BIO 252 Anatomy and Physiology Module 4
AP 4.1 – Lymphatic System
4.1 – Overview of Lymphatic Functions
Lymphatic System
● Lymphatic system: lymph, lymphatic vessels, lymphoid tissue, and lymphoid organs
○ Closely connected to the cardiovascular system
● Main functions → fluid balance, fat absorption, immunity
Key Functions
● Fluid Recovery
○ Collects excess interstitial fluid that leaks out of blood capillaries
○ Once inside the lymphatic vessels, the fluid is called lymph
○ Lymphatic vessels return lymph to the bloodstream to prevent tissue swelling (edema)
● Transport of Fats
○ Specialized lymphatic capillaries in the small intestine absorb fats from digestion ○ These
fats enter the lymph and eventually reach the bloodstream
● Immune Defense
○ Lymph passes through lymph nodes, which contain immune cells that:
■ Scan the lymph for foreign particles, pathogens, or
abnormal cells ■ Mount immune response when necessary
Lymphatic Vessels
● Flow of Lymph
○ One-way flow: lymphatic capillaries → larger lymphatic vessels → subclavian veins →
bloodstream → heart ○ Ultimately returns fluid to venous blood
● Where Lymph Comes From
○ Fluid begins as interstitial fluid between cells
○ Most interstitial fluid is reabsorbed by the capillaries, but not all
○ The remaining fluid enters lymphatic capillaries
● Lymphatic Capillaries
○ Extremely porous and positioned near cardiovascular capillaries
○ Take up plasma fluid that has leaked out of blood capillaries due to
high pressure ■ This fluid was not reabsorbed by the
circulatory system
● Purpose
, ○ The fluid in lymph bathes cells and helps supply them with glucose, oxygen, salts, amino
acids, and nutrients ○ Prevents fluid buildup in tissues
○ Ensures leaked plasma returns to circulation and maintains blood volume
Overview– Pathway Summaries
1. Excess interstitial fluid → lymphatic capillaries
2. Lymphatic capillaries → larger lymphatic vessels
3. Lymphatic vessels → subclavian veins
4. Lymph mixes with venous blood → returns to heart
Relationship Between Cardiovascular & Lymphatic Circulation
Structure of a Lymphatic Capillary
Lymphatic (Lymph) Vessels
● Location
○ Found throughout most of the body
○ Areas without blood vessels (e.g. bone marrow, epidermis) also lack lymphatic vessels
● Structure & Function
○ One-Way Flow
■ Have one-way valves, similar to veins
■ These valves prevent backward flow of lymph
● Movement of Lymph
○ Lymph movement does not rely on the heart ○ Instead, it depends on:
, ■ Skeletal muscle contraction (muscle pump)
■ Pressure changes, as the body moves
○ These actions squeeze lymph forward through vessels
Hierarchy of Lymphatic Vessels
1. Lymphatic capillaries
2. Larger lymphatic vessels
3. Lymphatic trunks
a. Formed by merging large lymphatic vessels
b. Drain major regions of the body
4. Lymphatic ducts
a. Final drainage point
b. Empty lymph into subclavian veins, returning it to venous circulation
Lymphoid Organs
● Contain lymphocytes (T cells and B cells), dendritic cells, and macrophages that defend the body
against pathogens
Lymph Nodes
● Hundreds found at junctions of lymphatic vessels
● Function as filtering centers
○ Remove infectious organisms, debris, and foreign material
from lymph ● Lymph flows through sinuses:
○ cortical → subcapsular → medullary
● Contain lymphocytes and macrophages for immune defense
Spleen
● Located on the left side of the stomach; largest lymphoid organ
● Functions:
○ Removes old or damaged RBCs and platelets
○ Filers out bacteria, viruses, toxins, and debris from blood
○ Stores lymphocytes and macrophages to mount immune response
Thymus
● Located in the thorax, above the heart ● Most active during childhood and adolescence ●
Produces hormones: ○ Thymosin
○ Thymoprotein
● Enables T lymphocytes (T cells) to mature and become functional
● Shrinks with age (replaced by fatty tissue)
