Controlling the flow rate

I am new to Electronics.
I'm working on a project which about mimics the Stomach function. To do this; I want to pump gastric juice (can bethink as water) in flow rate of 3ml/min from reservoir to stomach. (Arrangement.jpg)

I've bought this pump

I have attached my circuit diagram.

What I want is to control the flow rate. Please help me how to control the flow rate

Circuit Digram.png

2 litres per minute. seems like a very high rate.

look at peristaltic pumps. they are also called dosing pumps.
one rotation = 1 set volume.
you can count rotations to determine volume.
speed of rotations = flow rate

if that pump is really a peristaltic pump, then you are all set. just test it for time for 3/ml
you could run for a few seconds, pause, run, pause... etc

I want to pump gastric juice (can bethink as water) in flow rate of 3ml/min from reservoir

I've bought this pump
Dc6-12v r385 aquarium fish tank round water air dc diaphragm pump Sale - Banggood.com-arrival notice-arrival notice

The flow rate spec on that pump is 1.5-2L / Min(approximately)

You may be able to put the output of the pump through a needle valve to cut the flow, but basically that pump is not appropriate for for your use.

I think some more research on small pumps is required. As @dave-in-nj suggests a peristaltic dosing pump may be the way to go.

If I usethis pump;
it says it can pump 0-100ml/min

How do I control it on 3ml/min

Circuit Digram.png

You control the speed by using a pulse width modulation (PWM) signal. Unfortunately, you are driving your transistor from A0 which doesn't do PWM. You will need to change to another pin

run it on 12v for exactly 60 seconds.
measure the volume.

adust the PWM to something like 80%
run for exacctly 60 seconds
measure the volum.

if you want to measure revolutions, you can get more precise, but speed and time should get you pretty close.