IB Biology HLTransport in Animals & PlantsPaper 1 & 2~12 min read
Tissue Fluid
Your cells never touch blood. What actually bathes them is tissue fluid – plasma that has been squeezed out through the walls of a capillary. Whether it leaves the capillary or returns to it comes down to a tug of war between two forces.
📚 What you need to know
Plasma is a straw-coloured liquid making up about 55% of blood, and is about 95% water. Water is a good solvent, so many substances dissolve in it and are carried around the body.
As blood passes through capillaries, some plasma is forced out through gaps in the wall to form tissue fluid, which surrounds the cells.
Tissue fluid is similar to plasma but contains fewer proteins and cells, because these are too large to pass through the capillary wall.
At the arterial end, blood pressure (hydrostatic pressure) is high enough to force fluid out. This is pressure filtration.
At the venous end, blood pressure has fallen and the solute gradient created by the remaining plasma proteins pulls water back in by osmosis.
Roughly 90% of the fluid is reabsorbed. The other 10% drains into lymph vessels and is eventually returned to the circulatory system.
Lymph vessels contain valves, are moved by body movement, and pass through lymph nodes containing macrophages.
The tug of war at a capillary
Two opposing forces act on the fluid inside a capillary, and which one wins depends on where along the capillary you look.
Hydrostatic pressure – the blood pressure generated by the heart. It pushes fluid out through the gaps in the capillary wall.
The solute gradient – plasma proteins are too big to escape, so they stay in the capillary. That makes the blood a more concentrated solution than the tissue fluid, which pulls water back in by osmosis.
The solute gradient is roughly constant along the capillary. The hydrostatic pressure is not – it falls as blood travels through, because of distance from the heart and the slow, high-resistance flow through the capillary. So the balance flips.
The arrow thicknesses show the relative sizes of the two forces. Only the red one changes much along the capillary.
The whole idea in one line
fluid leaves where the push beats the pull, and returns where the pull beats the push
The specification only requires you to know about pressure filtration and the fall in pressure at the venous end. The solute gradient is included here because without it the reabsorption makes no sense – if only pressure mattered, fluid would simply leak out and stay out.
What is in tissue fluid
Tissue fluid is plasma minus the things that were too big to get through the gaps, and altered by what the cells have taken out and put in.
Substance
Blood plasma
Tissue fluid
Blood cells
Red blood cells, phagocytes, lymphocytes and platelets
Phagocytes only, which squeeze through the gaps
Proteins
Many large plasma proteins
Very few, and only smaller ones
Glucose
Higher concentration
Lower, because cells absorb it for respiration
Amino acids
Higher concentration
Lower, because cells absorb them for protein synthesis
Oxygen
Higher concentration
Lower, because cells use it in respiration
Carbon dioxide
Lower concentration
Higher, because cells produce it as waste
Exchange between cells and blood happens through this fluid. Carbon dioxide made in a cell diffuses out into the tissue fluid, and from there into the capillary.
Where the leftover fluid goes
About 90% of the fluid is reabsorbed at the venous end. The remaining 10% would build up and cause swelling if there were no way to collect it – and that is the job of the lymphatic system.
Lymph vessels have no pump either. Movement of the body squeezes them, and valves make sure the fluid can only travel one way.
Lymph capillaries are separate from the circulatory system, have closed ends and large pores that let big molecules in.
Fluid inside them is called lymph. It moves along the larger lymph ducts because body movement compresses them, and valves prevent backflow.
Lymph nodes contain macrophages that engulf bacteria, along with other immune cells.
Any plasma proteins that escaped from the blood are returned this way. If they were left in the tissue fluid they would raise its solute concentration and stop water being reabsorbed.
After digestion, lipids are carried from the intestines to the bloodstream by the lymph system, so lymph contains lipids.
Lymph finally re-enters the blood through veins close to the heart.
Why ankles swell on a long flight. Sitting still means no muscle movement, so lymph is not squeezed along and fluid accumulates in the lower limbs. The same reasoning explains swelling after an injury or when lymph nodes are removed.
Worked examples
WE 1
How tissue fluid forms
Explain how tissue fluid is formed at the arterial end of a capillary. (3 marks)
Point 1: the pressure
At the arterial end the hydrostatic pressure of the blood is high, because the capillary is close to the arteriole.
Point 2: the filtration
This forces plasma out through the gaps in the capillary wall; the process is called pressure filtration.
Point 3: what leaves and what stays
Water and small solutes such as glucose, amino acids and oxygen pass out, but plasma proteins and red blood cells are too large and remain in the blood.
High pressure pushes small molecules out; big ones stay behinduse the term hydrostatic pressure and name at least one substance that cannot leave
WE 2
Why fluid returns
Explain why most tissue fluid returns to the capillary at the venous end. (3 marks)
Point 1: pressure has fallen
Blood pressure is much lower at the venous end, because of the distance from the heart and the slow flow through the capillary, so less fluid is pushed out.
Point 2: the solute gradient
Plasma proteins stayed behind, so the blood has a higher solute concentration than the tissue fluid.
Point 3: osmosis wins
Water therefore moves back into the capillary by osmosis, and this inward pull is now greater than the outward push.
The push has weakened but the pull has notsay which force is greater at each end – that comparison is what earns the mark
WE 3
Swelling in the tissues
A patient has a very low concentration of plasma proteins. Suggest why their tissues become swollen. (3 marks)
Point 1: what proteins normally do
Plasma proteins stay in the capillary and create the solute gradient that draws water back in at the venous end.
Point 2: the effect of losing them
With fewer proteins the blood is less concentrated, so the inward osmotic pull is much weaker.
Point 3: the result
Less tissue fluid is reabsorbed, so fluid accumulates around the cells faster than the lymph system can drain it, and the tissues swell.
No solute gradient means no reabsorptionthe same reasoning explains swelling when lymph vessels are blocked – the drainage route is lost instead
💡 Exam tips
Use hydrostatic pressure for the push and solute gradient or osmosis for the pull.
Say which force is greater at each end, then state the net movement.
Name pressure filtration as the process forming tissue fluid.
When comparing plasma and tissue fluid, explain why each difference exists – too large to fit, or used up by cells.
Remember the approximate split: 90% reabsorbed, 10% into lymph.
Lymph moves by body movement and valves, not by the heart.
⚠ Common mistakes
Saying cells are bathed in blood. They are bathed in tissue fluid.
Saying tissue fluid contains red blood cells. They are far too large to fit through the gaps.
Claiming blood pressure pulls fluid back in. Pressure only ever pushes out; osmosis pulls in.
Forgetting that some phagocytes do get out. They are flexible and squeeze through.
Saying lymph is pumped by the heart. There is no pump in the lymphatic system.
Confusing lymph with plasma. Lymph is drained tissue fluid, and it also carries absorbed lipids.
Up next: Circulatory Systems – why a fish gets away with one circuit and a mammal needs two.
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