New safety measures had been introduced for this year
following a review,
with reductions in the height, and drops on the landing, of several fences made.
Despite the changes, only 15 of the 40-horse field reached the finishing post.
Two jockeys were also injured in this year's race. Noel Fehily broke his leg when his mount State of Play unseated him at the fifth and Brian Hughes, on board Viking Blond, suffered a suspected broken cheekbone when his horse fell at the first fence.
In a robust defence of the race Nicholls added: "We've got to be realistic about this. The horses have the best of everything they could have. They probably have better health care than we have.
"If people are going to continue to participate in sport, there is going to be both a human and animal risk.
"The worst thing you can do is to go too far. You make the fences smaller, they go faster and you get more fallers."
Did you know?
Twenty horses have died in races over Grand National fences since 2001. 10 have died in the race itself in the last 12 years
BBC commentator Richard Pitman believes changes must be made following Saturday's tragic events, which marred a thrilling race won by Neptune Collonges by a nose from Sunnyhillboy in a closest-ever finish to the race.
"There are questions to be asked and answers to be found and we cannot be complacent. We have to address the situation," the former jockey told BBC Radio 5 live.
"I think that things will have to be changed and I don't agree with the fences being smaller. That encourages horses to go faster. But I think the number of runners should be cut.
"The number of runners is the key point because quite a lot of horses were brought down by other horses. So if you've more room - say 25 horses instead of 40 - there will be a lot less interference from one to the other."
The RSPCA says there needs to be a number of drastic changes made to the race - including a reduction in the size of the field and the possibility of Becher's Brook being removed.
Becher's Brook is jumped twice in the Grand National as the sixth and 22nd fence
It is widely considered the most famous horseracing jump in the world
The approach height is only 4ft 10ins (1.47m), but the drop is between 5ft 2ins and 5ft 8ins
Chief executive Gavin Grant told BBC Breakfast: "Firstly, the scale of the field. Forty horses is a heck of a lot. Secondly, there are unique jumps there that horses aren't experienced in going over and I think we need to look at those jumps again.
"Becher's Brook has claimed another casualty [According To Pete] and perhaps it's time for that to go.
"We need to look at the landing areas. Some improvements have been made there, but when you've got a drop on the other side of the fence a horse isn't expecting that.
"And the going. The ground conditions are very important. Aintree has made a lot of progress making sure the going is softer because when it's hard the horses run faster.
"There is lots of work to be done to take the risks to horses out of this."
The British Horseracing Authority released a statement on Sunday saying it was still investigating the incidents in this year's race but pointed out Synchronised fractured his leg further down the course after unseating Tony McCoy at Becher's Brook whereas According to Pete was brought down by another horse.
The statement added: "We believe it would be premature to suggest that modifications to the course and other changes have not been effective or will not yet prove to be effective.
"The Grand National is a unique race and it represents a unique challenge for the sport and for its regulation.
"It is a thrilling spectacle, but there is a higher degree of risk involved in the Grand National than other races and for this reason everyone in the sport needs to be conscious of how the race is presented to the public, the general consumer perception and their views of how the race is run."
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