Blood pressure (BP) refers to the force of blood exerted on the walls of the arteries. This can be measured non-invasively with the use of a device called a sphygmomanometer. BP readings are composed of a top number (systolic reading) and a lower number (diastolic reading). The unit of measurement used is millimetres mercury or mmHg.
In some critical care settings, blood pressure will be monitored invasively and this can involve an insertion of an arterial line. It is done when haemodynamic monitoring is critical and the readings obtained needs to be very accurate.
Systolic pressure refers to the pressure exerted during each ventricular contraction (in the heart) while diastolic pressure is the pressure exerted when the ventricles are at rest.
Girl using a digital sphygmomanometer
BP readings showing the systolic
and diastolic blood pressure
There are several factors that affect blood pressure. These include activity, medications, age, diet, smoking, and genetic factors. An increase in activity like running will result to higher readings when compared with the readings at rest. Some factors like diet, medications, and genetics influence blood pressure even when in a relaxed state. That is why for some people, they are required to have their BP checked regularly for management and control because when blood pressure becomes consistently high (e.g. hypertension), the risk for developing cardiovascular disorders are increased.
While there are several factors that affect blood pressure, it is important to also note that our blood pressure can also fluctuate in response to a need to maintain a state of equilibrium or homeostasis. For example, when there is loss of blood (e.g. haemorrhage), the circulating blood volume decreases and the body activates certain mechanisms to hopefully maintain its fluid volume. As a result, the body will inhibit further urine formation and constricts its blood vessels to maintain blood pressure and perfuse the major organs with enough blood. This blog will explain some of those mechanisms involved such as the renin-angiotensin-aldosterone system, the role of the atrial natriuretic peptide hormone, the role of vasopressin or antidiuretic hormone, and the autonomic nervous system.
The Renin-Angiotensin-Aldosterone System (RAAS)
The renin-angiotensin-aldosterone system is activated in response to low blood pressure. The aim of RAAS is to increase BP in order for the body to compensate and perfuse the cells and tissues with adequate amount of blood thus supplying them with enough oxygen. Blood carries oxygen to the cells which is needed to function.
In the event of low blood pressure like in the case of dehydration, haemorrhage, or any other factors, the juxtaglomerular cells (JG cells) of the kidneys are stimulated to release renin in the blood. This renin then becomes angiotensin-1 once it comes in contact with the substance angiotensinogen which is released by the liver. Angiotensin-1 then travels throughout the circulatory system until at some point it comes in contact with an enzyme released by the lungs called the angiotensin-converting enzyme (ACE). Angiotensin-1 gets converted to Angiotensin-2 by ACE. Angiotensin-2 is a potent vasoconstrictor which causes the arteries to constrict and this increases the blood pressure. Furthermore, angiotensin-2 also stimulates the adrenal gland to release aldosterone. This hormone functions by promoting sodium and water reabsorption in the kidneys thus also facilitating an increase in blood pressure.
The Renin-Angiotensin-Aldosterone System
The atrial natriuretic peptide (ANP)
Atrial natriuretic peptide is a hormone released by the atrial cells of the heart when the atrial walls are stretched due to high blood pressure. The hormone’s action is to lower blood pressure by promoting an increase in sodium excretion by also increasing the kidney’s glomerular filtration rate. “Where sodium goes, water follows” thus ANP also promotes diuresis. Moreover, ANP also decreases renin and aldosterone secretion and this results to a lowering of the blood pressure (Germann and Stanfield, 2005).
The Antidiuretic Hormone
The posterior lobe of the pituitary gland stores and secretes antidiuretic hormone (ADH) or vasopressin produced by the hypothalamus in response to high osmolality in the blood. Osmolality refers to the concentration of particles in a dissolved solvent so in reference to high blood osmolality, that would simply mean that the particles circulating in blood plasma is too concentrated. This commonly occurs when the body’s circulating blood volume is decreased like in the case of dehydration. When a person is dehydrated, they usually have low blood pressure. The high osmolality in the blood stimulates the osmoreceptors of the hypothalamus to secrete ADH. ADH then travels through nerve axons to the posterior lobe of the pituitary gland where it is then released. The function of antidiuretic hormone is to decrease urine formation so that the body can reabsorb water and sodium. This then results to an increase in blood pressure in hopes of maintaining adequate circulating blood volume in the body.
The autonomic nervous system: sympathetic and parasympathetic
Most of the organs in the body are innervated by the autonomic nervous system (ANS), which consists of the sympathetic (fight and flight) and parasympathetic (rest and digest) nervous system. Organs such as the heart, lungs, the liver and so many others are dually innervated. When the body is at rest, the parasympathetic nervous system dominates which increases digestive activity but decreases activity of the other effector organs like the heart and the lungs thus decreasing heart rate and respirations.
When the body is excited or when physical activity occurs, the sympathetic nervous system is activated resulting to an increase in heart rate, respiration, and an increase in the activity of many of the effector organs. The stimulation of the sympathetic and parasympathetic nervous system of the ANS influences blood pressure. In addition, an increase in sympathetic activity like in the case of the fight and flight response also stimulates the adrenal gland to secrete catecholamines in the blood. Adrenaline and noradrenaline hormones are examples of catecholamines. They increase heart rate, respirations, and metabolism which also increases blood pressure.
Depending on which part of the ANS is stimulated, the sympathetic and parasympathetic nervous system plays a huge role in blood pressure regulation.
Sympathetic and Parasympathetic Nervous System.
Video by Allila Medical Media (2018)
There are many factors that influence blood pressure and the mechanisms explained here are the basic mechanisms commonly described in a human biology class. The management for abnormal blood pressure states involves a multi-disciplinary approach and should be a holistic one. Blood pressure can be affected by several physiologic factors but psychology, emotions, and how we deal with stress on a daily basis also have a huge impact on what our blood pressure readings will be like.
Disclaimer:
The content in this blog is for informational purposes only and should not be taken as medical advice. It is always best to consult your doctor for medical questions.
If you are a healthcare provider, the content here should not be used to make any diagnosis, give advice or prescribe treatment as this blog is for informational and educational purposes only. Healthcare is an everchanging field and each patient is unique. It is your responsibility as a healthcare provider to always refer to current care standards and practices.
References:
Alila Medical Media, 2018. Autonomic Nervous System: Sympathetic vs Parasympathetic, Animation. [video] Available at: <https://www.youtube.com/watch?v=D96mSg2_h0c>
Marieb, E. and Keller, S., 2018. Essentials of Human Anatomy & Physiology. 12th ed. Pearson Education Limited
Germann, W. and Stanfield, C., (2005). Principles of Human Physiology: Second Edition. San Francisco: Pearson Education, Inc.
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