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Can anyone explain why the preposition 'of' is used in one sentence and 'at' in another?

She presented with an elevated blood pressure OF 155/100. When she returned to the clinic, her blood pressure was AT 110/85.

2 Answers 2

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The reason they are different is that they are in different grammatical contexts.

In the first one it is in a PP (prepositional phrase) which directly specifies blood pressure. There is no particular semantic relationship - the number specifies the quantity, and of simply connects the two terms syntactically. We regularly use of in such cases: "a temperature of 30 degress", "a length of 5 metres", "a sequence of 50 events"; and also eg "a difference of opinion", "a case of mistaken identity", "the colour of blood".

In the second it is also a PP, but is the complement of the verb was. A preposition is not required here (we could say her blood pressure was 110/85) but it may be seen as a metaphorical location, perhaps on a vertical scale, so at invokes that metaphor. (We wouldn't usually use that metaphor for a measurement that was actually spatial, eg we wouldn't say "the length was at 5m") but it is possible for more abstract measurements that we can put on a scale.

We would not say her blood pressure was of 110/85: the of in your first example does not have any meaning of its own, but just serves to express the syntactic relationship between the bp and the number.

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In the first sentence, the prepositional phrase modifies a nominal phrase ("blood pressure"). If we think of 155/100 as occupying a position within a range of values, then "at" might be possible, but I think that "of" is more natural for most English speakers. For example:

A normal person has a temperature of 98 degrees Fahrenheit.
The Giants won the game by a score of 10 to 3.
The variable "x" has a value of -11.
I want you to buy the stock when it has a price of 26.

In the second sentence, the prepositional phrase actually seems to modify the verb "was" (not the nominal phrase "blood pressure"), answering the question "at what level". The preposition "of" is not usually used to modify a verb, but "at" works well. For example:

It has been getting colder all day; the temperature is now at 5 degrees.
Early in the game, the Giants' score stood at 7.
I increased the value of "x", but the value of "y" was kept at 2.
Keep an eye on the stock's price; I want you to buy at 26 and sell at 34.

(I included the last example to demonstrate that the prepositional phrase at the end of each sentence is not simply a predicative expression.)

Other terminology could certainly be used (for example, we could talk about "verb complements"), but I think that the reasoning would be similar.

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