Define capillary exchange
Capillaries are thin-walled vessels that easily permit fluids to move in and out with dissolved substances in them.
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
• 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.
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)
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.
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
As already mentioned in the lecture on blood, lymphocytes include the:
• T cells
• B cells
• Natural Killer cells
Lymphocytes wander through the body and concentrate in the
• Blood + Thymus gland
• 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 found in the mucus membranes of
o G.I. Tract
o Respiratory tract
o Urinary tract
o Reproductive tract
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 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
Where are lymph nodes highly concentrated?
• Mammary glands
• Axillae (arm pits)
• Digestive area
• Groin & neck
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
• 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.
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.