The Histology of Blood Pressure Regulation

It is a beautiful STEM day again, and today, I will be going into a detailed introduction of Blood Pressure. If you remember about two months ago, I wrote a post which is titled A Hypertension Experience and Research , where I explained a case of my uncle who had a high blood pressure and was also self-medicating. The post focused on hypertension, but in this post, I will be explaining Blood Pressure in full. A lot of us go to the hospital, and we are asked to check our blood pressure, but we do not know or understand how this thing work, or what Blood pressure itself mean. In this post, I will be breaking it down Poco a poco (a musical jargon, meaning little by little).

Let's Define Blood Pressure

In my previous posts, I have been able to discuss the heart, which I explained that the heart is a double-sided pump (It is important to know that the heart doesn't produce blood, it only pumps to the body and to the lung). I also explained in one of my previous posts that blood vessels allow for the flow of blood to and from the heart, and I emphasized in another post that the blood is responsible for transporting materials to the tissues and the body. The pumping of blood in the heart through the contraction of the ventricles will either send blood through the pulmonary artery to the lung or through the aorta to the body. Arteries are blood vessels that move blood away from the heart to the body. They are branched into arterioles, which then branch into capillaries which send the blood to the tissues. The waste from the tissues are returned through the venules, which sends the blood with waste to the vein, which then sends the blood to the heart and to lungs.

When the heart pumps, there is an increase in blood pressure, and when the heart relaxes, blood enters the heart to be pumped again.

To explain the blood flow and how the pressure works, the Arteries which move blood away from the heart to the tissues have a thick elastic wall with large radius. The Artery is made up of Endothelium, Elastin fibers, smooth muscles, and collagen fibers. Arteries are high pressure vessels closest to the heart, Large arteries expand, to absorb pressure and recoil to push the blood to vessels. The collagen fiber is for tensile strength, while the Elastin fiber allows for stretch capacity around the vessel wall. This allows the Arterial pressure fluctuation during systolic and diastolic events of the heart.

Arterioles which are made up of the endothelium, smooth muscles, collagen fiber/connective muscle coats, are smaller arteries, which are highly muscular with no elastic fiber. The smooth muscles can contract, also the diameter can increase and reduce to regulate blood flow. The Arterioles reduce the blood pressure from the arteries by slowing down the blood flow from the arteries via the sympathetic nervous system secreting neurotransmitter Epinephrine and Neoropinephrine, causing the arterioles to constrict, decreasing the diameter of the vessels.

Capillaries which are microscopic are made up of a single thin wall of endothelium, which is densely branched out to different tissues, and slows down the blood pressure, so the tissue can receive the adequate nutrients and exchange the adequate materials. Depending on the type of capillaries, they can be tighter than one another. You can read Fenestrated capillaries, continuous capillaries, and blood brain barriers capillaries.

Just in the same way are large vessels returning blood to the heart called Veins which have thin walls, large radius, consisting of smooth muscles, venous valve, endothelium, and connective tissue coat (Collagen fiber). They respond to the sympathetic nervous system, and can stretch to retain blood, but have little elastin fibers. The Venules are thin wall smaller veins, with small radius.

To understand Blood Pressure, we need to explain Cardiac Output as well as understand Total peripheral resistance. Cardiac Output, just as the name sound, is the volume of blood that can be pumped out by the heart in a circulation per minute. Cardiac output is the product (multiplication) of the Heart Rate (HR), and the Stroke Volume (SV) modulation. Heart Rate, is the number of heart contractions. Heart rate can be affected by a few things such as Parasympathetic nervous system, Sympathetic Nervous system, Hormones, and so on. Stroke Volume, regarded as end-diastolic volume minus end-systolic volume, is the total amount of blood pumped out of the ventricle at each systolic contraction.

End-Diastolic Volume, is refered to the amount of blood in the left ventricle before it pumps/contract

End-Systolic Volume is the volume of blood in the left ventricle after pump/contraction .

An increase of Cardiac Output will cause an increase in flow.

Mathematically:
Blood Pressure (BP) = Cardiac Output X Systemic Vascular Resistance / Total peripheral resistance (SVR).

Cardiac Output (CO) = Stroke Volume (SV) X Heart Rate (HR)

The Systemic Vascular Resistance / Total peripheral resistance, is the force exerted in resistance to blood flow by the systemic vasculature, but not the pulmonary vasculature. This force is affected by factors such as the length, and diameter of the blood vessel, and the viscosity of the blood within the blood vessel. Systemic vascular resistance can be altered by changing the diameter of the blood vessel, lengthen the blood vessel, and viscosity of the blood (using doping).

Matematically:

Systemic Vascular Resistance (SVR) = (mean arterial pressure (MAP) - Central venous pressure (CVP) ÷ Cardiac output (CO)

Mean arterial pressure is the average pressure of blood in the artery in a cardiac cycle (a diastolic and systolic effect) .

Mathematically
Mean arterial pressure (MAP) = Cardiac output (CO) X Total peripheral resistance (TPR)

Flow rate, I remember in my elementary physics, as the volume of liquid flowing past a point through an area, per unit time. In this case, I am looking into Blood flow rate, and it is just the same. The total volume of blood movement in a vessel in one minute.. Flow rate of blood will be a change in pressure over resistance.

Pulse Pressure is the difference between the systolic pressure and the diastolic pressure in the artery.

The pressure of the heart can go up to 120mmHg in the systolic events of the heart and can go down to almost zero, but this pressure can continue at this measurement all through the body. This type of pressure need to be slowed down in the body. The Large artery does this work. They ensure to receive the high pressured blood at 120mmg at systolic pressure, and then keep it low to around 80mmHg at diastolic pressure. At the arterioles, the blood pressure keeps going down to about 40mmHg, and to the capillaries, where it could go from 30mmHg to 10mmHg. The blood pressure cannot go to zero, because pressure is needed to move blood back to the heart..

Image References
Image 1 || Wikimedia commons