The Heart, Blood and Lungs In Depth
Our heart beats about 100,000 times a day and pumps about 7,000 liters of blood in a single day, through blood vessels that would stretch for tens of thousands of kilometers if you lined them up end to end. This chapter looks at how blood travels in loops, how the heart makes its own electricity, where the lungs swap gases, and why you cannot just mix blood groups.
Blood travels in two circles
The human heart is not one pump. It is two pumps joined together. The right side sends blood to the lungs to pick up oxygen. The left side sends oxygen-rich blood out to feed the whole body. Press the button to follow one drop of blood.
The heart beats on its own, without waiting for orders from the brain
In the wall of the right atrium there is a special group of cells called the SA node, and it works as the pacemaker. It fires electrical signals by itself in a steady rhythm. The signal spreads through the whole heart and makes the muscle squeeze in the correct order. An ECG machine is simply reading that signal from the skin.
So what exactly is blood pressure?
Blood pressure is the force of blood pushing on the walls of the arteries. The top number is the moment the heart squeezes. The bottom number is the moment the heart relaxes between beats. Tap through and see what makes the numbers change.
Try different situations
Why is high blood pressure dangerous over the long run?
Pressure that stays high all the time damages the walls of the blood vessels bit by bit, so fatty plaque sticks to them more easily, and it makes the heart work so hard that its wall grows abnormally thick. The end of that road is stroke, heart disease and failing kidneys.
The scary part is that high blood pressure usually has no symptoms at all. Many people only find out once something has already gone wrong. That is why measuring blood pressure regularly matters, instead of waiting for a headache before you check.
What children can do starting today is eat less salt, exercise regularly, sleep enough, and never smoke. All of these really do lower the long-term risk.
Tiny air sacs, enormous surface
The two lungs hold about 300 million alveoli. If you unfolded all of their walls you would get an area of about 70 square meters, bigger than an ordinary bedroom, and all of it is packed inside our chest. The wall of an alveolus is so thin that oxygen spreads straight through into the capillaries. Press the button to watch the gas exchange.
Gas exchange
Oxygen is more concentrated in the alveolus than in blood that has just come back from the body, so it spreads into the blood by itself. Carbon dioxide is more concentrated in the blood than in the alveolus, so it spreads out. There is no pump at all. It only uses the difference in concentration.
One hemoglobin molecule in a red blood cell can hold 4 oxygen molecules, and a single red blood cell contains about 270 million hemoglobin molecules. That is why blood can carry many dozens of times more oxygen than plain water could dissolve.
What is inside one drop of blood?
Blood is not just an ordinary red liquid. It is a liquid tissue with several kinds of cells floating in it. Tap them one at a time.
Blood group table: who can give to whom
Tap a cell in the table to see the reason. The rows are the donor, the columns are the recipient. Green means compatible.
Group O negative is the universal donor for red blood cells, and group AB positive is the universal recipient.
Test yourself
Fancy a game on this lesson?
Every question from this lesson is in the Body 101 arena. Race the clock, duel someone at home, play as a team, beat the bosses, or practise with no timer at all.