F TO A LEARNING LABAS Biology 9700 · Topic 8 Transport in mammals

Transport in mammals: the big picture

Every one of your cells needs a steady supply of oxygen and glucose, and needs its waste (such as carbon dioxide) taken away. In a small organism, is fast enough. You are far too big and too active for that, so you have a pump (the heart), a transport medium (blood) and a network of tubes (blood vessels).

Mammals have a , : blood stays inside vessels, and it passes through the heart twice on every complete trip round the body. This lets the heart send blood to the lungs gently, then pump it again at high pressure to the rest of the body.

The story in five sentences

  1. The heart is a double pump made of . The left side pumps oxygenated blood to the body; the right side pumps deoxygenated blood to the lungs.
  2. Valves open and close only because of pressure differences. Blood always flows from higher to lower pressure, and valves stop it flowing backwards.
  3. The heartbeat starts itself. Heart muscle is : the sets the rhythm, and the delays the wave so the atria empty before the ventricles contract.
  4. Vessels match their jobs. Arteries cope with high, pulsing pressure; capillaries let substances in and out; veins return blood at low pressure.
  5. Red blood cells carry the gases. loads oxygen in the lungs and unloads it in respiring tissue, helped by carbon dioxide (the ). Most carbon dioxide travels as hydrogencarbonate ions in the plasma.

Tip: any opens a plain-language definition.

mindset

  • Compare pressures on both sides of a valve. "The semilunar valve opens because pressure in the left ventricle rises above pressure in the aorta" earns the mark; "the valve opens to let blood out" does not.
  • Use the full, correct terms: sinoatrial node, Purkyne tissue, carbaminohaemoglobin, chloride shift, tunica media. Vague words ("signals", "strong walls") lose marks.
  • Give cause → effect chains in order, and quote data with units (kPa, %, s) when a graph is given.
  • Know the command words: describe (say what happens or what the data shows), explain (give reasons), compare (a point about both, in one sentence), suggest (apply ideas to a new situation).

Explore the lab

Heart & cardiac cycle8.3.1–8.3.3 · animated heart, live pressure graph, valves Heartbeat control8.3.4 · SAN → AVN delay → Purkyne tissue, ECG (extension) Double circulation8.1.1–8.1.2 · follow a red blood cell Blood vessels8.1.3–8.1.4 · identify, label and sort Blood cells8.1.5 · blood smear identification Tissue fluid & water8.1.6–8.1.7 · pressures along a capillary Oxygen transport8.2.4–8.2.6 · dissociation curve, Bohr shift CO₂ transport8.2.1–8.2.3 · inside a red blood cell Exam practiceAll outcomes · instant feedback, mark schemes

Numbers worth knowing (typical resting adult)

QuantityValueQuantityValue
Cardiac cycle at 75 beats per minute0.8 sAVN delayabout 0.1 s
Peak left ventricle pressureabout 16 kPaPeak right ventricle pressureabout 3 kPa
pO₂ in alveoli (lungs)about 13 kPapO₂ in respiring tissueabout 2–5 kPa
Red blood cell diameterabout 7 µmCO₂ carried as HCO₃⁻about 85%

1 kPa (kilopascal, a unit of pressure) ≈ 7.5 mmHg. Cambridge questions usually use kPa.

8.3.18.3.28.3.3

The heart and the cardiac cycle

The is the sequence of events in one heartbeat: (contraction) and (relaxation) of the atria and ventricles. Press play, or drag the time slider, and watch how the valves respond to the pressures on the graph.

Animated section through a mammalian heart, viewed from the frontThe heart's right side is on your left. Chambers contract and valves open or close in time with the pressure graph.

Viewed from the front, so the heart's right side is on your left. Blue = deoxygenated blood, red = oxygenated blood.

Atrial systole
AV valves: open Semilunar valves: closed
Pressure and volume changes in the left side of the heartLines show left atrium, left ventricle and aorta pressure in kilopascals against time, with left ventricle volume below. Shaded bands show when the valves are open.
0.00 s

Graph markers: 1 AV valve shuts · 2 semilunar valve opens · 3 semilunar valve shuts · 4 AV valve opens. Blue band = AV valves open; orange band = semilunar valves open.

