1. Brain
   The brain is affected first because neurons have a very high oxygen requirement and almost no oxygen reserve. Lack of oxygen causes dizziness, confusion, loss of consciousness, seizures, coma, and then permanent brain-cell death if it continues. Severe interruption of brain blood flow can begin killing neurons within minutes. Harrison’s Principles of Internal Medicine, 22e, p. 2389.
2. Heart during lack of oxygen
   Initially, the heart may beat faster as the body tries to compensate. With continuing hypoxia, heart muscle contraction becomes weak, leading to chest pain/ischemia, low blood pressure, dangerous arrhythmias, cardiac failure, and possibly cardiac arrest. Prolonged hypoxia depresses myocardial contractility. Harrison’s Principles of Internal Medicine, 22e, “Responses to Hypoxia.”
3. Short interruption versus long interruption of oxygen/blood supply
   • Short interruption: usually causes reversible cell injury. Cells switch to anaerobic metabolism, ATP falls, lactic acid accumulates, and cells swell. Function may return if oxygen and circulation are restored promptly.
   • Long interruption: causes irreversible cell injury and cell death, by necrosis and/or apoptosis. In the brain this can cause infarction, permanent neurologic disability, or death. Restoring blood flow late can also add reperfusion injury through oxidative stress and inflammation.
4. Irreversible damage
   Irreversible damage means the cells are permanently dead and cannot recover even after oxygen or blood flow returns. It occurs when oxygen deprivation is severe or lasts too long, causing irreversible mitochondrial failure, membrane damage, calcium influx, enzyme leakage, and DNA/protein damage.

Active voice

• Riya writes a letter.
  • Riya is doing the action.

Passive voice

• A letter is written by Riya.
  • The letter receives the action.

8. Fill in by or with

1. The lock was broken with a hammer.
2. This book was written by my favourite author.
3. The cake was decorated with icing.
4. The tiger was shot with a gun.
5. Claire was shouted at by her teacher.
6. He was hit on the head with an umbrella.

9. Rewrite the sentences in the passive

1. The garden fence is being repaired.
2. Is Latin taught at this school?
3. I don't like being pointed at.
4. He was hit on the head with a tennis racquet.
5. The preparations have been made by Michael.
6. Is the house being cleaned by Tim?
7. By whom were the Pyramids built?

1. We are going to be given a pay rise by the boss.
2. I expect my new car will be delivered soon.
3. The suspects are being questioned by the police.
4. Were the thieves seen by your next-door neighbours?
5. Paul remembers being asked to star in the school play by his teacher.
6. Computers are used by a lot of children nowadays.
7. Who was the kitchen window smashed by?
8. The work will not have been completed by the end of the month.
9. The letters will be posted by the children.
10. Pancakes are made from flour, eggs and milk.
11. Had the windows been closed by Helen before she left the house?
12. The housework has not been done by Jill yet.
13. The parcel may not be delivered today.