How Animals Move Things Around Inside Their Bodies
Every living cell needs food and oxygen to survive, and every cell produces waste that must be removed. For a single-celled organism like Amoeba, this is simple — the cell membrane handles everything. But animals are made of millions of cells, many of them deep inside the body, nowhere near food or oxygen sources.
That’s the problem transportation solves. The transport system in animals is a network that carries nutrients, gases, and waste between different parts of the body. Think of it like a delivery network — the heart is the distribution centre, blood is the delivery truck, and blood vessels are the roads.
In Class 7, we focus mainly on the human circulatory system, but we also compare how different animals (like fish, earthworms, and insects) solve the same basic problem in different ways.
Key Terms & Definitions
Blood — The fluid that carries materials through the body. It has four components: red blood cells (carry oxygen), white blood cells (fight infection), platelets (help clotting), and plasma (the liquid part that carries dissolved nutrients, hormones, and waste).
Heart — The muscular pump that keeps blood moving. In humans, it has four chambers.
Blood vessels — The tubes that carry blood. Three types:
- Arteries — carry blood away from the heart (remember: Artery, Away)
- Veins — carry blood towards the heart
- Capillaries — the tiniest vessels where actual exchange of materials happens
Circulatory system — The complete system: heart + blood vessels + blood working together.
Heartbeat — One complete cycle of the heart contracting and relaxing. Normal rate: 70-80 beats per minute in adults.
Pulse — The throbbing you feel in your wrist or neck, caused by the surge of blood each time the heart beats.
Excretion — The removal of waste products from the body. The kidneys filter blood and remove urea (a waste made from protein breakdown) as urine.
The Human Circulatory System
Structure of the Heart
The human heart is roughly the size of your fist and sits in the chest, slightly to the left. It is divided into four chambers:
- Upper chambers: right atrium and left atrium (atria = plural)
- Lower chambers: right ventricle and left ventricle
The right side and left side are completely separated by a wall called the septum. This is critical — oxygenated and deoxygenated blood must never mix.
A quick memory trick for the chambers: Atria are on top (Above), Ventricles are below. The ventricles do the heavy pumping, so their walls are thicker and more muscular.
How Blood Flows Through the Body
Let’s trace a single drop of blood on its complete journey:
Step 1: Deoxygenated blood from the body enters the right atrium.
Step 2: It moves down to the right ventricle, which pumps it to the lungs through the pulmonary artery.
Step 3: In the lungs, blood picks up oxygen and releases carbon dioxide. It is now oxygenated.
Step 4: Oxygenated blood returns to the left atrium through the pulmonary vein.
Step 5: It moves down to the left ventricle — the strongest chamber — which pumps it to the entire body through the aorta (the main artery).
Step 6: Blood delivers oxygen and nutrients to all cells through capillaries, picks up carbon dioxide and waste, and returns to the right atrium through veins.
Then the cycle repeats. This full journey happens once every minute approximately.
CBSE questions frequently ask: “Which chamber of the heart pumps blood to the entire body?” Answer: Left ventricle. They also ask to distinguish between pulmonary artery and pulmonary vein — remember, pulmonary artery carries deoxygenated blood (it’s going to the lungs to get oxygen), which is the exception to the “arteries carry oxygenated blood” rule.
Blood Vessels in Detail
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction | Away from heart | Towards heart | Connect arteries to veins |
| Wall thickness | Thick, muscular | Thinner | One cell thick |
| Blood type | Usually oxygenated | Usually deoxygenated | Both |
| Valves | No | Yes (prevent backflow) | No |
Capillaries are where the real work happens. Their walls are just one cell thick, which allows oxygen, glucose, and other nutrients to easily pass into surrounding tissues, and carbon dioxide and waste to pass back into the blood.
Components of Blood
Red Blood Cells (RBCs)
RBCs carry oxygen using a protein called haemoglobin, which contains iron. Haemoglobin is what makes blood red. RBCs are shaped like biconcave discs (dented on both sides) — this shape gives them more surface area to carry more oxygen.
Iron is why eating green vegetables and spinach is repeatedly recommended. Low iron → less haemoglobin → less oxygen carried → fatigue. This condition is called anaemia.
White Blood Cells (WBCs)
WBCs are the soldiers of the immune system. They identify and destroy bacteria, viruses, and other foreign particles. Unlike RBCs, WBCs have a nucleus and can change shape to engulf pathogens — a process called phagocytosis.
