An Overview of the Cardiovascular System

Arguably one of the most important, yet complicated systems in the human body is the cardiovascular system. It is made up of the blood, blood vessels and due to the nature of blood flow, it is affected by every organ system in the body, mostly indirectly. The renal system (the kidneys and adrenal medulla) have a very direct influence over the heart and cardiovascular system, though.




The Heart
The heart is a fist-shaped pump located slightly left of center in the chest. The job of the heart is to make sure that the blood makes it everywhere in the body that it needs to. It is made of cardiac muscle cells which are extremely responsive to changes in intracellular (intra-inside, cellular-cell) levels of Ca2+ (calcium ions). It is the flow of these calcium ions that allow the heart to expand and contract... that is to beat.

The Blood
Although an important part of the cardiovascular system, it is a little outside of the scope of this particular discussion. For now, just think of the blood as the major transport system in the body... how chemicals get from one area (the kidneys for example) to another (like the heart).

The Veins and Arteries
Most people think that veins carry oxygen-depleted blood and arteries carry oxygen-rich blood. This is usually true, but in the pulmonary circuit (blood flowing between the lungs and the heart), that particular model is reversed. It is actually more accurate to say that veins carry blood toward the heart and arteries carry blood away from the heart. Athleroscerosis is the stiffening of the blood vessels, often as a restult of high cholesterol which decreases the elasticisty of blood vessels and increases the risk for hypertension.

The Renal-Cardiovascular Relationship
The kidneys affect the cardiovascular system in a number of ways, but the two most important are through renin and aldosterone and through the adrenal gland. The renin-angiotensin system (RAS) is the more complicated of the two pathways, but I'll try to simplify it for you. The kidneys release renin, an enzyme which converts angiotensin I into angiotensin II. Angiotensin I is a relatively useless chemical in the blood stream, which angiotensin II is a powerful vasoconstrictor, which causes an increase in blood pressure. Aldosterone is a hormone very similar to testosterone which encourages the body to retain salt and therefore water. This increase in blood volume causes an increase in blood pressure.

The adrenal gland secretes epinephrine and norepinephrine, both of which stimulate the sympathetic nervous system and cause the heart to beat faster and blood pressure to rise. They are your "flight or fight" chemicals, most simply put and they bind with beta-adrenergic receptors to cause their effects.

Cardiovascular Disease

According to Somnath Pal, twelve percent of Americans suffer from diagnosed cardiovascular (CVD) disease of some type. Risk for developing heart attack (myocardial infarction), stroke, congestive heart failure (CHF) and atrial fibrillation all increase with an increase in blood pressure. High blood pressure, also called hypertension, affects approximately 22% of Americans, putting millions at risk for developing life-threatening diseases. According to the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC 7) as quoted by Hulisz, hypertension is defined as follows:

Pre-Hypertensive: SBP 120-139mmHg OR DBP 80-89mmHg
Stage 1 Hypertensive: SBP 140-159mmHg OR DBP 90-99mmHg
Stage 2 Hypertensive: SBP greater than 160mmHg or DBP greater than 100mmHg

SBP is systolic blood pressure, the measure of the blood pressure when the heart is contracted and DBP is diastolic blood pressure, which is when the heart is relaxed. It is usually read SBP over DBP, for example 120/80.

Causes of Hypertension
There is no short list of the causes of hypertension. Anything from family history (genetics) to diet can cause it. Some medications, like Non-Steroidal Anti-inflammatory drugs (NSAIDs), pseudoephedrine, phenylephrine, cocaine and amphetamines, oral contraceptives, some immunosuppressants and other herbal remedies can cause hypertension. Some diseases can cause or worsen it, most notably diabetes mellitus. Weight is also a significant risk factor for developing hypertension; the heavier you are, the more likely you are to be hypertensive. Other lifestyle issues, such as drinking, smoking, stress or drug use can also exacerbate the situation. As many as 90% of cases have no known cause.

Treating Cardiovascular Disease
There are a number of ways to treat cardiovascular disease, most of which are aimed at lowering the blood pressure and eliminating the predisposing factors from the equation. With this done, the risk of other cardiovascular issues decreases significantly. There are a number of ways to treat hypertension, and there is no one way that works for everyone. For example, blacks tend to have less success with measures aimed at the RAS.

For those considered pre-hypertensive, use of diuretics is usually the first choice. (I'll link this to the diuretics drug class profile when I get it up here, as with the other drug classes noted below). Diuretics focus on increasing urine output and decreasing blood volume to lower blood pressure. In patients with stage 1 or 2 hypertension however, diuretics often aren't enough. A number of other options exist, including combination products to increase compliance. ACE inhibitors, usually in combination with diuretics, are usually considered the first line of treatment in hypertension, with other options being used when ACE inhibitor therapy is ineffective.

• Beta-adrenergic Blockers prevent epinephrine and norepinephrine from binding with cardiac binding sites. They also inhibit the release of renin from the kidneys, which slows down the conversion for angiotensin I to angiotensin II.
• Angiotensin Converting Enzyme (ACE) inhibitors block the ability of renin to convert angiotensin I to angiotensin II. New research suggests that these medications may be more effective if taken at night.
• Angiotensin II receptor blockers (ARBs) compete with angiotensin II for cardiac binding sites, decreasing the effects of angiotensin II.
• Renin inhibitors are a relatively new drug class that directly disables renin, causing similar effects as ARBs.
• Calcium channel blockers prevent calcium ions from entering the cardiac muscle cells, which decreases the cells' ability to contract. High doses can cause a heart attack by essentially suffocating the cells of their energy source and preventing blood from reaching other cells.
• Alpha-Adrenoreceptor blockers help relax vascular and cardiac smooth muscle, but are rarely used anymore because of the risk of increased heart rate and a relatively low effectiveness. In fact, many of these drugs are now used to treat benign prostatic hyperplasia (BPH).

There is so much to say about hypertension and never enough time or space to say it in. There are a number of ways to help your blood pressure naturally, and medications are the way to go "when diet and exercise are not enough". Decreasing sodium intake and increasing potassium intake are great ways to help keep your blood pressure in check, as well as reducing or eliminating caffeine and exercising regularly. Weight loss of 5kg (a little over 10lbs) may help decrease blood pressure by 4.4/3.6mmHg. A diet high in whole grains, vegetables, fruits and low in fat is crucial to maintaining a healthy blood pressure. Moderate alcohol consumption (2 drinks for men and 1 for women) may also help lower systolic blood pressure.

Cheers!
C.Samuels

Resources

Finkel, Richard, Michelle Clark, Pamela Champe, and Luigi Cubeddu. Pharmacology. 4th. Baltimore, MD: Lippincott Williams & Wilkins, 2008. Print.

Hulisz, Darrell. "Understanding Hypertension: A Practical Guide for Increasing Antihypertensive Adherence." Powerpak CE. Powerpak CE, 01 May 2011. Web. 19 May 2011. <http://www.powerpak.com/index.asp?show=lesson&page=courses/107594/lesson.htm&lsn_id=107594>.

Pal, Somnath. "Prevalence and Impact of Cardiovascular Disease." USPharmacist.com. U.S. Pharmacist, 19 Feb 2010. Web. 17 May 2011. <http://www.uspharmacist.com/content/d/trendwatch/c/19187/>.

"Reduce Blood Pressure." High Blood Pressure - A Sign of Ill Health. Web. 17 May 2011. <http://www.reducebloodpressure.info/wp-content/uploads/2010/12/reduce-blood-pressure.jpg>.

University of Guelph. "Heart medication best at bedtime, animal study suggests." ScienceDaily, 10 May 2011. Web. 17 May 2011.