Human Transport
FoundationThe heart is a double pump driving a double circulation; blood travels through arteries, capillaries and veins, valves prevent backflow, haemoglobin in red cells carries oxygen, capillaries exchange materials with tissues to form tissue fluid, and the lymphatic system completes the network.
- Double circulation = systemic (left ventricle → body → right atrium) + pulmonary (right ventricle → lungs → left atrium): blood passes the heart twice per tour, balancing high-pressure delivery with lung protection.
- The four chambers divide the work — atria receive, ventricles pump; the tricuspid and bicuspid valves plus semilunar valves enforce one-way flow; coronary arteries feed the heart muscle.
- Arteries: thick-walled, high-pressure, away from the heart; veins: thin-walled with valves, returning to the heart; capillaries: one cell thick, slowest flow, built for exchange.
文體小知識：科學知識說明文——以心臟為泵、血管為管道、血液為載體，講解雙循環路線、瓣膜防倒流與微血管交換，並比較三類血管的結構與功能。
§Text Passage
1The circulatory system is a closed transport network powered by the heart. Human circulation is a double circulation: the systemic circulation pushes oxygenated blood from the left ventricle around the body and back to the right atrium, while the pulmonary circulation sends blood from the right ventricle to the lungs for oxygen and back to the left atrium. Blood passes through the heart twice on each full tour of the body, keeping the systemic side at high, efficient pressure while the pulmonary side stays gentle enough to protect the lung capillaries.
2The heart has four chambers: left and right atria above, receiving returning blood, and left and right ventricles below, pumping it out. The left ventricle has the thickest wall because it drives blood to the whole body. Atrioventricular valves (the tricuspid on the right, the bicuspid on the left) and the semilunar valves at the ventricle outlets keep blood flowing one way. The heart muscle itself is fed by the coronary arteries; when these are blocked by fatty deposits, the muscle is starved of oxygen and a heart attack follows.
3There are three kinds of blood vessel. Arteries carry blood away from the heart; their thick walls, rich in muscle and elastic fibres, withstand high pressure and help push blood along. Veins return blood to the heart at low pressure, with thinner walls and valves that stop backflow. Capillaries are the finest vessels: one flattened cell thick, with the slowest flow, they are the site of exchange, handing nutrients and oxygen to the cells and collecting metabolic wastes.
4Blood consists of plasma and blood cells. Plasma, about ninety per cent water, carries nutrients, metabolic wastes, hormones and carbon dioxide. Red blood cells are biconcave discs with no nucleus when mature; packed with haemoglobin, they pick up oxygen where it is concentrated — in the lungs — and release it where it is scarce — in the tissues. White blood cells engulf pathogens and mount immune responses, while platelets, with plasma proteins, clot blood at wounds.
5At the capillaries, blood pressure squeezes some of the plasma's water and dissolved substances through the wall, forming tissue fluid that bathes the cells and exchanges materials with them. Most tissue fluid drains back into the blood at the vein end of the capillaries; the rest enters lymph vessels as lymph and eventually returns to a vein. The lymphatic system also carries lipids absorbed in the small intestine into the blood and filters pathogens at lymph nodes.
1Summary 重點摘要
The heart is a double pump driving a double circulation; blood travels through arteries, capillaries and veins, valves prevent backflow, haemoglobin in red cells carries oxygen, capillaries exchange materials with tissues to form tissue fluid, and the lymphatic system completes the network.
2Vocabulary cards Vocabulary
The circulation in which blood passes through the heart twice per full tour, comprising the systemic and pulmonary circulations.
An upper chamber of the heart that receives returning blood and passes it into the ventricle.
A lower chamber of the heart that pumps blood out; the left ventricle has the thickest wall.
Structures ensuring one-way blood flow — the atrioventricular valves (tricuspid and bicuspid) and semilunar valves — preventing backflow.
The artery supplying the heart muscle with oxygen and nutrients; its blockage starves the muscle and causes a heart attack.
A vessel carrying blood away from the heart; thick-walled with much muscle and elastic fibre to withstand high pressure.
A vessel returning blood to the heart at low pressure, with thinner walls and valves that prevent backflow.
A tiny vessel whose wall is a single flattened cell; with the slowest flow, it is where blood and cells exchange materials.
The iron-containing pigment inside red blood cells; it binds oxygen in the lungs and releases it in tissues where oxygen is scarce.
Fluid formed as plasma water and solutes leak through the capillary wall, bathing the cells and mediating exchange.
3Step by step Steps
- Step 1: Why a double circulationHumans are warm-blooded; our cells consume oxygen at a high rate and need an efficient transport system. Human circulation is double: the systemic circulation sets out from the left ventricle, delivering oxygenated blood to every organ and returning to the right atrium; the pulmonary circulation sets out from the right ventricle, sending blood to the lungs for gas exchange and back to the left atrium. Blood passes through the heart twice on each full tour, so the systemic side stays at high pressure for efficient oxygen delivery while the pulmonary side remains gentle enough not to damage the lung capillaries. For example, fish use a single circulation in which pressure drops sharply after the gills, giving far less efficient oxygen delivery than the mammalian design.
- Step 2: Four chambers and the route of a blood dropThe heart has four chambers: left and right atria above to receive returning blood, and left and right ventricles below to pump it out. Trace one drop of blood: returning from the body it enters the right atrium through the vena cava, flows into the right ventricle and is pumped through the pulmonary artery to the lungs; freshly oxygenated, it returns through the pulmonary veins to the left atrium, flows into the left ventricle and is finally pumped out through the aorta to begin its tour of the body. For example, the left ventricle wall is the thickest of the four because it must drive blood to the most distant parts of the body, needing far higher pressure than the right ventricle, which only sends blood to the nearby lungs.
