Factors Affecting Resistance

Resistance Determined By

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Vessel Diameter

 On the right, a large-diameter vessel (or vasodilated vessel) allows blood to flow easily with less resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. and pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. Conversely, vasoconstrictionThe narrowing of blood vessels due to contraction of smooth muscle, increasing blood pressure and re increases friction, resistance, and pressure, slowing blood flow.

Blood vessel diameter is the most important factor in determining your blood pressure. Peripheral blood vessels are used to measure this since they are more numerous than centralized blood vessels. The smaller the vessel, the more pressure  to push the same volume of blood through. As a result, blood pressure increases.

Blood vessel diameter is the most variable of the factors affecting blood pressure and has the greatest impact on resistance. It changes often to adjust to internal cues. For example, if you get hot, the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. sends a message that you need to radiate away heat. So the peripheral blood vessels dilate to increase blood flow to the limbs. The increase in blood flow to the limbs helps to cool the blood. Combined with the cooling effect of sweating, this will reduce your core body temperature.

In contrast to this, if it is cold outside, your hypothalamus sends a signal for blood vessels to constrict. This reduces blood flow to the periphery. It limits the amount of cold blood entering the body core from the limbs. As your blood vessel diameter changes, so does the resistance, and so does the blood pressure.

The other reason that diameter impacts resistance so strongly is that the radiusLateral forearm bone (thumb side); rotates around ulna during pronation/supination. of the blood vessel inversely affects resistance. This variation follows the fourth power. That may sound complicated. In reality, it means that if the radius of the vessel doubles, the resistance will decrease to one-sixteenth of its original value. This is a dramatic drop. So while larger vessels reduce resistance, small vessels increase the resistance. And there are many more smaller vessels than there are larger ones.

Vessel Length

A longer vessel creates more resistance, which increases pressure, as seen in the left diagram. This is why blood pressure naturally increases as you grow from a baby to an adult due to longer vessels.

Resistance, as it pertains to total blood vessel length, is straightforward. The longer the blood vessel, the greater the resistance needed. As people gain weight, they need to build miles of small blood vessels to vascularizeTo invade a tissue with blood capillaries. all of the extra tissue. This means that peripheral resistance increases dramatically, which can increase blood pressure. That’s one of the reasons why obesity is correlated with high blood pressure.  I always remember this by remembering that a child’s blood pressure is quite low compared to that of an adult.  As we grow, our vessels elongate, which increases pressure.

Blood vessel diameter is the most important factor in determining your blood pressure. Peripheral blood vessels are used to measure this since they are more numerous than centralized blood vessels. The smaller the vessel, the more pressure  to push the same volume of blood through. As a result, blood pressure increases.

Blood Viscosity

Next is blood viscosityThe thickness or resistance to flow in a fluid, such as blood., which refers to the resistance to flow. Fluids like maple syrup, molasses, and motor oil are viscous. When blood viscosity is low, such as when you’re well-hydrated, red blood cellsThe basic structural and functional units of life. align. They flow smoothly through the vessel. This reduces friction and resistance. When you’re dehydrated, viscosity increases. The red blood cells become more concentrated, bumping into each other and the vessel walls, which increases resistance and pressure.

Blood is thicker than waterThe universal solvent essential for life., according to the saying. And it is a correct observation. Blood viscosity is defined as the internal resistance to flow related to the thickness or stickiness of the fluid. The thicker the fluid, the more difficult it is for the moleculesGroups of atoms bonded together. to keep flowing along. Maple syrup flows more slowly than water, because the syrup is more viscous. Generally speaking, blood thickness is pretty constant. However, if there is an abnormal increase in red blood cells as in the case of polycythemia, blood viscosity will change. This condition alters how the blood flows. If there are significantly fewer blood cells, like in the case of some anemias, it also affects viscosity. These changes directly affect the peripheral resistance.  Dehydration reduces the plasmaThe liquid component of blood. in your blood in comparison to the formed elementsThe cellular components of blood, including red blood cells, white blood cells, and platelets..  This increases viscosity, making it more difficult for the blood to flow.

Turbulence

Finally, turbulenceIrregular, chaotic blood flow that increases resistance and can contribute to clot formation., often confused with vasoconstriction, describes irregular blood flow through vessels. Turbulence can be caused by plaque deposits, as shown in the left diagram. It increases resistance and blood pressure, especially near the heart, where turbulence adds to afterload. Plaques create a situation like permanent vasoconstriction, combining high viscosity and small vessel diameter, which elevates pressure.

Turbulence is a factor that affects blood pressure.  However, anything that creates turbulence would also cause a smaller lumenThe inside space of a hollow organ or structure. of the blood vessel. Therefore, we can consider turbulence under the heading of vessel diameter.  If you acquire plaques of arteriosclerosis because you love bacon, your blood will no longer flow smoothly through your vessels.  Instead of a nice straight line of flow, the red blood cells hit the plaque and disrupt their blood flow.  This turbulence creates resistance to flow and increases blood pressure.  So, not only do plaques decrease the diameter of the blood vessel, they also cause turbulence.