Preload and Contractility

•  As blood volume returning to the heart
  increases, preload increases and there is enhanced
  filling with ventricular dilation. 

• According to Starling's Law, increased
  ventricular stretch usually leads to increased
  contractility. Increased preload and
  increased contractility lead to increased stroke volume
  and ultimately an increase in arterial pressure, all
  other factors remaining equal.    

• Some antihypertensive drugs decrease
  preload.

• Heart rate: Since the product of heart rate and
  stroke volume equals cardiac output, an increase in heart
  rate will increase arterial blood pressure, all other
  factors remaining equal.

• Some antihypertensive agents decrease heart rate
  (ß-adrenergic receptor antagonists, e.g.).

• For a given cardiac output, blood pressure
  depends only on peripheral resistance. Some
  antihypertensive drugs act to reduce peripheral
  resistance.