Lymphatic & Capillary Exchange

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Define capillary exchange

Capillaries are thin-walled vessels that easily permit fluids to move in and out with dissolved substances in them.

Capillary exchange is

Rapid exchange of plasma-dissolved nutrients, wastes and gases

Where does capillary exchange take place

between the blood and the interstitial fluids (fluids that bathe the tissues), through the capillary walls.

Main Forces that move fluids into and out of the capillaries

Hydrostatic pressures(HP)
• Blood pressure within the vessels
• Fluid pressures on the interstitial fluids
Osmotic Pressures (OP)
• Pressures that result from solutes (ions, sugars, proteins) dissolved in the blood and the interstitial fluid
• Osmotic pressure is highest where there are the most solutes
• Osmotic pressure draws fluids in

What happens as blood moves from the arteriolar end to the venule end of the capillary

changes in the hydrostatic blood pressure combine with the osmotic pressures of the blood and interstitial fluids to move fluids
out at the arteriolar end and
in at the venule end.

What happens to the hydrostatic (blood) pressure as blood moves through the capillary? Which way will this move fluids on the arterial end? Which way will this move fluids on the venule end?

Hydrostatic pressure drops as blood moves through the capillary.
At the arteriolar end this moves fluids out
Which way will this move fluids on the venule end?
At the venule end this moves fluids in

The combination of outward and inward forces causes

a net flow of fluids OUT at the arteriolar end of the capillary.

Arterial end: What are the outward forces

1. Blood pressure 30 mm Hg
This is a positive pressure. It forces plasma with O2 and Nutrients out of capillaries

2. Interstitial fluid pressure 3 mm Hg
This is a negative pressure. It sucks plasma out

3. Interstitial osmotic pressure 8 mm Hg
Osmotic pressure is due to solutes in the interstitial fluid. It draws fluid out from the blood

Arterial end: What are the inward forces

1. Blood osmotic pressure 28 mmHg
Osmotic pressure is due to solutes in the blood. It draws fluid into the blood

Arterial end: What is the net flow

The outward forces add up to 41 mmHg. The inward force is 28mmHg

Thus there is a net force of 13mmHg moving fluid out at the arterial end
This force moves plasma with 02 and nutrients into the tissues

Venule end:The combination of outward and inward forces causes

a net flow of fluids IN at the venule end of the capillary

Venule End: What are the outward Forces?
What are the inward Forces?
What is the net flow?

1. Blood pressure drops to 10 mm Hg
2. Interstitial hydrostatic pressure remains constant at 3 mm Hg
3. Tissue fluid osmotic pressure remains constant at 8 mm Hg
Blood osmotic pressure remains constant at 28 mm Hg
The outward forces add up to 21 mmHg. The inward force is 28mmHg

Thus there is a net force of 7mmHg moving fluid in at the venule end

This force draws plasma with C02 and wastes into capillaries

How does the amount of fluid leaving the capillaries compare with the amount of fluid entering? `

What might be the effect of this?

At the arteriolar end there is a net force of 13 mm HG moving fluids out
At the venule end there is a net force of 7 mm HG moving fluid back in

There is a difference of 6 mmHg
Thus more fluid moves out than returns.

This could lead to a gradual loss of blood volume. However, the lymphatic vessels returns this leaked fluid to the blood.

Define the lymphatic system

1.Low pressure
2.One-way system leading to circulatory system

List the functions of the lymphatic system

1. Returns leaked capillary fluid to blood
2. Transports digested lipids and vitamins - These molecules are too big to be absorbed into blood capillaries
3. Produces + Distributes Lymphocytes - part of the body's immune function

The lymphatic system is made up of:

1. Lymph Vessels - Filled with lymph fluid and lymphocytes
2. Lymphatic tissues - Lymph nodules
3. Lymphatic organs (ENCAPSULATED)
Lymph nodes

Cells associated with the lymphatic system include:


Lymph vessels include:

Veins and venules

Why aren't there any lymph arteries?

Lymph only flows in one direction, towards the heart. Arteries carry blood away from the heart.

Define lymph capillaries

How do they differ from the blood capillaries

small, single-layered vessels much like blood capillaries.

Lymph capillaries are
1. Larger than blood capillaries
2. Closed at one end
3. More permeable than blood capillaries - This allows large molecules such as lipids to penetrate the vessel
4. Endothelial cells that form the walls of the capillaries direct fluid inward only

Functions of lymph capillaries

1. Pick up excess plasma resulting from the capillary fluid exchange and carry it to the veins.
2. Pick up lipids from the digestive tract

Lymph Veins are designed to

Carry excess plasma and lipids back to the blood vessels emptying into the heart.

