1.1.2 Lymphoid System

Front 1

What constitutes the lymphatic system?

Back 1

1. Lymph: a fluid that resembles plasma, but with a lower concentration of suspended proteins
2. Lymphatic vessels: network beginning in peripheral tissues and connecting to veins
3. Lymphoid tissues and organs
4. Lymphocytes and other cells

Front 2

Primary function of lymphatic system

Back 2

To produce, maintain and distribute lymphocytes that provide defence against infections and other environmental hazards
- Lymphoid tissues (e.g. tonsils) and lymphoid organs (e.g. spleen, thymus) produce and store most of the body's lymphocytes

Front 3

Circulation

Back 3

Lymphocytes detect problems and can move to the site of injury or infection
- Lymphocytes, macrophages and microphages circulate within the blood
- Can enter or leave capillaries
- Carry excess fluid from peripheral tissues to the lymphatic vessels. From here it can return to the blood.

Front 4

Lymphatic vessels

Back 4

Carry lymph from the peripheral tissues to the venous system

Front 5

Lymphatic capillaries

Back 5

Lymphatic capillaries/terminal lymphatics branch through peripheral tissues.
- lined by endothelial cells, not tightly bound together. Some overlap
- Overlap acts as one-way valve, permitting fluids and solutes to enter, along with viruses, bacteria and cell debris.
- Incomplete or absent BM

Front 6

How lymphatic capillaries differ from blood capillaries

Back 6

1. Originate as pockets rather than forming continuous tubes
2. Have large diameters
3. Have thinner walls
4. Typically have flattened or irregular outline in sectional view

Front 7

Lacteals

Back 7

Prominant lymphatic capillaries in the intestine
- important in transport of lipids absorbed by the gut

Front 8

Small lymphatic capillaries

Back 8

From the lymphatic capillaries, lymph flows into larger lymphatic vessels that lead toward the body trunk.
- Comparable wall structure to veins, include valves

Front 9

Valves in small lymphatic capillaries

Back 9

Prevent the backflow of lymph, especially in the limbs.
- Valves are close together and produce noticable bulges
- Pressure within the lymphatic system is negligible, so valves are essential for maintaining normal lymph flow to thoracic cavity

Front 10

Major lymph-collecting vessels

Back 10

Two sets of vessels collect lymph from lymphatic capillaries:
- superficial and deep

Front 11

Superficial lymphatics

Back 11

- Located in subcutaneous layer, deep to the skin
- In the areolar tissues of the mucous membranes lining the digestive, respiratory, urinary and reproductive tracts
- In the areolar tissues of the serous membranes lining the pleural, pericardial and peritoneal cavities

Front 12

Deep lymphatics

Back 12

Larger lymphatic vessels that accompany deep arteries and veins supplying skeletal muscles and other organs of the neck, limbs and trunk, and walls of visceral organs

Front 13

Lymphatic trunks

Back 13

Superficial and deep lymphatics converge to form lymphatic trunks.
- These then empty into 2 large collecting vessels: thoracic duct and the right lymphatic duct

Front 14

Thoracic duct

Back 14

Collects lymph from the body inferior to the diaphragm and from the left side of the body superior to the diaphragm

Front 15

Right lymphatic duct

Back 15

Smaller, collects lymph from the right side of the body superior to the heart

Front 16

Lymphoid organs

Back 16

A fibrous tissue capsule seperates lymphoid organs - the lymph nodes, the thymus, the spleen - from surrounding tissues

Front 17

Lymph nodes

Back 17

- small lymphoid organs, most numerous in the neck, axilla and groin
- covered with a dense connective tissue, with bundles of collagen extending from the capsule in to the interior (trabeculae)

Front 18

Thymus

Back 18

Located in the mediastinum
- filled with T cells actively dividing
- Mature for 3 weeks then migrate into blood vessels
- Also produces several important hormones

Front 19

Spleen

Back 19

Largest collection of lymphoid tissue in the body:
- Removing abnormal blood cells and other blood components by phagocytosis
- Storing iron recycled by RBCs
- Initiating immune responses by B cells and T cells in response to antigens in circulating blood