatine tonsils, pharyngeal tonsils (adenoids), and lingual tonsils.

    • Subject to chronic infections; enlargement of pharyngeal tonsils may impair breathing.

Spleen

  • Largest lymphoid organ in the body:

    • Located in the upper left quadrant of the abdomen.

    • Functions include:

    • Phagocytosis of bacteria and old red blood cells (RBCs).

    • Acts as a reservoir for blood, stores iron recycled from RBCs and performs metabolic functions necessary for immunity.

  • Anatomy:

    • Composed of white pulp and red pulp, critical for filtering blood and immune function.

Lymphatic System Functions

  • Maintains constant communication between plasma and interstitial fluid.

  • Accelerates nutrient, hormone, and gas distribution through tissues.

  • Aids in returning insoluble lipids and tissue proteins to the bloodstream.

  • Carries bacterial toxins and chemical stimuli to lymphoid tissues and organs for immune function.