There are far fewer WBCs than RBCs in normal blood.
Platelets
Platelets are tiny cell fragments that help form clots when a blood vessel is injured. They clump together at the wound site and release chemicals that trigger clot formation, preventing excessive blood loss.
Plasma
Plasma is the pale yellow liquid part of blood (about 55% of total blood volume). It is mostly water but dissolves and transports nutrients (glucose, amino acids), hormones, carbon dioxide, urea, and proteins.
How Other Animals Handle Transportation
The CBSE syllabus at Class 7 requires awareness of transportation in a few other animals.
Earthworms
Earthworms have a closed circulatory system — blood flows through closed vessels, similar to humans. They have five pairs of structures that act as hearts (called aortic arches). Blood carries food and oxygen to all body cells.
Fish
Fish have a two-chambered heart — one atrium and one ventricle. Blood passes through the heart once per complete circuit (single circulation). Blood goes: heart → gills (for oxygen) → body → heart.
Insects
Insects (like cockroaches) have an open circulatory system. Blood (called haemolymph) is not confined to vessels — it sloshes around in open spaces called sinuses, bathing the organs directly. Insects use a different system (trachea) to transport oxygen, so their blood doesn’t need to carry it.
A classic CBSE question: “What type of circulatory system do insects have?” Answer: Open circulatory system. The follow-up trap question: “Do insects use blood to carry oxygen?” Answer: No — they use the tracheal system for gas exchange.
Excretion: Removing Waste
Transportation isn’t just about delivery — it’s also about collection of waste.
Cells constantly produce waste as they work:
- Carbon dioxide from respiration (removed via lungs)
- Urea from breakdown of proteins (removed via kidneys as urine)
- Excess water and salts (removed via kidneys and sweat)
The kidneys filter blood continuously. Each kidney contains millions of tiny filtering units called nephrons. Blood enters the kidney through the renal artery, gets filtered, and clean blood leaves through the renal vein. The filtered waste — urea dissolved in water — becomes urine, which collects in the urinary bladder and is passed out.
Sweat glands in the skin also help by excreting small amounts of urea, salts, and excess water as sweat. This is a secondary excretory function — the primary purpose of sweating is temperature regulation.
Solved Examples
Example 1 — Easy (CBSE)
Q: Why is the left ventricle wall thicker than the right ventricle wall?
The right ventricle only needs to pump blood to the lungs, which are nearby. The left ventricle must pump blood to the entire body, including distant parts like the legs and head. More distance requires more pressure, so the left ventricle needs stronger, thicker walls to generate that pressure.
Example 2 — Medium (CBSE)
Q: A student says “arteries always carry oxygenated blood.” Is this correct? Explain.
This is partially incorrect. Arteries carry blood away from the heart — that’s the defining feature. In most cases, blood leaving the heart (through the aorta) is oxygenated, so arteries usually do carry oxygenated blood.
However, the pulmonary artery is the exception. It carries deoxygenated blood from the right ventricle to the lungs to pick up oxygen. So the correct statement is: “Arteries carry blood away from the heart; they usually carry oxygenated blood, except the pulmonary artery.”
Example 3 — Medium (CBSE)
Q: Trace the path of a molecule of glucose from the small intestine to a muscle cell in the leg.
Glucose absorbed in the small intestine → enters capillaries in the intestine wall → travels through veins → reaches right atrium → right ventricle → lungs (for blood to get oxygenated) → left atrium → left ventricle → pumped through aorta → travels through arteries and smaller vessels → reaches capillaries near the muscle cell in the leg → passes through capillary wall into the muscle cell.
Example 4 — Harder (CBSE Higher Order Thinking)
Q: If a person has a very low platelet count, what problems might they face? Explain the role of platelets.
Low platelet count means the blood’s clotting ability is severely reduced. Even a small cut would bleed for a much longer time, and internal bleeding (from minor injuries) would be dangerous. Platelets normally rush to the site of injury, clump together, and trigger a cascade of reactions that form a clot (a mesh of fibrin protein) to seal the wound. Without adequate platelets, this clot cannot form properly, leading to excessive blood loss — a condition called thrombocytopenia.
