Which hormone increases blood pressure and why?
The biggest influence on blood pressure is blood volume, and therefore how much water the body retains. Assuming a beating heart, the organs with the next greatest influence are the kidneys. The amount of water they retain in the body, and the ratio of water to sodium, are regulated by antidiuretic hormone and aldosterone.
Estrogens can also promote water retention and elevate blood pressure, experienced by many women around the middle of the menstrual cycle.
Epinephrine and norepinephrine also affect blood pressure by stimulating the heart to beat faster and harder and stimulating the constriction of blood vessels.
Angiotensin II is a potent vasoconstrictor that raises blood pressure.
Natriuretic peptides promote sodium excretion in the urine, water follows sodium, and thus they lower blood pressure.
Thyroid hormones raises the heart rate and contraction force and can contribute to hypertension in hyperthyroidism (overactive thyroid).
Erythropoietin can raise blood pressure by elevating red blood cell count; a higher concentration of RBCs promotes retention of water in the circulatory system and thus raises BP. This happens in EPO doping by athletes and can occur when someone native to a low elevation spends an extended time at a high elevation such as the Rockies, Andes, or Alps.