CHAPTER 20: LYMPHATIC

SYSTEM, LYMPHOID

ORGANS & TISSUES

Human Anatomy and Physiology II –

BIOL153

The big picture

Fluid filters from capillaries at their arteriolar

end and flows through the interstitial space.

Most is reabsorbed at the venous end.

For all capillary beds, 20 L of fluid is filteredout per day—almost 7times the total plasmavolume!

• Due to fluid pressing against a

boundary• HP “pushes” fluid across the

boundary• In blood vessels, is due to blood

pressure

• Due to nondiffusible solutes that

cannot cross the boundary• OP “pulls” fluid across the

boundary• In blood vessels, is due to

plasma proteins

Piston

Boundary

Solutemolecules(proteins)

Boundary

“Pushes” “Pulls”

Hydrostatic pressure (HP) Osmotic pressure (OP)

17 L of fluid per day is reabsorbedinto the capillariesat the venous end.

LymphaticcapillaryVenule

About 3 L per day of fluid (and anyleaked proteins) areremoved by thelymphatic system(see Chapter 20).

Arteriole

Fluid moves through the interstitial space.

Net filtration pressure (NFP) determines the

direction of fluid movement. Two kinds of pressure drive fluid flow:

Goals/Objectives

Describe the structure and distribution of lymphatic vessels

Describe the source of lymph and mechanism(s) and pathway of lymph transport

Describe the general location, histological structure, and functions of lymph nodes

Describe the basic structure and cellular population of lymphoid tissue

Name and describe the other lymphoid organs of the body. Compare and contrast them with lymph nodes, structurally and functionally.

Lymphatic System

One-way system of vessels in which lymph flows only toward the heart

Consists of three parts

1. Network of lymphatic vessels (lymphatics)

2. Lymph – fluid in vessels

3. Lymph nodes –cleanse lymph

Lymphatic System

Lymphatic Capillaries

Lymphatic Capillaries

Filaments anchored

to connective tissue

Endothelial cell

Flaplike minivalve

Fibroblast in loose

connective tissue

Lymphatic capillaries are blind-ended tubes in which adjacent

endothelial cells overlap each other, forming flaplike minivalves.

Lymphatic Collecting Vessels

Lymph Transport – What Makes

Lymph Flow?

Clicker Question

Lymphatic vessels _______.

a) drain blood from lymph nodes

b) are part of the venous system

c) collect excess fluid from interstitial

spaces

d) all of the above

Clicker Question

Which of the following contribute(s) to the flow of

lymph through lymphatic vessels?

a) The pumping of the heart

b) The milking action of skeletal muscle

contraction

c) Pressure changes in the thorax that result

from breathing

d) Both b and c

Goals/Objectives

Describe the structure and distribution of lymphatic vessels

Describe the source of lymph and mechanism(s) and pathway of lymph transport

Describe the general location, histological structure, and functions of lymph nodes

Describe the basic structure and cellular population of lymphoid tissue

Name and describe the other lymphoid organs of the body. Compare and contrast them with lymph nodes, structurally and functionally.

Lymph Nodes

Lymph Nodes

Lymph Nodes

Functions

1. Filter lymph

• Macrophages destroy microorganisms

and debris

2. Immune system activation

• Lymphocytes activated and mount attack

against antigens

Structure of a Lymph Node

Afferentlymphaticvessels

Cortex

• Lymphoid follicle

• Germinal center

• Subcapsular sinus

Efferentlymphaticvessels Follicles

Trabecula

Subcapsularsinus

Capsule

Medullarycords

Medullarysinuses

Trabeculae

• Medullarysinus

• Medullarycord

Medulla

Hilum

Longitudinal view of the internal structure of a lymph

node and associated lymphatics

Photomicrograph of part of a lymph

node (72x)

Capsule

Circulation in the Lymph Nodes

Afferent lymphatics

↓

Subcapsular sinus

↓

Trabecular sinus

↓

Medullary sinus

↓

Efferent lymphatics

Lymphoid Cells - Lymphocytes

Lymphocytes

main warriors of

immune system;

• Arise in red

bone marrow

• Have a large

purple nucleus,

very little

cytoplasm.

