Mean arterial pressure is difficult to determine so brachial artery pressure is used for convenience. But keep in mind that it measures only the pressure in that one artery and other pressures in the circulatory system become progressively less and less as arteries branch into smaller diameter vessels as they continue back to the heart across capillaries and veins. However, since vessels of the circulatory system are interconnected, pressure changes in one are often a good indicator of pressure changes in others. There are important exceptions that will be dealt with later on. In any blood vessel of the circulatory system, hydrostatic pressure is directly related to volume.
More volume leads to higher pressures and vice versa.
Thus, understanding the factors that affect volume in a given vessel leads to an understanding of the pressures in that vessel. Specifically, volume in a given vessel depends on the amount of blood entering the vessel versus the amount running out into smaller branches. In this regard, a simple garden hose is a useful analogy. A spigot controls the amount entering the hose while a nozzle at the other end controls the amount leaving it. Varying either or both will change the volume in the hose and the pressure as well.