0.25×

Left atrium–
Left ventricle–
Aorta–
LV volume–

Change it: heart rate 8.3.3

75 bpm
Ventricular systole–
Rest of cycle (diastole)–

Change it: valve faults Extension

Valve disease is not a 9700 learning outcome, but it is a favourite "suggest" context and tests whether you really understand why valves open and shut.

= stiff, narrowed valve; = leaky valve.

Tap a part to see what it does and why

Pick a label

Tap a numbered marker on the heart or a label in the list. You will get what the part is, what it does and why it is built that way.

Reading the pressure graph 8.3.3

  • AV valves (the ) are open whenever atrial pressure is higher than ventricular pressure. They shut the moment the ventricle's pressure rises above the atrium's.
  • Semilunar valves ( at the base of the aorta and pulmonary artery) open only when ventricular pressure rises above the pressure in the artery, and shut when it falls below.
  • When both sets are shut, the ventricle is a sealed bag: pressure changes fast but volume stays the same ().
  • The small dip then bump on the aorta line is the dicrotic notch: a brief back-surge of blood snaps the semilunar valve shut, then the elastic wall recoils.

exam move

For "explain why the valve opens/closes at time X", always name both pressures: "At 0.12 s the pressure in the left ventricle becomes greater than in the left atrium, so the bicuspid (AV) valve closes, preventing backflow of blood into the atrium."

The right ventricle goes through the same cycle at the same time, but its peak pressure is only about 3 kPa (lungs need low pressure).

8.3.4 ECG = extension

Heartbeat control: SAN, AVN and Purkyne tissue

Cardiac muscle is : it contracts without needing an impulse from a nerve. The beat starts at the , a patch of specialised muscle in the wall of the right atrium. Watch the wave of spread.

Conduction pathway of the heartA wave of excitation spreads from the sinoatrial node across the atria, pauses at the atrioventricular node, then travels down the Purkyne tissue in the septum to the apex and up the ventricle walls.
1 · SAN fires

ECG-style trace for one heartbeatP wave, then a flat PR segment during the AVN delay, then the QRS complex, then the T wave. A cursor shows the current time.
0.00 s
0.1×

Change it: SAN rate 8.3.4

75 per min
One heartbeat lasts–

Change it: AVN delay 8.3.4

0.10 s
Atrial top-up delivered–
Ventricles filled (vs normal)–

The top-up and filling figures are a simple teaching model (the atria supply roughly the last fifth of ventricular filling at rest).

The sequence 8.3.4

  1. SAN (the pacemaker) sends out a wave of excitation at a regular rate.
  2. The wave spreads through the walls of both atria, so they contract together (atrial systole).
  3. A layer of between atria and ventricles stops the wave going straight into the ventricles.
  4. The wave passes only through the AVN, which delays it by about 0.1 s, so the atria finish emptying blood into the ventricles first.
  5. The wave travels fast down the in the septum (this part is called the ) to the apex (the bottom tip of the heart).
  6. Purkyne fibres carry it up through the ventricle walls, so the ventricles contract from the apex upwards, squeezing blood up into the aorta and pulmonary artery.

The syllabus says knowledge of nervous and hormonal control is not expected.

ECG-style trace Extension

An records the electrical activity of the heart through the skin. It is not a 9700 learning outcome, but it shows the timing beautifully:

  • P wave: excitation spreading across the atria (just before atrial systole).
  • Flat PR segment: the AVN delay.
  • QRS complex: excitation spreading through the ventricles via the Purkyne tissue (just before ventricular systole). It is tall because the ventricles have much more muscle.
  • T wave: the ventricles recovering (repolarising) before the next beat.
8.1.18.1.2

Follow a red blood cell round the double circulation

The takes blood from the heart to the lungs and back. The takes it from the heart to the rest of the body and back. Blood flows: artery → → capillary → → vein.