Exam-Specific Tips
CBSE Class 7 Marking Pattern
- 1-mark questions: Direct definitions (What is plasma? Name the chambers of the heart)
- 2-mark questions: Compare or explain (Difference between arteries and veins, why capillaries are important)
- 3-5 mark questions: Trace blood flow, explain excretion, label diagrams
High-scoring areas: The four-chamber diagram of the heart with correct labelling is a favourite. Practice drawing it with: right atrium, left atrium, right ventricle, left ventricle, septum, pulmonary artery, pulmonary vein, aorta, vena cava.
The most scoring topic in this chapter is the flow of blood — both through the heart and through the body. Students who can trace the complete path of blood (both oxygenated and deoxygenated) almost always score full marks on the long-answer question.
For diagram questions, draw clear arrows showing the direction of blood flow. Examiners specifically look for arrows — a diagram without them loses marks even if all parts are labelled correctly.
Common Mistakes to Avoid
Mistake 1: Confusing arteries and veins by blood type instead of direction
The rule is direction, not oxygen content. Arteries go AWAY from the heart, veins go TOWARDS it. The pulmonary artery carries deoxygenated blood — it’s still an artery because it’s going away from the heart. Students who memorize “arteries = oxygenated” will get this question wrong.
Mistake 2: Thinking the heart pumps only in one direction
The heart pumps in two directions simultaneously. The right side is pumping blood to the lungs while the left side is pumping blood to the body. These happen at the same time, not one after the other.
Mistake 3: Confusing excretion with egestion
Excretion = removal of metabolic waste produced by the body’s chemical reactions (urea, CO₂). Egestion = removal of undigested food through the anus. These are completely different processes. Urine is excretion; faeces is egestion.
Mistake 4: Saying insects have “no circulatory system”
Insects have an open circulatory system — blood (haemolymph) moves freely through body cavities. They do have a circulatory system; it’s just different from ours. Only say insects “don’t use blood for oxygen transport” (which is true — they use the tracheal system for that).
Mistake 5: Forgetting that plasma is not the same as blood
Plasma is the liquid component of blood. Blood = plasma + RBCs + WBCs + platelets. When a question asks “what does plasma carry?”, the answer is nutrients, hormones, CO₂, urea, and proteins — not oxygen (that’s haemoglobin’s job).
Practice Questions
Q1. Name the four chambers of the human heart and state the function of each.
Right atrium: receives deoxygenated blood from the body via the vena cava. Right ventricle: pumps deoxygenated blood to the lungs via the pulmonary artery. Left atrium: receives oxygenated blood from the lungs via the pulmonary vein. Left ventricle: pumps oxygenated blood to the entire body via the aorta. The left ventricle has the thickest walls because it does the most work.
Q2. What is the difference between plasma and serum?
Plasma is blood minus the cells (RBCs, WBCs, platelets) — it still contains clotting proteins (like fibrinogen). Serum is plasma minus the clotting proteins — it’s what remains after blood has clotted and the clot is removed. At Class 7, plasma is the key term. Serum is important for Class 9 onwards and in NEET preparation.
Q3. Why do capillaries have walls only one cell thick?
Capillaries are the sites where actual exchange of materials happens between blood and body cells. For this exchange to happen efficiently, the walls must be as thin as possible so that oxygen, glucose, and nutrients can pass out easily, and carbon dioxide and waste can pass in easily. Thicker walls would slow or prevent this exchange.
Q4. A person donates 350 mL of blood. After a few weeks, they feel completely normal. Why doesn’t the body suffer permanently?
The body continuously produces new blood cells. Red blood cells are manufactured in the bone marrow and have a lifespan of about 120 days, after which they are replaced. After donation, the body increases its production rate temporarily to restore normal blood volume and cell counts. This is why blood donation is safe for healthy adults when done in moderation — the body has the capacity to replenish the loss.
Q5. Compare the circulatory systems of fish and humans. Which is more efficient and why?
Fish have a two-chambered heart and single circulation — blood passes through the heart once per full body circuit. After the gills oxygenate the blood, it goes directly to the body under lower pressure (because pressure drops in the gill capillaries).
Humans have a four-chambered heart and double circulation — blood passes through the heart twice per circuit (once for the lungs, once for the body). This means oxygenated blood is pumped to the body at full pressure.
Human circulation is more efficient because the heart re-pressurises oxygenated blood before sending it to the body, allowing faster delivery to all cells.