AP 4.1 – Lymphatic System
4.1 – Overview of Lymphatic Functions
Lymphatic System
● Lymphatic system: lymph, lymphatic vessels, lymphoid tissue, and lymphoid organs
○ Closely connected to the cardiovascular system
● Main functions → fluid balance, fat absorption, immunity
Key Functions
● Fluid Recovery
○ Collects excess interstitial fluid that leaks out of blood capillaries
○ Once inside the lymphatic vessels, the fluid is called lymph
○ Lymphatic vessels return lymph to the bloodstream to prevent tissue swelling (edema)
● Transport of Fats
○ Specialized lymphatic capillaries in the small intestine absorb fats from digestion ○ These
fats enter the lymph and eventually reach the bloodstream
● Immune Defense
○ Lymph passes through lymph nodes, which contain immune cells that:
■ Scan the lymph for foreign particles, pathogens, or
abnormal cells ■ Mount immune response when necessary
Lymphatic Vessels
● Flow of Lymph
○ One-way flow: lymphatic capillaries → larger lymphatic vessels → subclavian veins →
bloodstream → heart ○ Ultimately returns fluid to venous blood
● Where Lymph Comes From
○ Fluid begins as interstitial fluid between cells
○ Most interstitial fluid is reabsorbed by the capillaries, but not all
○ The remaining fluid enters lymphatic capillaries
● Lymphatic Capillaries
○ Extremely porous and positioned near cardiovascular capillaries
○ Take up plasma fluid that has leaked out of blood capillaries due to
high pressure ■ This fluid was not reabsorbed by the
circulatory system
● Purpose
, ○ The fluid in lymph bathes cells and helps supply them with glucose, oxygen, salts, amino
acids, and nutrients ○ Prevents fluid buildup in tissues
○ Ensures leaked plasma returns to circulation and maintains blood volume
Overview– Pathway Summaries
1. Excess interstitial fluid → lymphatic capillaries
2. Lymphatic capillaries → larger lymphatic vessels
3. Lymphatic vessels → subclavian veins
4. Lymph mixes with venous blood → returns to heart
Relationship Between Cardiovascular & Lymphatic Circulation
Structure of a Lymphatic Capillary
Lymphatic (Lymph) Vessels
● Location
○ Found throughout most of the body
○ Areas without blood vessels (e.g. bone marrow, epidermis) also lack lymphatic vessels
● Structure & Function
○ One-Way Flow
■ Have one-way valves, similar to veins
■ These valves prevent backward flow of lymph
● Movement of Lymph
○ Lymph movement does not rely on the heart ○ Instead, it depends on:
, ■ Skeletal muscle contraction (muscle pump)
■ Pressure changes, as the body moves
○ These actions squeeze lymph forward through vessels
Hierarchy of Lymphatic Vessels
1. Lymphatic capillaries
2. Larger lymphatic vessels
3. Lymphatic trunks
a. Formed by merging large lymphatic vessels
b. Drain major regions of the body
4. Lymphatic ducts
a. Final drainage point
b. Empty lymph into subclavian veins, returning it to venous circulation
Lymphoid Organs
● Contain lymphocytes (T cells and B cells), dendritic cells, and macrophages that defend the body
against pathogens
Lymph Nodes
● Hundreds found at junctions of lymphatic vessels
● Function as filtering centers
○ Remove infectious organisms, debris, and foreign material
from lymph ● Lymph flows through sinuses:
○ cortical → subcapsular → medullary
● Contain lymphocytes and macrophages for immune defense
Spleen
● Located on the left side of the stomach; largest lymphoid organ
● Functions:
○ Removes old or damaged RBCs and platelets
○ Filers out bacteria, viruses, toxins, and debris from blood
○ Stores lymphocytes and macrophages to mount immune response
Thymus
● Located in the thorax, above the heart ● Most active during childhood and adolescence ●
Produces hormones: ○ Thymosin
○ Thymoprotein
● Enables T lymphocytes (T cells) to mature and become functional
● Shrinks with age (replaced by fatty tissue)