Other Lymphoid Cells

Macrophages

phagocytize foreign

substances; help

activate T cells

Dendritic cells capture

antigens and deliver

them to lymph nodes;

activate

T cells

Reticular cells produce

reticular fiber stroma

that supports other cells

in lymphoid organs

Structure of a Lymph Node

Afferentlymphaticvessels

Cortex

• Lymphoid follicle

• Germinal center

• Subcapsular sinus

Efferentlymphaticvessels Follicles

Trabecula

Subcapsularsinus

Capsule

Medullarycords

Medullarysinuses

Trabeculae

• Medullarysinus

• Medullarycord

Medulla

Hilum

Longitudinal view of the internal structure of a lymph

node and associated lymphatics

Photomicrograph of part of a lymph

node (72x)

Capsule

Goals/Objectives

Describe the structure and distribution of lymphatic vessels

Describe the source of lymph and mechanism(s) and pathway of lymph transport

Describe the general location, histological structure, and functions of lymph nodes

Describe the basic structure and cellular population of lymphoid tissue

Name and describe the other lymphoid organs of the body. Compare and contrast them with lymph nodes, structurally and functionally.

Other Lymphoid Tissue and

Organs

Houses, and provides proliferation site for,

lymphocytes

Surveillance vantage point for

lymphocytes and macrophages

Largely reticular connective tissue –

type of loose connective tissue

Two main types of lymphoid tissue

Diffuse lymphoid tissue (more scattered);

Lymphoid follicles (tight collections of B/T

cells)

Other Lymphoid Tissue and

Organs

© 2013 Pearson Education, Inc.

Tonsils (in pharyngealregion)

Thymus (in thorax; most active during youth)

Spleen (curves around left side of stomach)

Peyer’s patches

(aggregated lymphoidnodules in smallintestine)

Appendix

Spleen

Functions

• Like LNs - Site of lymphocyte

proliferation and immune

surveillance and response

• Cleanses blood of aged cells

and platelets, macrophages

remove debris

Spleen - Structure

Spleen - Structure

Spleen - Structure

Spleen: Additional Functions

Stores breakdown products of

RBCs (e.g., iron) for later reuse

Stores blood platelets and

monocytes

May be site of fetal erythrocyte

production (normally ceases

before birth)

Thymus

Thymus - Structure

Capsule

Cortex

Medulla

Thymiccorpuscles

• The main function of the thymus is to “educate” naïve T cells to recognize what is foreign and was is not.

• Differs from other lymphoid

organs in important ways:

• Has no follicles because

it lacks B cells

• Does not directly fight

antigens

• Functions strictly in

T lymphocyte

maturation

Thymus

Stroma of epithelial cells (not reticular fibers)

• Provide environment in which T

lymphocytes become immunocompetent

Mucosa-associated Lymphoid Tissue

(MALT)

Lymphoid tissues in mucous membranes

throughout body

Protects from pathogens trying to enter

body

Largest collections of MALT in tonsils,

Peyer's patches, appendix

Also in mucosa of respiratory and

genitourinary organs; rest of digestive

tract

Tonsils

Pharyngeal tonsil

Palatine tonsilLingual tonsil

Germinal centersin lymphoid follicles

Tonsillarcrypt

Contain follicles with

germinal centers

Are not fully

encapsulated

Overlying epithelium

invaginates forming

tonsillar crypts

Trap and destroy bacteria

and particulate matter

Allow immune cells to

build memory for

pathogens

Aggregates of Lymphoid

Follicles

Follicles of aPeyer’s patch(aggregatedlymphoid nodules)

Smooth muscle inthe intestinal wall

• In wall of distal portion of small

intestine (great location because there is

a lot of bacteria here)

• Similar structures are also found in the

appendix

• In Peyer’s patch and appendix:

• Destroy bacteria, preventing them

from breaching intestinal wall

• Generate "memory" lymphocytes

Appendix

Appendix

Clicker Question

The thymus is important for _______.

a) removal of foreign antigens

b) B lymphocyte maturation

c) T lymphocyte maturation

d) secretion of hormones that promote B

lymphocyte immunocompetence