Map of the double circulationLungs at the top, heart in the middle and body tissues at the bottom. A red blood cell moves along the path and changes colour when it gains or loses oxygen.
Now atVena cava
Blood isDeoxygenated
Passes through heart0
Pressure hereLow

The four named vessels 8.1.2

Tap a vessel name for its function.
8.1.38.1.4

Arteries, veins and capillaries

Artery and vein walls have three layers. The (inner layer, lined by a smooth ), the (middle layer of smooth muscle and elastic fibres) and the (outer layer, mainly collagen fibres). The is the space blood flows through. Drag cards to their boxes, or tap a card then tap a box (keyboard: Enter on a card, then Enter on a box).

1 · Identify the transverse sections 8.1.3

These are drawn like stained light-microscope sections (, a slice straight across the vessel). They are not to the same scale: a capillary is far smaller.

2 · Label the artery 8.1.3

3 · Match each feature to its vessel 8.1.4

(such as the aorta) are close to the heart; carry blood to organs. When you check, each card tells you why the feature suits the job.

Change it: wall elasticity 8.1.4

How elastic is the wall of a large artery such as the aorta? 100% = healthy young adult.

100%
Highest pressure–
Lowest pressure–
Pressure swing–
Flow to tissues between beats (vs peak)–

Change it: lumen width 8.1.4

The smooth muscle of a muscular artery or arteriole contracts or relaxes to change its lumen.

100%

Plan diagrams: TS and LS 8.1.3

Plan diagrams of an artery and a veinTransverse sections show the artery with a thick tunica media and small round lumen, and the vein with a thin wall and wide lumen. A longitudinal section of a vein shows a pocket valve.

Rules Cambridge rewards in a plan diagram

  • A shows tissue layers only: no individual cells.
  • Use sharp, clear, continuous lines; no shading, no sketchy or overlapping lines.
  • Keep proportions correct (for example, the thickness of the tunica media compared with the lumen).
  • Draw large (at least half the space), and label with ruled lines that touch the layer and do not cross.
  • An (longitudinal section, a slice along the vessel) of a vein can show a valve.
8.1.5

Blood cells: what is on the slide?

A stained blood smear (a thin layer of blood on a glass slide) shows mostly red blood cells and a few . The stain colours nuclei purple, which is the key to telling the white cells apart.

Stained blood smearMany pink red blood cells without nuclei and one highlighted white blood cell to identify.

Round 1 of 8

Which cell is ringed in orange?

Score 0/0

Drawing blood cells 8.1.5

Draw the outline and the nucleus with single, sharp lines; show the nucleus shape and its size compared with the cell (a lymphocyte nucleus almost fills the cell; a monocyte nucleus is bean-shaped). Do not shade. A red blood cell has no nucleus: draw its pale centre as a second circle to show the biconcave shape.

8.1.68.1.7

Tissue fluid: formed in capillaries

is the liquid that bathes your cells. It forms when some is pushed out through gaps in capillary walls. Two pressures compete along the capillary: (blood pressure pushing fluid out) and (the pull of water back in by osmosis, caused by plasma proteins).

Capillary from arteriole end to venule endArrows show fluid leaving at the arteriole end and returning at the venule end. Their size shows the net pressure. Pressures along the capillaryHydrostatic pressure falls along the capillary; oncotic pressure stays almost constant. Where hydrostatic is higher, fluid leaves; where it is lower, fluid returns.
10%
4.7 kPa
2.0 kPa
70 g dm⁻³
Hydrostatic here–
Oncotic here–
Net pressure–
Fluid balance–

Functions of tissue fluid 8.1.7

  • Supplies cells with oxygen, glucose, amino acids, fatty acids and ions.
  • Removes waste such as carbon dioxide and urea from cells.
  • Gives cells a stable surrounding (constant pH, temperature and water potential); part of .

Tissue fluid is like plasma but has almost no large plasma proteins and no red blood cells: they are too big to leave through the gaps. Some white blood cells can squeeze out.

