Grade 8

The Human Body and Its Systems

The systems that keep a body of trillions of cells supplied, cleaned and coordinated, taken in the order they depend on each other.

6 lessons

  1. 1Why does a large body need organ systems at all?A single cell takes in oxygen and food straight from its surroundings by diffusion, because no point inside it is far from the membrane. As a body grows, volume grows faster than surface area and diffusion becomes far too slow to keep up. So a large body is built in levels — cell, tissue, organ, system — with delivery systems that bring supplies to every cell.30m
  2. 2What happens to a sandwich between the mouth and the bloodstream?It goes through two kinds of breakdown. Mechanical breakdown cuts it into smaller pieces and increases the contact area; chemical breakdown uses enzymes to split large molecules into small ones. Only at the end of that, in the small intestine, are the molecules small enough to cross the wall into the blood. Everything else carries on to the large intestine.35m
  3. 3What is the difference between breathing, gas exchange and cellular respiration?Three different things that share a word. Breathing is air moving in and out, driven by muscles. Gas exchange is oxygen crossing from an alveolus into the blood and carbon dioxide crossing the other way, and it is nothing but diffusion. Cellular respiration is the chemical reaction inside the cell that releases energy from oxygen and sugar.35m
  4. 4Why does blood pass through the heart twice on every complete circuit?Because there are two separate circuits, not one. In the pulmonary circuit blood leaves the heart for the lungs, collects oxygen and returns. In the systemic circuit it leaves again and travels to the whole body. The two circuits need very different pressures, so each one gets a ventricle of its own.35m
  5. 5How does a kidney decide what to remove from the blood and what to keep?It does not decide in advance. First it filters an enormous volume of liquid out of the blood, carrying everything small enough to pass, and only then reabsorbs what the body still needs — water, sugar and salts. Whatever is not reabsorbed leaves as urine. Filtering everything and returning the useful is simpler than choosing in advance.30m
  6. 6When does the body send an instruction by nerve and when by hormone?It depends on how fast and for how long. The nervous system sends an electrical signal along a defined route to one organ; it arrives in a fraction of a second and acts for a moment. The hormonal system releases a chemical into the blood; it arrives slowly, reaches the whole body, and goes on acting. Speed against persistence.40m