Regulation of Blood Pressure

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The regulation of blood pressure is a complex process that involves several mechanisms:

1. Renal regulation: The kidneys help regulate blood pressure through sodium and water balance. When blood pressure drops, the kidneys compensate by retaining sodium and water. This increases the blood volume and blood pressure. When blood pressure rises, the kidneys increase sodium and water excretion to reduce the blood volume and bring blood pressure down.
2. Hormonal regulation: Several hormones help regulate blood pressure, including:
   • Renin-angiotensin-aldosterone system: When blood pressure drops, the kidneys release renin which converts angiotensinogen to angiotensin I and II. Angiotensin II constricts blood vessels and stimulates aldosterone release from the adrenal glands. Aldosterone causes sodium retention by the kidneys.
   • Antidiuretic hormone (ADH): Released from the posterior pituitary gland. It causes water retention by the kidneys to increase blood volume and blood pressure.
   • Atrial natriuretic peptide (ANP): Released from the atria of the heart in response to volume expansion. It causes sodium and water excretion by the kidneys to decrease blood volume and blood pressure.
3. Autonomic nervous system: The autonomic nervous system helps regulate blood pressure through adjustments in heart rate, contractility, and peripheral resistance:
   • Sympathetic activation: Increased heart rate, heart contractility and vasoconstriction. This increases blood pressure.
   • Parasympathetic activation: Decreased heart rate, decreased heart contractility. This decreases blood pressure.
   • Baroreceptors: Pressure receptors located in the carotid sinus, aortic arch and cardiac chambers. They help detect changes in blood pressure and activate the autonomic system to restore appropriate levels.
4. Local regulation: Local metabolic changes can also affect blood vessels and influence blood pressure, especially in highly metabolic organs like the kidneys. When oxygen or nutrient needs increase in an organ, blood flow to that organ increases, increasing overall blood pressure.

So in summary, multiple mechanisms including renal regulation, hormonal regulation, autonomic regulation and local regulation work together in a concerted fashion to help maintain blood pressure at appropriate levels based on the body's needs.

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Topic Highlights:-

• Blood pressure or BP is the pressure created by the blood pushing against the walls of blood vessels.
• BP is regulated mainly by two systems – the Renin-Angiotensin-Aldosterone-System or the RAAS through enzymes and hormones, and the sympathetic nervous system involving signal transductions, along with other micronutrients and hormones.
• This presentation provides an insight into the factors regulating blood pressure, blood pressure measurement, types of changes in BP, and complications arising from it.
• The presentation also includes lifestyle modifications that help maintain normal BP.

Transcript:-
Blood pressure (BP) is the pressure created by the blood pushing against the walls of blood vessels , particularly the arteries . Blood pressure is created by two mechanisms – pumping of blood into the arteries and through the circulatory system , by the heart, and resistance exerted by the arteries to the flow of blood.

There are various factors that determine blood pressure. It varies based on the strength of the heartbeat, the volume of blood being pumped and the elasticity of the blood vessels. Blood pressure also changes depending on the temperature, posture, and physical state of the body, diet, emotional state, and medications taken. Blood pressure is lowest during sleep and highest during exercise.

Blood pressure is measured and expressed at two points – high point or systolic blood pressure and low point or diastolic blood pressure.

Systolic pressure determines the value at which the heart contracts to pump blood during circulation. Diastolic pressure is the point at which the heart relaxes to fill the blood. Systolic pressure is always higher and is mentioned first, followed by the diastolic pressure.

Blood pressure is measured in terms of millimeters of mercury (mm Hg). A pressure of 140/80 mm Hg refers to systolic pressure being 140 and the diastolic pressure being 80 in the column of mercury. The optimal recommended systolic pressure is less than or equal to 120 mm of Hg and diastolic pressure less than or equal to 80 mm of Hg. It is expressed as 120/80 mm Hg. Sphygmomanometer is used to measure the arterial blood pressure.

There are two systems in our body responsible for maintaining normal blood pressure levels at all times. These are the sympathetic nervous system and the renin-angiotensin–aldosterone system.

The sympathetic nervous system , which is a part of the autonomic nervous system , produces two types of chemicals or hormones – adrenaline, also known as epinephrine , and noradrenaline, also known as norepinephrine .

Release of norepinephrine leads to increased vasoconstriction and release of epinephrine results in the dilation of blood vessels near to the muscle cells. This process controls blood flow to various parts of the body. These hormones also act during stressful conditions. Hence they play an important role in controlling blood pressure and preventing hypertension .

The renin-angiotensin-aldosterone system or RAAS cascade helps maintain the blood pressure and blood volume in equilibrium. Any change in this system will result in hypertension. Low blood pressures in the arteries induce the kidneys to release the enzyme renin .

