In most people with established essential hypertension , increased resistance to blood flow ( total peripheral resistance ) accounts for the high pressure while cardiac output remains normal.  There is evidence that some younger people with prehypertension or 'borderline hypertension' have high cardiac output, an elevated heart rate and normal peripheral resistance, termed hyperkinetic borderline hypertension.  These individuals develop the typical features of established essential hypertension in later life as their cardiac output falls and peripheral resistance rises with age.  Whether this pattern is typical of all people who ultimately develop hypertension is disputed.  The increased peripheral resistance in established hypertension is mainly attributable to structural narrowing of small arteries and arterioles ,  although a reduction in the number or density of capillaries may also contribute. 
Blood tests may be done to assess risk factors for heart disease and stroke as well as looking for complications of hypertension. These include complete blood count ( CBC ), electrolytes , BUN (blood urea nitrogen), and creatinine and GFR (glomerular filtration rate) to measure kidney function. A fasting lipid profile will measure cholesterol and triglyceride levels in the blood. If appropriate, blood tests may be considered to look for an underlying cause of high blood pressure (secondary hypertension)including abnormal thyroid or adrenal gland function.
For older people, often the first number (systolic) is 140 or higher, but the second number (diastolic) is less than 90. This problem is called isolated systolic hypertension, which is due to age-related stiffening of the major arteries. It is the most common form of high blood pressure in older people and can lead to serious health problems. Isolated systolic hypertension is treated in the same way as regular high blood pressure but may require more than one type of blood pressure medication. If your doctor determines that your systolic pressure is above a normal level for your age, ask how you can lower it.