Q6. What would happen if the septum in the human heart had a hole in it?
A hole in the septum (called a ventricular septal defect or “hole in the heart”) allows oxygenated blood from the left side to mix with deoxygenated blood on the right side. This means the body receives blood with less oxygen than normal. The person would feel tired easily, be short of breath, and if severe, their skin might appear slightly bluish (cyanosis) due to insufficient oxygen in the blood. This is why this condition requires medical treatment.
Q7. Kidney filters blood continuously. Estimate roughly how many times the total blood volume passes through the kidneys in a day. (Hint: kidneys filter about 180 litres of fluid per day; total blood volume ≈ 5 litres)
180 litres filtered per day ÷ 5 litres total blood volume = 36 times per day.
This seems surprisingly high, but it shows how critical the kidneys are. The 180 litres is not all urine — the body reabsorbs most of it. Only about 1.5 litres actually becomes urine. This question is excellent for understanding why kidney failure is a medical emergency.
Q8. A cockroach and a dog are both injured and bleeding. In which animal does the blood loss pose a greater immediate threat to oxygen supply to cells? Explain.
The dog faces a greater threat to oxygen supply. In the dog (mammal), blood carries oxygen via haemoglobin directly to every cell. Blood loss means less oxygen-carrying capacity immediately.
In the cockroach (insect), blood (haemolymph) does not carry oxygen — that role is handled by the tracheal system (air tubes that reach every cell directly). So blood loss in a cockroach does not directly reduce oxygen delivery to cells, though it still disrupts nutrient transport and causes other problems.
FAQs
Why does the heart never get tired even though it beats continuously?
The heart is made of a special type of muscle called cardiac muscle, which is different from the skeletal muscles in your arms and legs. Cardiac muscle has an exceptional blood supply (from the coronary arteries), extremely efficient energy use, and built-in electrical signals that keep it rhythmic. It also rests briefly between each beat (the relaxation phase). It does “get tired” when its own blood supply is cut off — that’s exactly what a heart attack is.
What is blood pressure and why does it matter at Class 7?
Blood pressure is the force exerted by blood against the walls of arteries. It’s expressed as two numbers (e.g., 120/80 mmHg) — the higher number is when the heart contracts (systolic) and the lower is when it relaxes (diastolic). At Class 7, knowing the concept is enough. High blood pressure (hypertension) damages blood vessel walls over time and increases risk of heart attack and stroke.
Why is blood red?
The red colour comes from haemoglobin, specifically from the iron-containing part called haem. When iron in haem binds with oxygen, it forms oxyhaemoglobin, which is bright red. Deoxygenated blood (without bound oxygen) is darker red — not blue, despite what some textbooks imply with their diagrams. Blood is never truly blue inside the body.
Is sweating a form of excretion?
Yes, technically. Sweat contains water, salts (NaCl), and small amounts of urea. Since urea is a metabolic waste product, sweating does constitute excretion. However, the primary purpose of sweating is thermoregulation (cooling the body), not excretion. The kidneys are the primary excretory organs.
How do we measure heartbeat at home?
Feel your pulse at the wrist (radial pulse) by pressing two fingers lightly on the inside of your wrist below the thumb. Count beats for 15 seconds and multiply by 4 to get beats per minute. Normal resting rate is 60-100 bpm for adults; children typically have slightly higher rates (70-100+ bpm). After exercise, the rate increases significantly because muscles need more oxygen.
Why does blood clot when we get a cut but not inside our blood vessels?
Blood clotting requires a cascade of chemical signals triggered by contact with damaged tissue and exposure to air. Inside healthy blood vessels, the inner lining (endothelium) actively prevents clotting by releasing anticoagulant chemicals. When a vessel is cut, this lining is disrupted, the cascade begins, and platelets clump at the site. Abnormal clotting inside vessels (thrombosis) does occur in disease states — it’s a leading cause of heart attacks and strokes.
What is the difference between blood and lymph?
Blood flows through blood vessels and carries RBCs, WBCs, platelets, and plasma. Lymph is a colourless fluid that leaks out of capillaries into the spaces between cells, collecting waste and excess fluid, and returning it to the bloodstream through the lymphatic system. Lymph contains WBCs but no RBCs. This topic becomes more important from Class 9 onwards, but knowing lymph exists as a related fluid is useful.