The system that produces the pressure necessary to deliver blood to cells and then return the blood for re-oxygenation is one of the most complicated and vital systems on earth. Each individual human body has on average between 60,000 (96,000K) and 100,000 (160,000K) miles of blood vessels.
Blood pressure is the pressure exerted by the blood against the walls of the blood vessels, especially the arteries. This system is complex but basically has five parts. The first is the pressure sensory system. The body has sensors throughout the system which keep the brain constantly aware of the pressure. If the pressure is too low or high the body, under the brain’s direction, can adjust to change the pressure. These adjustments help us understand the other parts.
The pump or heart can speed up or increase its blood output to increase pressure. The opposite will decrease the pressure.
The walls of the blood vessels in a healthy person can dilate or contract. This change in resistance may decrease or increase the pressure on the walls. The length and size of blood vessels also affect the pressure of blood flow. The longer the vessel the higher the resistance, so the circulatory system is engineered for size and length to carry blood at a pressure below the level that would rupture vessel walls.
The brain can send more blood to the kidneys which will increase urine output. Urine is primarily water that is removed from the blood. When the kidneys make more urine the volume of blood decreases which can lower blood pressure. When we loose blood our brain can slow the blood flow to the kidneys to increase blood volume and, therefore, blood pressure.
I am told that salt has an effect on blood volume in some people. It tends to increase volume and may increase blood pressure. The kidneys can also regulate sodium retention in the blood.
The concentration of blood sugar may change the viscosity (thickness of fluid) of the blood. The more sugar (think of syrup) the thicker the blood.
How would life exist for long without all of the above at the same time? The human circulatory system is a marvel of creation engineering. Our Creator, the Chief Engineer, is amazing.