Why We Need Transport — Blood, Tissue Fluid and Lymph

What you do

• Follow one water molecule: plasma → tissue fluid → lymph → back into a vein. Name the vessel it is in at each step.
• Explain why a red blood cell can never turn up in lymph but a lymphocyte can.

Challenges to clear

• Fill the tissue fluid bin first — it is the one most people get wrong.
• Now sort all eleven clues into the right fluid.
• Fill the Lymph box — the fluid that only ever flows one way.

The task

Three fluids move around inside you. Sort each clue into the fluid it belongs to.

What this lesson covers

• Blood — Sealed inside the heart and the blood vessels: Held in the heart, arteries, veins and capillaries (Blood never leaves its vessels, which is why ours is called a closed blood circulatory system. In insects the blood flows through open body spaces instead.); About 5 to 6 litres in an average adult (Blood is the only one of the three given a volume in the book. Tissue fluid and lymph are never quoted as a single tidy amount.); Bright red from an artery, dark red from a vein (Only blood carries haemoglobin, so only blood is red. Deoxygenated blood is dark red, never blue, whatever the diagrams suggest.); Carries red cells, white cells and platelets (All three formed elements travel in blood. Lymph carries white cells only, so this clue can belong nowhere else.)
• Tissue fluid — In the spaces between the cells of an organ: Formed when plasma and leukocytes leak out through capillary walls (A capillary wall is a single layer of cells, so plasma seeps out into the spaces between the body cells. Red cells are too large to follow.); The fluid a body cell actually takes its oxygen from (No cell ever touches blood directly. Oxygen crosses from the capillary into this fluid, the cell absorbs it from there and returns carbon dioxide the same way.); Fills the spaces between the cells of an organ (It is also called intercellular or extracellular fluid. It has no vessels of its own — it simply bathes the cells.)
• Lymph — Inside lymph vessels, lymph nodes and lymphatic organs: Contains leukocytes but no red cells and no platelets (Only white cells, mostly lymphocytes, get through into the lymph vessels. That is why lymph is colourless.); 94 per cent water and 6 per cent solids (The book gives exactly this split for lymph. Blood plasma is quoted differently, at 90 to 92 per cent water.); Passes through nodes that trap germs on its way back (The nodes localise an infection and stop it spreading, which is why the nodes in your groin or armpit swell when a limb is injured.); Poured into the large veins close to the right auricle (This fluid flows one way only, from the tissues back to the blood. It rejoins the circulation just before the blood enters the heart.)

Hint

Tissue fluid is plasma that has escaped; lymph is tissue fluid that has been collected up. One fluid literally becomes the next. Trap: synovial fluid and vitreous humour are body fluids too, but they do not circulate, so they belong in none of these bins.