What is Blood Pressure?
Blood pressure is perhaps one of the most commonly monitored clinical parameters among medical patients. Blood pressure can be defined as the force generated by the blood flow per unit area of the systemic vessels. Arterial blood pressure is dependent upon a few major physiological indicators namely, cardiac output, the resistance of the peripheral vessels, and vascular tone. Systemic blood pressure must be maintained within an acceptable range, and this requires the utilization of certain regulatory mechanisms. Any values of blood pressure surpassing or falling behind the normal range can lead to irreversible changes in the localized organ perfusion. Therefore, arterial blood pressure is strictly controlled through a variety of mechanisms.
Regulatory Mechanisms of Blood Pressure
1. Baroreceptor Reflex (BR)
Baroreceptors are blood pressure sensitive bodies located either in the carotid artery or in the aortic arch system. These bodies become activated when blood flow through the said vessels produces a stretch response in the arterial wall. This property of the baroreceptors can be utilized in the modulation of peripheral vascular resistance as well as the cardiac stroke volume.
As the blood pressure rises beyond the normal physiological range, the carotid or aortic arterial walls gradually become overstretched. This is followed by stimulation of the sensory neuronal fibers found in these baroreceptors. These events result in increased activation of the parasympathetic centers of the central nervous system which reduces the sympathetic tone of the blood vessels as well as myocardium. This leads to a drop in the peripheral arterial resistance and cardiac output respectively. However, this process becomes reversed when blood pressure falls below the normal limits, which is accompanied by an increase in the vascular tone and cardiac output.
2. Renin-Angiotensin System (RAS)
Renin is an enzyme secreted by the kidney cells. This enzyme, along with the angiotensin-converting enzyme (ACE), cleaves and activates the angiotensin molecule. The latter functions as a potent peripheral vasoconstrictor molecule. Together, these substances constitute the specialized renin-angiotensin system which is considered to be one of the most effective blood pressure regulatory mechanisms.
Angiotensin II plays an integral role in the maintenance of systemic blood pressure. Moreover, it is also involved in the autoregulation of renal blood flow. In addition, angiotensin II also potentiates the production of aldosterone hormone from the adrenal gland, which plays a significant role in the reabsorption of sodium from the kidneys. Sodium retention is also accompanied by increased water reabsorption which helps maintain the blood volume and pressure. In this way, the RAS is responsible for keeping the body hemodynamically stable under severely hypotensive conditions.
3. Anti-diuretic Hormone (ADH)
The antidiuretic hormone (ADH) is synthesized by the hypothalamus of the brain and released from the pituitary gland into circulation. The ADH-secreting mechanism is directly correlated with the degree of thirst. In the case of volume-depleted individuals, the blood levels of this substance become drastically elevated. It exerts its actions on the tubular system of kidneys where it increases the overall reabsorption of water. Besides, ADH is also known for its direct actions on the vascular channels where it promotes vasoconstriction, thereby sustaining the arterial blood pressure.
These regulatory processes enable the body to maintain the normal levels of organ perfusion which is highly essential for the prevention of organ ischemia.
Hypertension
Most of the classification systems define hypertension as the blood pressure levels recorded above 140/90 mmHg. Hypertension is associated with gradually rising morbidity and mortality rates in the developed world. Hypertension is now strongly considered as a multi-system disorder while it can also be an underlying etiological factor for a multitude of chronic disease processes, including chronic heart failure and renal failure, etc.
Classification
Hypertension could be classified as primary or secondary, as elaborated below:
- Primary or essential hypertension: This form is usually of idiopathic nature.
- Secondary hypertension: This occurs secondary to underlying disorders such as adrenal gland tumors (e.g., pheochromocytoma, etc.).
Risk Factors
Numerous factors are responsible for the development of hypertension. Some of these are modifiable while others are irreversible.
- Irreversible factors:
- Old age
- Racial factors
- Male gender
- Positive family history
- Reversible factors:
- Smoking
- High fat-containing diet and deranged blood lipid profile
- Sedentary lifestyle
- Drug abuse.
Clinical Manifestations
There are hardly any identifiable clinical symptoms that have been attributed to hypertension. It is often labeled as a “silent killer”. However, there might be a few presenting complaints associated with the disease which may well correlate with the underlying etiology.
Diagnosis
The blood pressure needs to be adequately monitored with the aid of a sphygmomanometer. This could be done in a clinical setting or at home. ‘White coat hypertension’ is the term used for describing the falsely increased reading of mean arterial pressure during a medical checkup at the doctor’s office. This mainly arises due to the patient’s own anxiety. Before confirming the diagnosis of hypertension, repeated readings ought to be taken so as to avoid any false-positive results.
Management
Hypertension can be prevented as well as substantially minimized with the help of a well-balanced lifestyle. Apart from the conservative measures including physical exercise and dietary modification, severe and persistent hypertension usually requires pharmacological intervention. For this purpose, a number of drug therapies have been utilized. These include the following:
- ACE inhibitors and angiotensin receptor blockers
- Beta-blockers
- Calcium channel antagonists
- Diuretics
It is noteworthy that hypertension can be best prevented by routine screening of blood pressure which can help identify the clinical issue at the earliest possible stage. This, in turn, can help achieve optimal control of hypertension by means of conservative measures only.
References
- Shahoud JS, Sanvictores T, Aeddula NR. Physiology, Arterial Pressure Regulation. [Updated 2020 Sep 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538509/
- Magder S. The meaning of blood pressure. Crit Care. 2018;22(1):257.
- Wolff CB, Collier DJ, Shah M, et al. A Discussion on the Regulation of Blood Flow and Pressure. Adv Exp Med Biol. 2016;876:129-135.