Renin acts on the protein angiotensinogen produced in the liver, converting it to angiotensin-I , which is a mild vasoconstrictor. Angiotensin-I reaches the lungs through the blood stream where the angiotensin-converting enzyme (ACE) converts it into an active form – angiotensin-II a powerful vasoconstrictor.

Constriction of blood vessels due to the formation of angiotensin-II results in an increase in blood pressure. In addition, it stimulates the adrenal glands to release the hormone called aldosterone into the blood stream. Aldosterone acts on the kidneys, increasing sodium and water reabsorption back into the blood stream.

Angiotensin-II also stimulates the release of vasopressin from the posterior pituitary , leading to an increase in fluid retention. These processes increase the volume of blood and hence control blood pressure. The kidneys receive a negative feedback as the blood pressure normalizes, decreasing the secretion of renin and maintaining homeostasis.

Many micronutrients are found to play an important role in maintaining normal blood pressure. Regulation of these micronutrients by other factors or nutrients has a direct effect on blood pressure. For example, vitamins that regulate the concentration of Ca in blood, vitamins B, C, D and E, antioxidant vitamins and related factors that regulate NO ( nitric oxide ) have a significant role to play in maintaining blood pressure.

Hemoglobin plays a vital role in regulating blood pressure. Hemoglobin contains nitric oxide that helps in relaxing the walls of the blood vessel and increases the rate of blood flow. Regulating the concentrations of nitric oxide controls the expansion and contraction of blood vessels, thereby directly altering the blood pressure.

Dopamine is a component produced by the hypothalamus and plays an important role in regulating the cardiovascular , hormonal , renal and central nervous systems . Dopamine receptors are found in a number of organs and tissues. There are five types of receptors labeled from D1 to D5, and grouped under D1-like D2-like subfamilies.

Among the five types of receptors D1, D3 and D4 receptors are found to interact with the RAAS cascade, while D2 and D5 influence the central and/or peripheral nervous system. A change in the action of these receptors has a direct influence on the blood pressure.

High Blood Pressure (Hypertension) – Occurs due to elevated blood pressure in the arteries that carry blood from the heart to all parts of the body.

There are various causes for an increase in blood pressure. They include heredity, kidney abnormalities, adrenal gland tumor , diabetes , hormonal changes, narrowing of the arteries due to cholesterol deposition, use of birth control pills or by congenital narrowing of the aorta.
Low Blood Pressure ( Hypotension ) – Occurs as a result of decreased blood flow through the arteries.
The causes of hypotension include slowed or abnormal heartbeat, weakened heart muscle, heart diseases such as myocardial infarction , dilated blood vessels, loss of blood, dehydration, kidney disorders and certain medications. When the normal blood supply to the brain is affected patients with hypotension show symptoms of weakness, dizziness and fainting.

Low or high blood pressure can result in heart disease and stroke. During hyper or hypotension, the heart works harder than normal. Both the heart and the arteries are under greater strain as more or less blood flows through them. When the heart is forced to work harder for a long period of time, the heart enlarges. A chance of a heart attack and myocardial infraction increases with a drastic change in blood pressure.

Kidneys fail to function normally during high or low blood pressure. Changes in blood pressure directly affect the filtering process of kidneys, leading to an increase in concentration of wastes in the body.

High or low blood pressure can produce life-threatening complications on the brain. They may lead to stroke, gradually causing failure of major organs like heart, kidneys, liver and the brain.

High blood pressure may lead to damage of the retina by damage to blood vessels. Damaged blood vessels of the retina cause blurred vision and eventually blindness. It can also lead to osteoporosis as high level of calcium is excreted in the urine.

Hypertension and hypotension are easily treated and managed. There are several ways to lead a healthy life and maintain normal levels of blood pressure. Lifestyle modifications including weight control and exercise are essential for controlling blood pressure. Exercises may include simple lifestyle changes like using the stairs instead of an elevator, cleaning the house or working in the garden.

It’s also important to maintain a well-balanced diet with lots of fruits and vegetables. Those who suffer from hypertension and hypotension should also reduce the intake of fat and cholesterol in their diet. They should choose unsaturated fats such as vegetable oils over saturated fats found in animal products and transfatty acids found in fast foods and commercial products as well as maintain a proper intake of salt.

They should also quit smoking and reduce alcohol intake. Manage stress levels. Learn relaxation therapies such as meditation and yoga for effective management of stress.

Blood glucose levels should also be controlled. There is an increased prevalence of hypertension among diabetics. Both these conditions increase the chances of cardiovascular and renal diseases.

One of the most important preventive measures is to take the medications for hypertension or hypotension as prescribed by the physician.