The shape of the lesson
By the end of the lesson, learners can
- Four cells to recognise: Click each cell in turn: the red blood cell, the neutrophil, the monocyte and the lymphocyte.
- Tissue fluid: out, then back: Watch the blue water particles leave the capillary near the arterial end and return near the venous end.
- Neutrophil: lobed nucleus. Monocyte: largest, kidney-shaped nucleus.
- Lymphocyte: small, round nucleus filling the cell. Red cell: no nucleus.
- Water: solvent action and high specific heat capacity.
- Arterial end hydrostatic pressure out; venous end osmosis back in.
One picture every idea returns to
Blood contains red blood cells (biconcave, no nucleus, full of haemoglobin) and white cells: neutrophils (lobed nucleus, granular cytoplasm, phagocytes), monocytes (the largest, with a kidney-shaped nucleus, becoming macrophages) and lymphocytes (small, with a large round nucleus filling most of the cell). Water is the main component of blood plasma and of tissue fluid. Its solvent action lets it carry glucose, ions, amino acids, urea and hormones in solution, and its high specific heat capacity lets blood carry heat without large temperature changes. Tissue fluid bathes the cells, supplying them with oxygen, glucose and ions and removing wastes such as carbon dioxide. It forms at the arterial end of a capillary, where hydrostatic pressure forces fluid and small solutes out; plasma proteins stay in, so at the venous end, where hydrostatic pressure has fallen, water returns by osmosis. Excess fluid drains into lymph vessels.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Most of blood is water, and some of it leaks out.. Blood is cells carried in plasma, and plasma is mainly water. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Four cells to recognise | Click each cell in turn: the red blood cell, the neutrophil, the monocyte and the lymphocyte. Project tab 3 of the lab (Transport in mammals lab, blood cells) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Tissue fluid: out, then back | Watch the blue water particles leave the capillary near the arterial end and return near the venous end. Project tab 4 of the lab (Transport in mammals lab, tissue fluid) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 22 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Tissue fluid: out, then back, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A lymphocyte's nucleus nearly fills the cell, leaving a thin rim of cytoplasm; a monocyte is much bigger with a bean-shaped nucleus. | Do not call any cell with a large nucleus a monocyte. |
| Tissue fluid is not the same as plasma. | It has almost no plasma proteins and no red blood cells, because these are too large to leave the capillary. |
Blood cells, water and tissue fluid: worksheet
6 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced.
State three functions of tissue fluid.
Describe how you would distinguish a neutrophil, a monocyte and a lymphocyte on a stained blood smear, and state one way all three differ from a red blood cell.
Relate two properties of water to its role in transport in mammals.
Describe and explain how tissue fluid forms at the arterial end of a capillary and returns at the venous end.
A person with a liver disease makes too little plasma protein. Suggest and explain why tissue fluid builds up in the tissues of this person.
In a capillary, the hydrostatic pressure is 4.3 kPa at the arterial end and 1.7 kPa at the venous end. The opposing effect of the water potential of the blood is 3.3 kPa all along. Calculate the net pressure at each end and state the direction of fluid movement at each.
Answer key and marking notes
- It bathes the cells, providing the medium through which substances are exchanged.
- It supplies cells with oxygen, glucose, amino acids and ions from the blood.
- It removes waste products such as carbon dioxide and urea from the cells back into the blood (or lymph).
- Neutrophil: nucleus with several lobes; granular cytoplasm.
- Monocyte: the largest white cell; a large kidney- or bean-shaped nucleus.
- Lymphocyte: small; a large round nucleus that fills most of the cell, with a thin rim of cytoplasm.
- All three have a nucleus and are larger than a red blood cell, which has no nucleus and is a biconcave disc.
- Water is polar, so it is a good solvent.
- Glucose, amino acids, ions, urea and hormones dissolve in plasma and are carried in solution to and from cells.
- Water has a high specific heat capacity: a lot of energy is needed to raise its temperature.
- So blood can absorb and carry heat from active organs to the rest of the body without a large change in its own temperature.
- At the arterial end the hydrostatic pressure of the blood is high (from the contraction of the ventricles).
- It is greater than the effect of the lower water potential of the blood, so water and small solutes are forced out through gaps in the capillary wall.
- Cells and large plasma proteins remain in the capillary, so the water potential of the blood stays lower than that of the tissue fluid.
- Along the capillary the hydrostatic pressure falls as fluid is lost and resistance slows the blood.
