Circulatory & Respiratory Systems · Grade 7 Science
Chapter: Body Organization, Transport & MovementCirculatory & Respiratory Systems
Learning goals
- I can explain circulatory & respiratory systems.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Hold your breath and the discomfort you feel within a minute is not a shortage of oxygen. It is a rise in carbon dioxide, which your body detects far more sensitively. That distinction is the right way into this unit, because it separates two things the word breathing runs together. Ventilation is the mechanical movement of air in and out of your lungs — ribs lifting, the diaphragm flattening, pressure dropping so that air flows in. Gas exchange is the chemical traffic that ventilation serves: oxygen crossing from air into blood, carbon dioxide crossing the other way. Ventilation is a pump; gas exchange is a transfer across a surface; and cellular respiration, from Chapter 2, is a third thing again, happening in mitochondria a long way from either.
Gas exchange happens across a surface built for it. Your lungs divide into roughly three hundred million tiny air sacs, each wrapped in blood vessels, and the resulting surface is somewhere around seventy square metres — comparable to a badminton court folded into your chest. The barrier between air and blood is about a thousandth of a millimetre thick. Large area, short distance, and a constant difference in concentration maintained at both ends: those are the same three conditions that made the small intestine's lining work, and they are what any fast exchange surface needs.
Delivering the gas is the circulatory system's job. Blood leaves the heart in an artery, which has thick muscular walls to withstand the pressure of each beat. It returns in a vein, which is wider, thinner-walled and carries valves to stop backflow in the low-pressure return. Between them lie the capillaries, so narrow that red blood cells pass almost single file, with walls one cell thick — and that is where every exchange with tissue actually happens. Arteries and veins are plumbing; capillaries are the point of the whole system. Blood runs in two connected circuits: heart to lungs and back, then heart to body and back, which is why the heart has four chambers rather than two.
Build your understanding
Where this fits
Digestion delivered food molecules to the gut wall and absorption put them into the blood. Energy release also needs oxygen, and oxygen enters somewhere else entirely. This unit covers the two systems that collect oxygen and deliver both it and the absorbed food to every cell — and separates breathing, which you can feel, from the chemistry it serves.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Breathing is not gas exchange, and neither is respiration
Three different things wear overlapping names, and separating them fixes most of the confusion in this topic. Ventilation moves air: the diaphragm contracts and flattens, the rib cage lifts, the volume of the chest increases, pressure inside falls below atmospheric, and air flows in. Nothing chemical has happened yet.
Gas exchange is the transfer at the air sac surface. Oxygen is more concentrated in the fresh air than in the blood arriving, so it diffuses into the blood; carbon dioxide is more concentrated in that blood than in the air, so it diffuses out. This is the diffusion from Chapter 2 — no pumping, no energy spent, just movement down concentration gradients across a thin wet surface.
Cellular respiration is the chemistry that creates those gradients in the first place. Cells consume oxygen and produce carbon dioxide, which keeps blood oxygen low and blood carbon dioxide high, which keeps the diffusion running at the lungs. Ventilation exists to keep the air side of the gradient fresh. A plant, which has no lungs and does not ventilate, still respires.
Pause and explain
Distinguish ventilation, gas exchange and cellular respiration in one sentence each.
Check your reasoning after answering
Ventilation is the mechanical movement of air into and out of the lungs, driven by the diaphragm and ribs changing the volume of the chest. Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out of it, across the air sac surface. Cellular respiration is the chemical release of energy from glucose inside mitochondria, which is what consumes the oxygen and produces the carbon dioxide.
Part 2 · What an exchange surface has to be
Three features make a surface good at exchange, and the lungs have all three to an extreme degree. A large area: three hundred million air sacs reaching perhaps seventy square metres. A short diffusion distance: the wall between air and blood is roughly one micrometre, two cells thick. And a maintained concentration difference: ventilation refreshes the air on one side while circulation carries away the blood on the other, so the gradient never has a chance to even out.
That third condition is the one usually forgotten, and without it the other two would not help for long. If blood sat still in the capillaries it would reach equilibrium with the air and exchange would stop, exactly as the diffusion unit predicted. A continuous flow on both sides keeps the system permanently far from equilibrium, which is what keeps it moving.
