Nitric Oxide is called an endothelium derived relaxing factor (EDRF) as it is released by the endothelium of the blood vessel. EDRF cause relaxation of the vascular smooth muscle, and as a result cause vasodilation of the blood vessel. The following factors contribute to the release of nitric oxide from the endothelium:

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  1. Blood travelling at high velocity causes a shearing effect on the wall of the blood vessels. As the endothelial cells endure a drag force produced due to friction. This results in a mechanical trigger which stimulates release of nitric oxide.

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TYPES OF CHANNELS AND CONCENTRATIONS OF VARIOUS IONS

  1. Sodium Channels: The concentration of sodium ions outside the cell membrane is greater than that inside the cell. Therefore, there is a passive movement of sodium ions into the cell as the channels open. The sodium channels are voltage gated as they undergo conformational change in response to differences in potential across the membrane. The sodium channels have two types of gates that control the passage of sodium ions; the ‘H’ gate and the ‘M’ gate. At resting stage, the M gate is closed and the H gate is open. Upon stimulation by an action potential, the M gate opens and the channels become active, allowing sodium ions to travel into the cell. This opening of the channels is limited by time. After a fraction of a second, the H gates close spontaneously rendering the channels inactive. The sodium channels enter a refractory period during which they cannot be activated no matter how strong is the stimulus. At the same time, the M gate closes as well. As soon as the refractory period ends, the H channels open and the sodium channels are restored to their initial inactive state. The M gates remain closed till the arrival of the next action potential and the cycle is repeated.

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Different states of the fast sodium channels and relative conformational states of the H and M gates are summarized in the table below:

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Resistance can be integrated into two types of circuits; parallel and serial circuit.

Series Circuit

The heart is placed in series with the pulmonary vasculature and as well as the rest of the body. It pumps blood to the lungs, from where the blood travels back to the left side of the heart. The left heart pumps blood to the rest of the body after which the blood is returned back to the right heart. Therefore, the pulmonary and systemic circuits are connected in series via two pumps, which are the right heart and left heart respectively.

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Calculating Resistance in Series

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Definition: Reynolds Number is used to predict the type of blood flow in a blood vessel. There are two types of blood flow:

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  • Laminar Flow: The laminar flow is described as the flow of fluid which is travelling in a calm, layered fashion. The layer of fluid flowing in the center most region of the blood vessel is said to have the highest velocity. Moving peripherally, the velocity of the layers decrease and the outer most layer, which is running along the vessel wall, is said to be travelling with the lowest velocity. This is due to friction which results in a backward drag produced by the wall on the layer adjacent to it. It is important for a blood vessel to exhibit laminar flow in order to maintain its physical integrity and carry out various cardiovascular functions. Laminar flow allows margination to occur efficiently among other functions. Margination is the process of adherence of blood cells to the vessel wall and their subsequent exit through the wall of the blood vessel to areas of need. This process gets largely disrupted if blood flow is not laminar.
  • Turbulent Flow:

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Capacitance is defined as the distensibilty of a blood vessel. In other words, capacitance is the ability of an object to get stretched. In contrast, elasticity is defined as the object’s ability to recoil or return to its previous shape after being stretched. Elastance is produced by elastin fibers present in the vascular wall. Take for example the aorta; it has the most layers (around 50 layers) of elastin fibers in its tunica media which makes it the most elastic blood vessel in the body. If the aorta is compressed or stretched, it will recoil back to its normal shape.

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Compliance of a vessel is the opposite of its elastance. The veins are said to be compliant because if you keep increasing the volume of blood in the veins, their walls will distend allowing for more blood to be accommodated.

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The Poisuilles Equation takes into account factors such as blood viscosity, length and cross sectional area of a blood vessel and uses it to determine the resistance to the flow of blood.

R=8 n Lπr^4  (Assuming that the flow is laminar and the volume is constant)

  • Where: n: Viscosity of the blood

              L: Length of the blood vessel

              r: Radius of the blood vessel

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FACTORS CONTRIBUTING TO THE POISUILLES EQUATION

Blood viscosity

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