What would happen if the lymph vessels were blocked and couldn't return the extra leaked fluid to the blood?

The excess fluid from capillary exchange would start to build up in the tissues of the body.
The fluid-filled tissues would swell. A condition known as edema.

Like blood veins, lymph veins have

1 way valves

Lymph flows in one direction through the following pathway.

• From Capillaries to Veins
• From Veins to the
o Thoracic Duct or the
o Right Lymphatic Duct
• The lymph is dumped into the Subclavian Veins of the circulatory system

Why is lymph dumped into the Subclavian veins? What would happen if instead it were dumped into the aorta?

The Subclavian veins are the lowest pressure points in the circulatory system. Thus fluid can easily be added too blood here.

If lymph were dumped into the aorta, the high pressure of the blood in the aorta would make it squirt right back out.

Lymph is pushed towards the subclavian vein, but there is no lymph heart behind it.
What mechanisms do you think move the lymph up through the veins?

Lymph is moved by the same mechanisms that move venous blood:
• Contraction of skeletal muscle
• Inhalation

The production and maturation of lymphocytes takes place in the

• Red bone marrow
• Thymus
• Spleen

The lymph is filtered for pathogens in the

• Lymph nodes
• Lymphatic nodules
• Spleen

As already mentioned in the lecture on blood, lymphocytes include the:

• T cells
• B cells
• Natural Killer cells

Most lymphocytes live how long

4 years, some 20 years

Lymphocytes wander through the body and concentrate in the

• Blood + Thymus gland
• Spleen
• Bone marrow

The primary site of lymphocyte formation is the

Red Bone Marrow

Hemocytoblasts form 2 types of stem cells

1. One group forms B + NK cells
• B cells enter blood
o Go to lymph nodes + spleen
• NK cells migrate through body

2. One group goes to the thymus
• Here they form T cells
o These then enter blood

Lymphatic tissues are

clumps of lymphocytes (B cells and T cells) within the connective tissues and mucus membranes of many organs

Lymphocytes are not covered in

a connective tissue capsule

Lymphocytes are found in the mucus membranes of

o G.I. Tract
o Respiratory tract
o Urinary tract
o Reproductive tract

Lymphocytes are called MALT

Mucosa-Associated Lymphatic Tissue

Why are lymphatic tissues clustered at these particular regions? What do they all have in common?

These tissues tracts all open to the outside. They are thus the places where pathogens would gain entry to the outside.
Lymphatic tissues guard the entryways.

In some places lymphatic tissues form dense masses or nodules.
Large clusters of nodules are found in the

o Mouth-Tonsils
o Intestines-Peyers patches

In some places lymphatic tissues form dense masses or nodules. What do they contain

• Contain T + B cells
• Help fight infection

Lymphatic organs differ from lymphatic tissues in that

they are enclosed in a connective tissue capsule

Lymphatic organs

o Lymph nodes
o Thymus
o Spleen

Lymph nodes are

small, encapsulated bean-shaped clumps of connective tissue

Where are lymph nodes highly concentrated?

• Mammary glands
• Axillae (arm pits)
• Digestive area
• Groin & neck

Lymph Node Function

These cells filter lymph and destroy foreign microorganisms

Filtering Process of lymph nodes

• Lymph passes through the layers of the node
• Immune cells remove and destroy antigens that pass through

Why do the lymphatic vessels have valves?`

These are one-way valves that direct the flow of lymph into the afferent and out of the efferent vessels.

trace the pathway of lymph through a node.

Lymph enters the node through the Afferent lymphatic vessels.
It then passes through the Medullary sinus.
It moves out through the Efferent lymphatic vessel.

Immune Cells In Lymph Nodes

• Engulf debris and pathogens
• Process them and present pathogens to lymphocytes for destruction
Dendritic cells
• Bind pathogens present them to lymphocytes for destruction
• Destroy and create antigens against pathogens

The thymus is located

just above the heart
It is large through age 12 and then gradually shrinks with age.

Thymus function

Developing T cells cluster in the thymus.
• They develop the ability to distinguish between the body's own cells and foreign invading cells.
• Mature T cells normally attack only invaders.

The spleen is located

in the left hypochondriac region, superior to the stomach

Spleen functions

• Filters blood
o Macrophages phagocytize bacteria and worn RBCs
o Lymphocytes destroy pathogens
• Stores and releases Blood at times of need (Sympathetic Stimulation)

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