The same three features appear wherever fast exchange is needed, which is a good sign that the explanation is right rather than a story about lungs. Fish gills have filaments and lamellae giving a huge area, a thin barrier, and water flowing one way past blood flowing the other. The small intestine has villi, a one-cell lining and a capillary carrying absorbed molecules away. Different organs, same requirements.
Pause and explain
Why would gas exchange stop if blood stopped flowing through the lung capillaries, even with the lungs still ventilating?
Check your reasoning after answering
Because the blood in the capillaries would quickly reach equilibrium with the air — taking up oxygen until its concentration matched, and giving up carbon dioxide until the same. With no difference left across the surface, diffusion would have no net direction. Flow on the blood side is what keeps the gradient steep, just as ventilation does on the air side.
Part 3 · Two circuits, and why the heart has four chambers
Blood makes two journeys, not one. In the first, the right side of the heart pumps blood low in oxygen to the lungs, where it takes up oxygen and releases carbon dioxide, and it returns to the left side. In the second, the left side pumps that oxygen-rich blood to the whole body, where it delivers oxygen and collects carbon dioxide, and it returns to the right side. Two circuits, each beginning and ending at the heart.
This is why four chambers are needed: two to receive returning blood and two to pump it out, with the two circuits kept separate so that oxygen-rich and oxygen-poor blood do not mix. The left side pumps against far more resistance, since it must reach the feet and the brain, and its muscular wall is correspondingly thicker — a structural difference you can see, explained by a functional demand.
Keeping the two sides straight also explains the vessels. An artery carries blood away from the heart and a vein carries it back, and that is the definition — not whether the blood is rich in oxygen. The pulmonary artery carries oxygen-poor blood to the lungs, and the pulmonary vein carries oxygen-rich blood back. Defining the vessels by direction rather than by content is what makes the exception stop being confusing.
Pause and explain
The pulmonary artery carries blood low in oxygen. Does that break the definition of an artery?
Check your reasoning after answering
No, because an artery is defined by direction: it carries blood away from the heart. The pulmonary artery carries oxygen-poor blood away from the right side towards the lungs, and the pulmonary vein brings oxygen-rich blood back. Defining arteries and veins by what they carry rather than which way they go is what creates the apparent exception.
Part 4 · Connecting transport to what cells actually do
Everything above is delivery. The purpose only appears at the capillary, where oxygen and glucose leave the blood, cross into tissue fluid, and pass through a cell's membrane; and where carbon dioxide makes the reverse trip. Capillary walls are one cell thick and the vessels are so fine that no cell in your body is far from one — that short distance is what makes diffusion fast enough to serve a working cell.
Inside the cell, the mitochondria consume the oxygen and the glucose and release the energy. This is the step people skip when they say that breathing gives you energy. Breathing moves air. The energy comes from a chemical reaction in cells, which the air movement supplies one reactant for, and which would also need the glucose that arrived by an entirely separate route through the digestive system.
A clean way to check your own explanation is to ask where each substance was at each stage and what moved it. Oxygen: air, then lung capillary blood, then heart, then body capillary, then tissue fluid, then cell, then mitochondrion. Each step has a mechanism — bulk flow driven by the heart, or diffusion down a gradient. If a step in your account has no mechanism, that is the part to work on.
Pause and explain
A draft says oxygen moves directly from the lungs to every muscle cell. What is missing?
Check your reasoning after answering
The whole transport stage. Oxygen diffuses from the air sacs into blood in the lung capillaries, is carried to the left side of the heart, pumped out through arteries, and reaches tissues through capillaries, where it diffuses into tissue fluid and then into cells. Only then is it used, in mitochondria. Without the heart and vessels there is no route.
Apply the whole unit · Repair a systems diagram
Illustrative draft, not a measured record: a student's diagram shows air entering the lungs, then an arrow labelled oxygen going straight from the lungs to every muscle cell. No vessels and no heart are drawn.
Your task
Repair the pathway, label where each gas exchange happens, and distinguish the pathway from cellular respiration.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Insert the missing stage: oxygen diffuses from the air sacs into blood in the lung capillaries, which the draft omits entirely.
Add the transport: that blood returns to the left side of the heart and is pumped through arteries to capillaries throughout the body.
Label the second exchange: at a body capillary, oxygen diffuses into tissue fluid and then into cells, while carbon dioxide diffuses the other way and is carried back for release at the lungs.