Why water? 8.1.6

Water is the main component of blood and tissue fluid.

Solvent action: water molecules are , so ions and polar molecules (glucose, amino acids, urea, hydrogencarbonate ions) dissolve in it and are carried in solution.

High : lots of energy is needed to raise its temperature, because hydrogen bonds must be broken. Blood can carry heat from very active organs (liver, muscles) to the rest of the body with only a small change in its own temperature.

Where does the extra fluid go? Extension

Usually slightly more fluid leaves than returns. The extra drains into blind-ended lymph vessels and becomes lymph, which is returned to the blood near the heart. Lymph is not a 9700 learning outcome.

Oedema (swelling) happens when tissue fluid builds up faster than it drains. A diet very low in protein (kwashiorkor, still seen in parts of Southern Africa in times of drought) lowers plasma protein, so less fluid returns.

8.2.48.2.58.2.6

The oxygen dissociation curve and the Bohr shift

Each molecule can carry four oxygen molecules. The curve shows the of haemoglobin with oxygen at each . Drag the orange tissue marker along the curve, and move the pCO₂ slider to see the .

Oxygen dissociation curve of adult haemoglobinAn S-shaped curve of percentage saturation against partial pressure of oxygen. Markers show the lungs and a respiring tissue.
5.3 kPa
7.39
37.0 °C
5.0 kPa
Saturation in lungs (pO₂ 13 kPa)–
Saturation in tissue–
Oxygen unloaded–
Extra unloaded due to the shift–
P₅₀ (pO₂ at 50% saturation)–

Extension: not in 9700 Topic 8

Why is the curve S-shaped? 8.2.4

At low pO₂ it is hard for the first oxygen molecule to bind to haemoglobin. Once it binds, the haemoglobin molecule changes shape, which makes it easier for the second and third to bind (), so the curve becomes steep. The last site is harder to fill, so the curve levels off near 100%.

Why it matters 8.2.5 8.2.6

  • Lungs (pO₂ ≈ 13 kPa): the curve is flat and high, so haemoglobin becomes almost fully saturated even if pO₂ falls a little.
  • Respiring tissue (pO₂ ≈ 2–5 kPa): the curve is steep, so a small fall in pO₂ makes haemoglobin release a lot of oxygen.
  • Bohr shift: more CO₂ means more H⁺ in red blood cells; H⁺ lowers haemoglobin's for oxygen, so the curve moves right. More oxygen is released exactly where respiration is fastest.
8.2.18.2.28.2.3

Carbon dioxide transport: inside a red blood cell

Step through what happens as blood passes a respiring tissue, then watch it reverse in the lungs. Carbon dioxide travels in three ways: about 5% dissolved in plasma, about 10% as , and about 85% as in the plasma.

Red blood cell next to a respiring tissue cellMolecules move between the tissue cell, plasma and red blood cell as each step is shown.

The chloride shift and why it matters 8.2.2

Hydrogencarbonate ions (HCO₃⁻) diffuse out of the red blood cell through a transport protein, and chloride ions (Cl⁻) move in at the same time. This is the .

  • It keeps the electrical charge balanced (each negative ion leaving is replaced by one entering).
  • It keeps HCO₃⁻ low inside the cell, so carbonic acid keeps forming and more CO₂ can be taken up.

The role of plasma 8.2.3

Plasma carries most of the CO₂: as hydrogencarbonate ions (made in red blood cells, then diffused out) and a small amount as dissolved CO₂. In the lungs the HCO₃⁻ goes back into red blood cells to be turned back into CO₂, which diffuses into the alveoli.

All Topic 8 outcomes

Exam practice

Original Cambridge-style questions written for F to A. Multiple-choice answers are marked instantly with the reason. For written answers, write your answer first: the mark scheme opens once you have written at least a sentence. Tick the points you made to mark yourself.

Score: 0 / 0

Page not found

That address does not match a screen in this lab. Go to the big picture or choose a screen above.