You do this with the tank empty. First adjust the air from the nipple usually on top, to 28 psi, then switch on pump to fill tank. Now open 2-3 faucets and watch the gauge very carefully as pressure reduces. Adjust main spring on pressure switch to cut ON at 30-31 psi.
If your pressure exceeds the setting of the PRV or is below the setting
The recommended amount of air pressure in a bladder tank is typically around 2 psi below the cut-in pressure of the water pump.
The air charge in a bladder tank should be 2 PSI below the pressure switch ON setting. In other words if the pressure switch is a 30-50, then the tank air should be 28. -If the pressure switch is the higher 40-60, then the tank air is 38. These tanks are all charged at the top and you can check air pressure there also.
The tank air pressure mustbe checked and made compatible with the pressure switch. (2 PSI below 'on' setting). Most bladder tanks can have air released OR pumped in on a nipple at top of tank. Most home pressure switches are pre-set at 30-50 PSI. So tank pressure should be 28. ( You can adjust the pressure switch setting with a 3/8 nut driver. Detailed instructions in lid of pressure switch.) - If the pressure switch is old or if someone has fooled around with settings -get a new one, only about $25 .
the bladder
Modern well water pressure tanks are made of thin steel with a thick rubber bladder inside. The mouth of this bladder is open to the water inlet pipe. The tank has an air charge outside this bladder, with an inflation nipple (just like on a bicycle tire) at the top.) The standard pressure switch is fitted to the TEE at water opening with a gauge and a shutoff valve on house side and a non-return valve on well side. The pressure switch should be set at 30/50 or 40 /65 -these are factory settings bur can be adjusted. If the PS is at 30/50 you will need an air charge of 28 psi outside the bladder, when bladder is empty. (Raise or lower this pressure by the nipple at top) When the pump is turned on water will enter the bladder until it gets to 50 psi. then the pressure switch will open. The residual air pressure will squeeze the bladder forcing water through the system. When water level gets down to 30 psi, then the switch will close, turning the pump on again. Having tank air pressure 2 pounds below switch setting is an important part of this.
Bladder is not situated below the kidney. It was situated posterior to the kidney. You have adapted the erect posture and so bladder came below the kidney. Actually it was posterior because it was convenient to have it near the outlet.
The recommended pressure setting for a well pump air tank is typically around 2 psi below the cut-in pressure of the pump. This helps ensure optimal performance and efficiency of the system.
The pubis bone is located just below the urinary bladder.
It's located below the bladder, around the urethra. See link below for pics.
Generally the pressure in the tank air bladder should be 2 psi below the water pressure switch kick in (pump turn on) point. In this example of 48 to 68 Lbs the air bladder pressure should be 46 psi. This pressure should be measured with the tank empty of water, that is to say NO water pressure applied. NEVER over pressurize the air bladder - it would be better to under pressurize if in doubt. Some tank manufacturers may recommend a different PSI differential so check with the tank literature or the manufacturer.
Frogs do have bladders, but I don't think there is such a thing as a "small bladder" in a frog. I suspect that the asker may be referring to the "gall bladder". If so, please see the related link below which shows a frog's gall bladder, below its liver.