Blood pressure is the force at which blood is propelled through the blood vessels to all the parts of our body.
Blood pressure levels are frequently regulated by our body to prioritize blood flow to aid specific functions. E.g. After a heavy meal, blood delivery is directed to digestive organs to facilitate efficient digestion.
Regulation Of Blood Pressure
The body can regulate blood pressure with a combination of neural networks and hormonal mechanisms.
These blood pressure regulatory functions can be categorized into 2 types: – Short-term regulation and Long-term regulation.
Short-Term Regulation of Blood Pressure
This is the quickest form of blood pressure regulation performed by our bodies. It is controlled by the cardiovascular center in our brain and it forms a part of our autonomic nervous system.
The cardiovascular center is located in the medulla oblongata of the brain stem and it provides a rapid neural mechanism for the regulation of blood pressure.
- Cardioaccelerator center: It increases the cardiac output by increasing the heart rate and contractility. This in turn increases the blood pressure in the body. Sympathetic cardiac nerves carry the nerve impulses for carrying out these functions.
- Cardioinhibitory center: This region sends signals via the parasympathetic vagus nerves to decrease heart rate and thereby inhibit cardiac output. This lowers the blood pressure levels in the body.
- Vasomotor center: This region regulates the diameter of blood vessels. Sympathetic motor neurons called vasomotor nerves signals the smooth muscles in arterioles throughout the body to maintain a steady-state of vasoconstriction that is appropriate to the particular region. Blood pressure can be increased or decreased by this region.
It consists of the following 3 distinct regions which are responsible for performing certain functions: –
The cardiovascular center is connected to certain sensory neurons in the body which signals the cardiovascular center to control the blood pressure levels. These signals are sent from: –
They are sensors located in the carotid sinus near the throat and the aortic arch in the heart.
They are a type of neuron, namely, ‘Mechanoreceptor sensory neuron’, that are excited by a stretch of the blood vessel.
When there is an increase or decrease in the mean arterial blood pressure, it alerts the central nervous system to trigger an appropriate response.
These receptors are a part of the ‘baroreflex’ which is an autonomic reflex system that influences the cardiac output and helps to regulate short-term blood pressure.
Also known as chemosensor, are also sensory neurons that monitor the levels of carbon dioxide and oxygen in the blood. They alert the cardiovascular center when the levels of oxygen drop below a certain point or when the level of carbon dioxide rises above normal.
They are also situated in the carotid sinus near the throat and the aortic arch in the heart.
The cardiovascular center also receives signals from the higher brain regions such as the cerebral cortex and hypothalamus, for triggering certain responses (stress control, fight-or-flight response, temperature adjustments, etc) that might require adjustments in the blood pressure.
The short-term regulation of blood pressure is designed for making immediate adjustments to the blood pressure levels for the sudden drops or changes that occur normally all the time.
E.g. standing up from a sitting position, stopping in an elevator, jumping from a high point, etc can all affect blood pressure levels that need quick adjustments.
The body has other mechanisms for regulating blood pressure in the long run.
Long-Term Regulation of Blood Pressure
The long-term regulation of blood pressure in the body is carried out by various functions performed by the kidneys.
Kidneys directly influence the blood pressure by: –
- Causing the arteries and veins to constrict
- Increasing the circulating blood volume
The kidneys can do this with the help of a very important hormonal system called ‘Renin-Angiotensin System (RAS)’.
Renin–Angiotensin System (RAS)
It is also known as the ‘Renin–Angiotensin–Aldosterone System (RAAS)’.
When blood flow to the kidneys is reduced due to lowered blood pressure levels, certain cells inside the kidneys called ‘juxtaglomerular cells’ start converting prorenin (a protein that is a precursor to renin) into renin. This ‘renin’ is immediately secreted into the blood circulation.
It reacts with blood cells to form plasma renin that again reacts with the hormone ‘angiotensinogen’, released by the liver.
This reaction converts the angiotensinogen hormones into a peptide hormone called ‘Angiotensin I’.
The vascular endothelial cells, usually found in the lungs, have a particular enzyme called ‘angiotensin-converting enzyme’ that, as it sounds like, – converts the Angiotensin I hormones into ‘Angiotensin II’.
Angiotensin II is a strong vasoconstrictive peptide that is known to cause blood vessels to narrow. This narrowing of the blood vessels will increase the blood pressure back to normal levels.
Angiotensin II is also known to stimulate the secretion of another important hormone called ‘Aldosterone’ from the adrenal cortex.
Aldosterone causes the renal tubules to increase the sodium reabsorption in the body. This consequently causes the reabsorption of water in the blood while simultaneously causing the excretion of potassium.
This process increases the volume of extracellular fluid in the body which in turn, also raises the blood pressure levels back to normal.
Angiotensin II levels in the blood can sometimes be too high. So, to maintain an optimal blood pressure level, the body releases another hormone to counter angiotensin II, called ‘Atrial Natriuretic peptide’.
Atrial Natriuretic Peptide (ANP)
This hormone is secreted by specific cardiac muscle cells in the walls of the ‘atria’ inside the heart. Low-pressure baroreceptors situated inside the heart sense the stretching of the atrial wall due to increased atrial blood volume.
This triggers the release of ANP which influences the kidneys in the following ways:
- ANP affects the sodium channels formed inside its collecting duct and induces sodium excretion reduces re-absorption.
- ANP inhibits the effect of Angiotensin II on the mesangial cells of the kidneys, thereby relaxing them and eventually lowering and balancing out the blood pressure level.
- ANP increases blood flow through the vasa recta which washes sodium chloride and urea out of the system. This lowers the osmolarity of the medullary interstitium and leads to less reabsorption of tubular fluid while increasing the excretion.
- ANP inhibits renin secretion which counters the RAAS process.