Blood Flow and Pressure

Blood Flow is Caused By

Students can sometimes get the concepts of blood flow, blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange., and resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. all mixed up. Blood pressure and resistance rise and fall together. They are proportional to each other. Blood flow rises and falls in the opposite direction as blood pressure and or resistance. Blood flow is inverse to either blood pressure or resistance. I usually have one person who emails me during the semester. They say, wait a minute. When I put my thumb over the hose, it sprays out at a much faster speed. This is compared to if I just let it flow from the garden hose. No one here is talking about speed. When we talk about blood flow, we mean the blood amount that can enter the capillary beds of tissue.

The pressure in your arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr is about 120mm of mercury. Meanwhile, the pressure in your veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge is somewhere close to 15mm of mercury. This creates a pressure gradient down which blood would flow. As though the heart doesn’t trust blood to flow the heart also creates systole which forces blood through the circuits. Systole depends on the heart’s contractilityThe ability of muscle tissue to shorten with force., or the force it generates to push blood into the aorta. Additionally, it depends on the elasticity of your arteries. If you’re young and healthy, your aorta and elastic arteries expand to allow pressure to dissipate through the systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu. However, if your aorta has calcifications and cannot expand when blood enters, the pressure increases due to reduced elasticity.

Blood Flow is Opposed by

The previous slide talked about things that encourage blood to flow through your systemic circuit.  Here are all (or most) of the things that discourage blood to flow.  These things are called resistance. Together, they create friction as blood passes through and touches the vessel walls. That friction is reflected in the pressure of the blood.  These items and how they specifically oppose blood flow are discussed in another minilecture.

For Blood to Flow

The concept of blood flow involves creating enough pressure to overcome resistance. This is kinda of like pulling a box that’s too heavy to lift along the ground.  You need to use a lot of muscle to pull or push. This effort is required to overcome the friction between the box and the ground. 

Blood flow can be calculated.  The top part of this equation here is what encourages blood flow.  That’s the pressure gradient.  Pressure is high in the aorta and low in the vena cava.  It’s this number here that usually confuses students.  I want the pulse pressureThe difference between systolic and diastolic blood pressure, indicating the force of each heartbeat or the difference in the gradient.  So, it’s not one number I seek, it’s the difference between two.  Let’s just look at this 120 minus 80.  If your heart contracts with more force, systole raises to 140. That top number gets larger. This encourages blood flow.  If your diastolic blood pressure rises to 100, that makes this gradient smaller and reduces blood flow.

What happens if?

In math and science, we use a triangle symbol (Δ) to indicate difference. Thus, the difference in pressure, ΔP, encourages blood flow, while resistance, R, discourages it.

.  I want you to prove this for yourself.  Start out with 120-80 over 1.  Increase each number and see what happens to blood flow.  Then, decrease each number and see what happens to blood flow.  This will not be the last time we see this equation, we see if for air flowThe movement of air into and out of the respiratory tract, driven by pressure differences between th too.  I urge you to understand this equation here.

What do we mean when we say that resistance decreases as the blood flows down the pressure gradient? Well, resistance is a measure of the amount of friction blood encounters as it travels through the vessels. Resistance is also used to describe any opposition to blood flow. Most of the resistance is located in the systemic circuit or the peripheral circulation. We often call this peripheral or systemic resistance.