Every fact in the three heart and lungs lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications and care points: this is general science education, not medical advice. The resting heart-rate range of about 60–100 beats per minute is for adults; children's resting rates are usually higher, and every person's normal is different, so pupils should not compare themselves or worry about their numbers. Pupils with a heart or breathing condition, or who feel unwell, should sit out the exercise parts or record a classmate's results instead; anyone who feels dizzy or short of breath stops at once and tells an adult. Questions about a pupil's own health go to a parent, the school nurse or a doctor. The 'blue and red blood' colouring in diagrams is a convention: blood low in oxygen is darker red, not blue. Stroke volume and cardiac output values are typical textbook figures for adults at rest.
All sources
[1] How the Heart Works: Anatomy
National Heart, Lung, and Blood Institute (NIH, USA) · Reliability: High
Type: National health research agency
The heart sits in the centre of the chest near the lungs and has four chambers: two upper atria receive blood and two lower ventricles pump it out. The right side pumps oxygen-poor blood to the lungs, the left side pumps oxygen-rich blood to the rest of the body. Valves make sure the blood keeps flowing in the right direction; a wall (the septum) divides the right and left sides.
National Heart, Lung, and Blood Institute (NIH, USA) · Reliability: High
Type: National health research agency
Air enters through the nose or mouth, which moistens and warms it, and goes down the windpipe (trachea), which branches into bronchial tubes, then bronchioles ending in clusters of tiny air sacs (alveoli). Each alveolus is surrounded by capillaries; oxygen moves from the air into the blood and carbon dioxide moves from the blood into the alveoli.
[3] How the Lungs Work: What Breathing Does for the Body
National Heart, Lung, and Blood Institute (NIH, USA) · Reliability: High
Type: National health research agency
When you breathe in, the diaphragm tightens and moves down, making room in the chest for the lungs to fill; when you breathe out, it relaxes and moves up and the lungs deflate. The body uses oxygen from the air and gets rid of carbon dioxide, a waste gas.
[4] Biology, 40.3 Mammalian Heart and Blood Vessels
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Mammals have a double circulation: a pulmonary circuit to the lungs and a systemic circuit to the body. Arteries take blood away from the heart, veins bring it back, and capillaries are where nutrients, waste and oxygen are exchanged with the tissues. Systole is the contraction phase that pushes blood out; diastole is the relaxation phase in which the heart fills.
[5] Anatomy and Physiology, 19.4 Cardiac Physiology
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Cardiac output is the volume of blood pumped by each ventricle in one minute: CO = HR × SV. At rest a typical stroke volume is about 70 mL and heart rate about 75 beats per minute, giving about 5.25 L/min; during exercise both rise and CO can reach about 19.5 L/min.
[6] Watch your heart rate, but don't obsess about it
American Heart Association News · Reliability: Medium-high
Type: Medical charity news article
Most adults have a resting heart rate between about 60 and 100 beats per minute, with fit people often lower and natural variation through the day. Warns not to press hard on the arteries in the neck to check the pulse. Maximum heart rate is often estimated as 220 minus age.