Baroreceptor reflex: Baroreceptors in various organs can detect changes in blood pressure, and adjust the mean arterial pressure by altering both the force and speed of the heart's contractions, as well as the total peripheral resistance.
Renin-angiotensin system (RAS): This system is generally known for its long-term adjustment of blood pressure. This system allows the kidney to compensate for loss in blood volume or drops in blood pressure by activating an endogenous vasoconstrictor known as angiotensin II.
Aldosterone release: This steroid hormone is released from the adrenal gland in response to either high serum potassium levels or if angiotensin II is present. This hormone increases the excretion of potassium by the kidneys, while increasing sodium retention. Since sodium is the main ion which determines the amount of fluid in the blood vessels by osmosis, aldosterone will increase fluid retention, and indirectly, blood pressure.
These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; however, the antihypertensive effect of diuretics is not due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and suffer from fainting.

Effects of high blood pressure

Blood pressure exceeding normal values is called arterial hypertension. It itself is only rarely an acute problem; see hypertensive crisis. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.

All level of blood pressure puts mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.

Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, arterial aneurysms and the second leading cause of chronic renal failure after Diabetes Mellitus.

Effects of low blood pressure

Blood pressure that is too low is known as hypotension.

Low blood pressure may be a sign of severe disease and requires more urgent medical attention.

When blood pressure and blood flow is very low, the perfusion of the brain may be critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.

However, people who function well while maintaining low blood pressures have lower rates of cardiovascular disease events than people with normal blood pressures; i.e. normal does not mean best health.

Factors influencing blood pressure
The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence blood pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs, etc.

Blood Pressure Factors

Rate of pumping. In the circulatory system, this rate is called heart rate, the rate at which blood (the fluid) is pumped by the heart. The higher the heart rate, the higher (potentially, assuming no change in stroke volume) the blood pressure.
Volume of fluid. In the case of the circulatory system, this is blood volume, the amount of blood present in the body. The more blood present in the body, the higher the rate of blood return to the heart and the resulting cardiac output. There is some relationship between dietary salt intake and increased blood volume, potentially resulting in higher blood pressure, though this varies with the individual and is highly dependant on autonomic nervous system response.

In cardiac physiology, the rate and volume of flow are accounted for in a combined fashion by cardiac output. Cardiac output is the product of the heart rate, or the rate of contraction, multiplied by the stroke volume, the amount of blood pumped out from the heart with each contraction. Basically, it represents the efficiency with which the heart circulates the blood throughout the body.
Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the blood pressure. Resistance is related to size (The larger the blood vessel, the lower the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing blood pressure.

Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in blood pressure. Certain medical conditions can change the viscosity of the blood. For instance, low red blood cell concentration, anemia, reduces viscosity, whereas increased red blood cell conentration increases viscosity. (The effect of so-called blood thinners are not on viscosity but on ability of the blood to clot, thus a misnomer.)
In practice, each individuals autonomic nervous system responds to and highly regulates all these interacting factors so although the above issues are basic critially important issues, the actual blood pressure response of a given individual varies widely because of both split second on ongoing slower responses of the nervous system and end organs which powerfully change the variables and actual resulting blood pressure moment to moment.