Separate the chemistry: cellular respiration happens inside the cells' mitochondria, consuming the delivered oxygen and glucose — it is not part of the transport pathway.
The draft is missing everything between the lungs and the cells. Repaired, the pathway runs: air is drawn into the air sacs by ventilation; oxygen diffuses across the thin sac wall into blood in the surrounding capillaries, because oxygen is more concentrated in fresh air than in the arriving blood; that blood returns to the left side of the heart and is pumped out through arteries, which branch until they become capillaries in the tissues; at a body capillary oxygen diffuses out into tissue fluid and across the membranes of nearby cells. Carbon dioxide follows the reverse route — produced in cells, diffusing into the capillary blood, carried back to the right side of the heart, pumped to the lungs, and diffusing into the air sacs to be removed by ventilation. Two exchanges, both by diffusion down concentration gradients, with bulk flow driven by the heart in between. Cellular respiration is a separate thing and belongs inside the cell: it is the reaction in which mitochondria use the delivered oxygen and glucose to release energy, and it is what produces the carbon dioxide and keeps both gradients steep. The draft's single arrow collapses a pump, two exchange surfaces and a chemical reaction into one step. Worth noting too that the glucose arrives by a different route entirely, absorbed in the small intestine and carried by the same circulation.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
The respiratory system moves air and exchanges gases; the circulatory system transports materials between organs and cells. In lungs, oxygen moves from air spaces into blood while carbon dioxide can move in the other direction. The heart pumps blood through vessels. These tasks interact, but ventilation, gas exchange, and cellular respiration are distinct processes.
Worked examples
Example 1 · Explain a structure from a demand
- Identify what each side pumps to: the right side sends blood to the lungs, which are close by; the left side sends it to the entire body, including the brain and feet.
- Compare the resistance: a long branching network reaching the extremities offers far more resistance than the short pulmonary circuit.
- Connect structure to demand: generating the higher pressure needed requires more muscle, so the left wall is thicker.
A visible structural difference usually has a functional demand behind it. Name the demand.
Example 2 · Find the missing mechanism
- List the stages actually required: ventilation, gas exchange at the lungs, transport by blood, exchange at a body capillary, and cellular respiration.
- Attach a mechanism to each: pressure difference for ventilation, diffusion for both exchanges, bulk flow driven by the heart for transport, and enzyme-controlled chemistry in mitochondria.
- Note the second reactant: glucose from digestion, delivered by the same circulation, without which oxygen releases nothing.
If a step in an account has no mechanism, that is the part that needs work.
Avoid these mistakes
- Mistake: breathing is respiration. Ventilation moves air mechanically; cellular respiration is a chemical reaction in mitochondria. A plant respires and does not breathe.
- Mistake: arteries carry oxygen-rich blood by definition. They carry blood away from the heart — the pulmonary artery carries oxygen-poor blood to the lungs.
- Mistake: oxygen goes straight from lungs to muscles. It crosses into blood, travels through the heart and arteries, and diffuses out at a capillary; without the transport stage there is no route.
- Mistake: a big surface is enough for fast exchange. Area, a short diffusion distance and a maintained concentration difference are all needed — blood that stopped flowing would reach equilibrium and exchange would halt.
Check your understanding
Is exhaled air pure carbon dioxide?
Discuss the answer
No. It remains a mixture including nitrogen, oxygen, carbon dioxide, and water vapor in different proportions.
Why are thin surfaces relevant?
Discuss the answer
They shorten the distance materials must cross and support exchange under suitable conditions.
Words to know
- ventilation
- The mechanical movement of air in and out of the lungs as the diaphragm and ribs change the volume of the chest; nothing chemical happens yet.
- gas exchange
- Diffusion of oxygen into the blood and carbon dioxide out of it across a thin, moist surface with a maintained concentration difference.
- capillary
- A vessel one cell thick and barely wider than a red blood cell, where every exchange with tissue actually happens.
- artery
- A vessel carrying blood away from the heart, with thick muscular walls for the pressure of each beat — defined by direction, not by oxygen content.
- vein
- A vessel carrying blood back towards the heart, wider and thinner-walled, with valves preventing backflow at low pressure.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Why is inhalation alone not the whole oxygen pathway?
Check your explanation after answering
Oxygen must cross exchange surfaces, travel in blood, and reach cells; inhalation is only one step.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.