Lymphatic System and Body Defenses Chapter 12

The Lymphatic System Two semi-independent parts – Lymphatic vessels – Lymphoid tissues and organs Function of lymphatic system – Transport fluids back to the blood – Essential role in body defense and disease resistance

Lymphatic Vessels Function – Pick up excess tissue fluid – Lymph – and return it to the blood stream Also known as lymphatics One way system flows only to the heart Characteristics – Thin-walled – Valved – pumpless

Lymph capillaries Web between tissue cells and blood capillaries Remarkably permeable Held by fine collagen fibres Act as one way doors – Flap like mini valves close as high pressure builds up inside – Keeps lymph from leaking back

Lymphatic collecting vessels Collects lymph from lymph capillaries Carries lymph to and away from lymph node Returns fluid to the heart through two ducts – Right lymphatic duct Drains lymph from right arm and right side of head and thorax – Thoracic duct Receives lymph from rest of the body

Lymphatic Vessels

Lymph Nodes Function – Remove foreign material from lymph Cluster along vessels in three main areas – Inguinal – Axillary – Cervical Contain 2 types of cells – Macrophages Engulf and destroy bacteria, viruses and foreign substances – Lymphocytes (white blood cells) Respond to foreign substances

Lymph Nodes (continued) Structure – Kidney shaped, less than 1” long – “buried” in connective tissue – Cortex Outer part Contains follicles (collections of lymphocytes) and T- cells – Medulla Inner part Contains phagocytic macrophages

Flow of Lymph through Nodes Lymph enters through afferent lymph vessels Flows through sinuses Exits at indented region - hilus – through efferent lymphatic vessels Slow process – Allows time for lymphocytes and macrophages to perform

Other Lymphoid Organs Spleen Thymus Tonsils Peyer’s patch Common features – Predominance of reticular connective tissues and lymphocytes

Other Lymphoid Organs Spleen – Filters blood of bacteria, viruses – Located on left side of abdominal cavity – Destroys worn out red blood cells – Acts as blood reservoire – Produces lymphocytes Thymus – Function peaks at youth – Found low in throat over heart – Produces hormones that program lymphocytes

Other Lymphoid Organs Tonsils – Small masses of lymphatic tissue around pharynx – Trap and removes bacteria or pathogens entering throat – So efficient can become congested tonsillitis Peyer’s patches – Like tonsils but in small intestine – Captures and destroys bacteria

Mucosa-associated Lymphatic Tissue (MALT) Composed of – Peyer’s patches – Tonsils – Other small collections of lymphatic tissue Protects respiratory and digestive tracts

Body Defenses

Nonspecific Body Defenses

Specific Body Defenses: The Immune System