Should I need to use the word "TO", or maybe I can say
Keep standing for a long time causes to blood pressure BE low.
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I can't make sense of the word "keep". It seems to be a verb that lacks a subject. We could make it an infinitive if we want it to serve as the subject of the sentence. We could simply lose it and let the gerund "standing" act as the subject on its own.
"Standing for a long time" is a perfectly suitable subject. "Causes" is a perfectly suitable verb.
"To blood pressure" seems to be a prepositional phrase that doesn't make any sense. "Blood pressure" is a noun, so this "to" can't be an infinitive marker. I have no idea what the preposition "to" is trying to do.
Just "Blood pressure", without a leading preposition, is a perfectly suitable direct object.
The rest of the sentence needs to be an object complement. The verb "cause" licenses a normal infinitive as its object complement. The verb "make", on the other hand, licenses a bare infinitive in the same role.
Standing for a long time causes blood pressure to be low.
Standing for a long time makes blood pressure be low.
There is no clear pattern to which verbs license ordinary infinitives and which license bare infinitives. The verbs that license bare infinitives are fairly rare, and pretty much have to be learned one at a time.
There is nothing wrong with the infinitive phrase "to be low", but there are better choices available. Two options you may want to consider are "to decrease" and "to drop".
Standing for a long time causes blood pressure to drop.
Standing for a long time makes blood pressure decrease.
The governing verb (in this case, "causes" or "makes") determines what kind of object complement, if any, can be used. If the verb is "causes", then the object complement needs to be an infinitive. If the verb is "makes", then the object complement can be a bare infinitive, an adjective, or a noun.
Do not say "to blood pressure" in this sentence. The prepositional phrase has no useful job to do, and the verb requires a direct object.
If the verb is "causes", do use an infinitive with the "to" marker as the object complement. Pick a clear and useful verb for this purpose.
Remove 'keep' from the start.
I would say better write it this way-
Standing for a long time causes low blood pressure.
Because the word 'keep' in this context means 'to remain' and is followed by the clause for 'for a long time', I would probably construct the sentence using one or the other but not both. The sentence would then be, "Standing for a long time causes blood pressure to be low."; or better still, "causes low blood-pressure"; or, "To keep standing [will] cause [the subject's] blood-pressure to be low"; or again, "To keep standing causes low blood-pressure."
Some verbs in English may be followed by an object and then by an infinitive with the particle to.
Prolonged standing causes the blood pressure to decrease.
The key verb here is "causes": it takes the object blood pressure and then the so-called to-infinitive "to decrease".
The same applies to your sentence:
Standing for a long time causes the blood pressure to be low.
Some verbs indeed can take another verb without to. These are:
Let my blood pressure be low.
This pill makes your blood pressure decrease.
It helped me decrease my blood pressure.
The nurse saw the patient's blood pressure decrease and, relieved, went on to help others.
Reference: Cambridge Dictionaries Online, "Verb Patterns".