- Step 3: Valves stop backflowFour sets of valves keep the blood moving one way. Between the right atrium and right ventricle sits the tricuspid valve, and between the left atrium and left ventricle the bicuspid valve; both atrioventricular valves close when the ventricles contract, preventing blood from flowing back into the atria. At the outlets into the pulmonary artery and aorta sit the semilunar valves, closing as the ventricles relax so arterial blood cannot fall back. The opening and closing of these valves is the sound of the heartbeat. For example, a stethoscope picks up two sounds: the first is mainly the atrioventricular valves closing, the second mainly the semilunar valves; damaged valves let blood leak backwards and weaken the pumping.
- Step 4: Comparing the three vesselsBlood vessels fall into three classes by flow direction. Arteries carry blood away from the heart; thick walls rich in muscle and elastic fibre withstand the high pumping pressure, and elastic recoil helps push blood along. Veins return blood to the heart at low pressure with thinner walls, using valves to stop backflow and relying on surrounding skeletal muscles to squeeze blood upward. Capillaries link the two: extremely narrow, one flattened cell thick, with the slowest flow — the ideal site of exchange. For example, the pulse felt at the wrist is the artery's rhythm of expansion and recoil; slight swelling of the ankles after standing long reflects sluggish venous return.
- Step 5: Components of blood and their jobsBlood consists of plasma and three kinds of blood cell. Plasma, about ninety per cent water, carries nutrients, carbon dioxide, hormones and metabolic wastes. Red blood cells are biconcave discs without a nucleus when mature; packed with haemoglobin, they transport oxygen, and the biconcave shape enlarges the surface while shortening the diffusion path. White blood cells of several kinds engulf pathogens and mount immune responses. Platelets, tiny cell fragments, work with plasma proteins to clot blood at wounds. For example, people living at high altitude carry more red blood cells — an adaptation to thin air that boosts oxygen transport.
- Step 6: Capillary exchange and tissue fluidAt the arterial end of a capillary, blood pressure squeezes water and small dissolved molecules through the wall, forming tissue fluid that bathes the body's cells; nutrients and oxygen diffuse through it into the cells while metabolic wastes diffuse the other way. At the venous end the pressure has fallen, and most tissue fluid seeps back into the capillary; the rest drains into lymph vessels as lymph, eventually rejoining the blood through a large vein. The lymphatic system also carries lipids absorbed in the small intestine into the blood and filters pathogens at lymph nodes. For example, standing for hours lets tissue fluid pool at the ankles, causing mild swelling that fades once you walk.
- Step 7: Transport working with the whole bodyThe circulatory system links digestion, breathing and excretion into one whole. Glucose and amino acids absorbed by the small intestine travel by the hepatic portal vein to the liver for processing before being distributed around the body; oxygen is loaded in the lungs and unloaded in the tissues; the kidneys filter metabolic wastes from the blood into urine, keeping the internal environment stable. During exercise, cardiac output and heart rate rise and blood is redistributed, sending a larger share to the muscles and skin. For example, after a big meal blood flows preferentially to the digestive tract, which is why vigorous exercise straight after eating makes the gut and muscles compete for blood and often feels uncomfortable.
4Diagrams Diagrams
5Worked example Example
List, in order, the chambers and vessels a drop of blood passes through from the vena cava to the aorta, naming the valves along the way and where each prevents backflow; then explain why the left ventricle wall is thicker than the right.
- Route: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.
- The tricuspid valve lies between the right atrium and right ventricle and closes as the ventricle contracts, preventing backflow into the right atrium; the bicuspid valve between the left atrium and left ventricle does the same on the left.
- The semilunar valves sit at the outlets of the right ventricle into the pulmonary artery and of the left ventricle into the aorta, closing as the ventricles relax so arterial blood cannot fall back into them.
- The left ventricle wall is thicker because it pumps blood to every organ of the body — a long journey with high resistance requiring stronger contraction and higher pressure — whereas the right ventricle only sends blood to the nearby lungs.
- Answer: The full route is: vena cava → right atrium (via the tricuspid valve) → right ventricle (via a semilunar valve) → pulmonary artery → lungs → pulmonary vein → left atrium (via the bicuspid valve) → left ventricle (via a semilunar valve) → aorta. The atrioventricular valves prevent backflow into the atria; the semilunar valves prevent backflow into the ventricles. The left ventricle wall is thickest because it must drive blood around the whole body at the highest pressure, while the right ventricle needs only enough pressure to reach the lungs.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the two circuits of the double circulation, and from which chamber does each begin?
Q2Why is the wall of the left ventricle thicker than that of the right?
Q3What structural adaptations suit capillaries for exchange?
Q4Which blood component carries oxygen, and what happens to it in the lungs and in the tissues?
Q5How is tissue fluid formed, and where does the part that does not re-enter the capillaries go?
8Recap Recap
- Double circulation = systemic (left ventricle → body → right atrium) + pulmonary (right ventricle → lungs → left atrium): blood passes the heart twice per tour, balancing high-pressure delivery with lung protection.
- The four chambers divide the work — atria receive, ventricles pump; the tricuspid and bicuspid valves plus semilunar valves enforce one-way flow; coronary arteries feed the heart muscle.
- Arteries: thick-walled, high-pressure, away from the heart; veins: thin-walled with valves, returning to the heart; capillaries: one cell thick, slowest flow, built for exchange.
- Blood = plasma (carrying nutrients, wastes and hormones) + red cells (haemoglobin for oxygen) + white cells (defence) + platelets (clotting).
- Capillary pressure squeezes out tissue fluid that bathes the cells; the surplus returns via lymph vessels, and the lymphatic system also transports lipids and filters pathogens at lymph nodes.