- At the venous end water moves back into the capillary by osmosis; excess tissue fluid drains into lymph vessels.
- Fewer plasma proteins means the water potential of the blood is higher (less negative) than normal.
- The water potential gradient from tissue fluid into the capillary at the venous end is smaller.
- Less water returns by osmosis, while fluid still leaves at the arterial end, so fluid accumulates (oedema).
- Arterial end: 4.3 - 3.3 = 1.0 kPa outwards.
- Venous end: 3.3 - 1.7 = 1.6 kPa inwards.
- So fluid leaves the capillary at the arterial end and water returns by osmosis at the venous end.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which is the main component of both blood plasma and tissue fluid?
- Awater
- Bplasma proteins
- Cglucose
- Dsodium chloride
Why can blood carry heat from the liver and muscles to the rest of the body without its own temperature changing much?
- Awater has a high specific heat capacity
- Bwater is a good solvent for ions
- Cwater molecules are held together by cohesion
- Dhaemoglobin absorbs the heat released
A cell on a blood smear is about 8 μm across. Almost all of it is taken up by a dark, round nucleus with a thin ring of cytoplasm. What is it?
- Aa lymphocyte
- Ba neutrophil
- Ca monocyte
- Da red blood cell
Which substance passes out of a capillary into the tissue fluid at the arterial end?
- Aglucose
- Bred blood cells
- Calbumin and other plasma proteins
- Dplatelets
Which feature of a red blood cell gives it a large surface area to volume ratio for loading and unloading oxygen?
- Aits biconcave disc shape
- Bits lack of a nucleus
- Cits flexible cell membrane
- Dits haemoglobin content
The graph shows hydrostatic pressure along a capillary (solid line) and the opposing effect of the blood's water potential (dashed line). At which point is fluid returning to the capillary fastest?
- AD
- BA
- CB
- DC
A blockage in the lymph vessels draining a leg causes the leg to swell. Which explanation is correct?
- Amore tissue fluid forms than returns to the capillaries, and the excess cannot drain away
- Bplasma proteins leak out of the capillaries into the tissues
- Chydrostatic pressure in the capillaries falls, so fluid stops leaving
- Dred blood cells collect in the tissue fluid of the leg
In one capillary bed, 20 cm3 of fluid leaves the capillaries each minute and 18 cm3 returns by osmosis. What percentage of the fluid that leaves is drained by the lymph vessels?
- A10%
- B90%
- C2%
- D11%
Quiz key
Water makes up about 90% of plasma and most of tissue fluid. Plasma proteins are a major solute of plasma but almost absent from tissue fluid. Glucose and sodium chloride are dissolved in the water in small amounts.
A high specific heat capacity means a lot of energy is needed to raise water's temperature, so blood absorbs heat with only a small rise. Solvent action concerns carrying dissolved substances, cohesion matters in xylem, and haemoglobin carries oxygen, not heat.
A small cell with a large round nucleus and very little cytoplasm is a lymphocyte. A neutrophil has a lobed nucleus and granular cytoplasm, a monocyte is much larger with a kidney-shaped nucleus, and a red blood cell has no nucleus.
Glucose is a small, soluble molecule forced out with water by hydrostatic pressure. Red blood cells, platelets and large plasma proteins such as albumin are too large to pass through the gaps, so they stay in the blood.
The biconcave disc has more surface for its volume than a sphere, and no part of the cytoplasm is far from the membrane. Having no nucleus leaves more room for haemoglobin; flexibility lets the cell squeeze through capillaries; haemoglobin carries the oxygen. None of these changes the surface area ratio.
At D the hydrostatic pressure is furthest below the water potential effect, so the net inward pressure, and the return of water by osmosis, is greatest. At A fluid is leaving fastest. At B the two are equal, so there is no net movement. At C water is returning, but the gap is smaller than at D.
Normally slightly more fluid leaves the capillaries than returns, and lymph vessels drain the excess. If they are blocked, tissue fluid accumulates and the leg swells. The blockage does not make proteins or red cells leave the capillaries, and a fall in hydrostatic pressure would reduce, not increase, tissue fluid.
20 - 18 = 2 cm3 per minute is not returned; 2 ÷ 20 × 100 = 10%, which drains into the lymph. 90% is the fraction that returns to the capillaries. 2% confuses the volume with a percentage. 11% divides by 18, the returned volume, instead of 20.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.