Blood Pressure Regulation and Hypertension

By Sergey Skudaev


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Extracellular liquid volume, vascular system capacity and ion concentration.

From physics, you may recall that liquid cannot be compressed. You cannot force 10 liters of water into a one liter box. Imagine a pump that pumps water from a pool via a pipe to a field of vegetables. Common sense tells us that the longer and the thinner that pipe is the more force is required to move water through the pipe. Since water cannot be compressed, the pressure of water at the end of the pipe will be the same as at the beginning of the pipe, assuming that no leakage exists somewhere in the middle of the pipe.

The human cardiovascular system is more complicated than the sprinkler system, but the law of physics still applies. The thinner the blood vessels are, and the greater the volume of blood that is to be pumped through them, the more force is required to do the job.

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The resistance of vascular system is controlled by the Nervous and Endocrine Systems. The same is true about blood volume, though it can be changed voluntarily by consuming water and minerals.

If for some reason the blood pressure dropps, the kidney afferent arterioles detect that drop using their baroreceptors and this starts a chain of reactions needed for the correction of the problem.

How a Nephron works

To understand blood pressure regulation, you have to know how kidneys filter blood to produce urine. I will give you a very simplified explanation of how it occurrs. The functional unit of the kidney is called a nephron. See the picture of the nephron below.

nephron

This picture borrowed from http://www.lakemichigancollege.edu/liberal/bio/anat/urin.html

The nephron is comprised of glomerular (Bowman's) capsule (1), Proximal Convoluted Tubule (2), Distal Convoluted Tubule (3), Nephron loop of Henle (4), and Collecting duck (5).

Via an afferent arteriole blood flows into the capillaries inside the glomerular capsule and via an efferent arteriole blood flows out of the glomerular capsule. The afferent arteriole is greater in diameter than the efferent arteriole. As a result, the blood pressure inside the glomerular capsule is increased. The glomerular capsule capillaries is porous. Plasma of the blood, small molecules and ions are pressure filtered from capillaries inside glomerular capsule space. The filtrate flows into the proximal tubule, where selective reabsorption of glucose, amino acids and some other nutrients takes place.

The active reabsorption of sodium takes place in the ascending limb of the nephron loop. A high concentration of sodium in the kidney medulla tissue around of the nephron loop is helpful because it pulls water from the descending limb of the nephron loop and increases the concentration of the filtrate that flow through the nephron loop due to osmosis. Only the descending limb is permeable to water. The ascending limb is not permeable to water and water cannot go through the ascending limb wall following the sodium ions.

In the distal convoluted tubule, active secretion of creatinine and some other compounds from the blood to the urine takes place. Reabsorption of the bicarbonate ions occurs here with secretion of hydrogen ions.

Omron HEM-780 Automatic Blood Pressure Monitor with ComFit Cuff

Renin - Angiotensin - Aldosterone System (RAAS)

Let us return to blood pressure regulation. When blood pressure of afferent arterioles decreases, the juxtaglomerular cells (JG cells ) of kidney start producing hormone renin. Additionally, the renin may be produced by the brain, adrenal glands, ovaries, and visceral adipose tissue (abdominal fat). Renin secretion is stimulated not only by afferent arterioles´ baroreceptors, but also by decreasing of Cl- ion concentration in the distal tubule of the nephron, by sympathetic nerve stimulation and by signals from baroreceptors of the carotid sinuses and some other large arteries.

The renin-angiotensin-aldosterone hormonal cascade begins. Renin itself does not affect blood pressure. It converts a large globulin angiotensinogen to decapeptide angiotensin I. The angiotensinogen always is present in blood. It is produced mainly by the liver. Also, it is produced by kidney, ovary, adrenal glands and adipose (fat) tissue in smaller amount.

Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE). Angiotensin II is an active peptide that increases the peripheral resistance by constriction of arterioles and as a result, increases blood pressure.

Additionally, angiotensin II stimulates the aldosterone secretion. The aldosterone is a steroid hormone that is produced by the outer layer of the adrenal cortex (zona glomerulosa). The aldosterone retains sodium and water in the distal tubules and collected ducts. As a result, the volume of blood is increased and thus the blood pressure. Normally, the distal tubule is not permeable to water, but aldosterone changes its permeability to water and water is diffused out of tubule following the sodium ions.

There is a negative feedback from angiotensin II that decreases renin secretion.

Drugs that control blood pressure

Angiotensin-Converting Enzyme inhibitors (ACE inhibitors) such as lisinopril, benazepril, ramipril, perindopril, cilazapril, trandolapril, fosinopril) competitively block the action of ACE and as a result, the conversion of the Angiotensin I to the Angiotensin II. That explains why ACE inhibitors are used to control blood pressure.

The other drugs that control the blood pressure are diuretics and beta blockers. Diuretics increase excretion of the water and as a result, decrease volume of the blood and the blood pressure. Beta-blockers block the beta adrenal receptors and ease the heart´s pumping action. A heart beat rate and a force of constriction is controlled by the nervous system. The parasympathetic nervous system decreases the beat rate, the heart tissue conductivity and the contractility. In opposite, the sympathetic nervous system increases the beat rate, the heart tissue conductivity and the contractility. Sympathetic nerves release adrenaline that affects the heart functions via adrenaline receptors. Beta blockers block adrenaline beta receptors and adrenaline effect on the heart. There are also adrenaline alpha 1 receptors and adrenaline alpha 2 receptors that affect the blood pressure. Alpha 1 agonists, for example phenylephrine, cause vasoconstriction used as vasopressors and nasal decongestants. They raise the blood pressure. Alpha2-adrenergic drugs, for example clonidine, used as antihypertensive agents due to their inhibition effect on the brainstem vasomotor center.

Healthy Life style

You should notice that renin as well as angiotensinogen is produced by adipose (fat) tissue. That explains why overweight is a risk factor for hypertension and maintaining a normal weight improves conditions of hypertensive individuals.

Reducing sodium intake may decrease the blood pressure because high concentration of sodium in blood and extracellular fluid retains water and increases the volume of the extracellular fluid including the volume of blood. Every one knows what healthy life style is, but, unfortunately, few of us follow it.

Patients with hypertension must understand that having healthy life style is good but, unfortunately, it is not enough. The blood pressure must be constantly controlled by medications to avoid life threatening conditions.