The cardiovascular system has many function, however the most important function is circulation. Circulation is the process of transporting materials around the body – these materials may be taken in or exported out depending on the necessity of the body.

 

The organism is build with the appropriate mechanisms to allow efficient transporting depending on their distances. For small distances, the mechanism of diffusion is used, however for larger distances diffusion isn’t effective. Larger distances use the circulatory system as a mechanism to efficiently transport materials; this is seen mainly in large organisms (i.e. mammals) as opposed to smaller ones (i.e. bacteria.)

 

The transport of carbon dioxide and oxygen through the pulmonary and systemic circulations requires bulk flow, however the exchange of oxygen and carbon dioxide within the red blood cells occur through diffusion due to the small distance. The process of diffusion can be explained by a physical equation:

 

Fick’s First Law of Diffusion:

Flux = D x [(C outside – C inside)/d]

d = thickness

D = diffusion constant

 

The diffusion constant depends on

• Substance diffusing

  • Solubility of the gases depends on their partial pressure

  • Carbon dioxide is more soluble than oxygen

• Substance within which diffusion occurs

  • Diffusion is easier through air than water

  • The gas phase has smaller particles with more kinetic energy in comparison to the liquid phase

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The rate of diffusion depends on

• Distance

  • Diffusion increases with smaller distances

  • The increase in rate occurs in smaller distances, as more energy is required to travel longer distances, also – particles are small and require a long time to travel, thus will get lost in the process of long distances

• Surface area

  • Diffusion increases with increasing surface area

  • The increase in rate will occur because more particles will be able to travel and spread more due to increasing area 

• Thickness

  • Diffusion will increase when the thickness of the membrane is decreased

  • The increase in rate will occur due to the thinner membrane, which will allow the particles to diffuse more easily and readily due to less interactions which results in less energy and heat required to cause the movement

  • Note: similar to small molecular size

• Concentration gradient

  • Diffusion will increase when concentration gradient increases

  • ​The increase occurs due to a shift in balance, where one side of the membrane will have a higher concentration than the other side, which increases pressure and results in the movement of particles from the area of higher concentration to the area of lower concentration in order to set an equilibrium.

 

Mammals have evolved to form a circulation system consistent of four chambers and two closed circulation loops. The chambers refer to the right atrium, which pumps blood to the lungs and the left atrium, which pumps blood to the rest of the body – as well as, the right and left ventricles, which will be discussed later in more details.  The circulatory system consists of two loops that work in parallel, which means that blood is pumped to the lungs and to the rest of the body at the same time, however both are is separate circulatory loops – pulmonary and systemic respectively.

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The cardiovascular system is also referred to as the series – parallel system, which is a crucial component to the organism survival. Despite the fact that mammals have one heart, it is referred to as the left heart and the right heart in order to best understand the processes of the series-parallel system. The right heart, left heart, and lungs are in series and the organs are in parallel due to the branches off the aorta that supply the organs. The organs that are in series have a physically distinct blood supply in comparison to the organs in parallel, which is understood by the electrical circuits taught in physics (see physics overview.)

 

Flow

The structures in series all have the same flow; this is because of the single path for the flow to go through. This single path for the right heart, left heart and lungs does not require the flow to be ‘split’ and redirected, therefore all have the same flow. The idea of flow is similar to current in a circuit, where a circuit in series will have the same current throughout.

 

The structures in parallel all have different flow rate, which is adjusted by the weight of the organ. The organs in parallel have different flow rates due to the many different paths they have requiring them to ‘split’ to accommodate all paths.

 

Flow may vary depending on the bodily requirements (i.e. increased activity) and it also depends on the pressure for flow direction and resistance for magnitude of flow.

 

Flow = cross sectional area x mean velocity

 

• To increase the flow through a vessel, the cross sectional area or mean velocity must increase.

• As the vessels go down the hierarchical chain (i.e. aorta, arteries, capillaries), the cross sectional area of the vessels increase, while flow remains constant. This is physiologically advantageous, as the mean velocity is low to allow sufficient time for diffusion/exchange of gases and other materials.

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Pressure

 

Pressure = force/area

 

The flow rate in the vessels of the cardiovascular system depends on pressure. As mentioned earlier, the pressure determines the direction of the flow. The direction of the flow can be physically explained through the perfusion pressure calculation, which explains that blood flows from an area of high pressure towards the area of low pressure.

 

Perfusion pressure = P inlet – P outlet

 

The perfusion pressure can be used to explain why arteries bring blood in and veins take blood away. The direction of oxygenated and deoxygenated blood flow in the arteries and veins is due to a high arterial pressure and a low venous pressure, therefore directing the flow in that manner.

 

Perfusion pressure is necessary for blood flow

 

Flow = perfusion pressure/ resistance

 

Resistance

 

Vessels in series have a larger total resistance than their resistance in part, which can be understood by electrical circuits in series:

 

R = R1+R2+R3…

 

Vessels in parallel have a lower total resistance than their individual parts. The lower total resistance of the organs in parallel is physiologically advantageous, as a higher total resistance similar to the organs in series would require the heart to pump out blood at a much higher pressure in order to achieve adequate flow. The lowered resistance allows the heart to pump enough blood to all the organs at a low pressure.

 

R=1/R1+1/R2+1/R3

 

Ohm’s law states that the lower the resistance of the circuit, the higher the current flows through the circuit. Therefore, adding more parallel branches to the circuit will reduce the resistance and consequently increase the current. This can be thought of in a physiological perspective, where adding more parallel branches that feed the organs will reduce the total resistance of the vessel and then increase the flow rate.

 

I total = V (1/R1+1/R2+…+1/Rn)

 

The vessels in the body follow through laminar flow, meaning that they travel in a smooth, unidirectional manner through the vessel. Laminar flow rate is similar to a parabolic shape, where the blood by the sides of the vessel moves slower in comparison to the blood in the center of the vessel. In order to affect the resistance of a vessel, the radius, viscosity or length must change – however, a change in the radius will cause the greatest impact.

 

Resistance = 8vL/pir^4

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Cardiac Cycle: How Does the Cardiovascular System Work?

• Ventricular Filling

  • During diastole the left ventricle is filling up with blood because the pressure in the atrium is greater than the pressure of the ventricles causing the atrio-ventricular valves to passively open

    • Recall that blood flows from area of high pressure to area of low pressure

• Atrial Kick

  • At the end of diastole, a slight increase is seen in the pressure of the atrium due to the final amount of blood passed from the atrium to the ventricle

    • Corresponds to the P wave (seen later)

• Isovolumetric Ventricular Contraction

  • The pressure at the ventricles is greater than the pressure in the atrium, which causes atrio-ventricular close

    • First heart sound and start of QRS

  • The pressure in the ventricles is rising (no change in volume)

  • When ventricular pressure is greater than aortic pressure, the aortic and pulmonary valves open and the QRS complex is over.

• Ventricular Ejection

  • The pressure in the ventricles is greater than the pressure in the aorta causing blood ejection through the aorta

  • As the blood goes through the aorta; a decrease in ventricular pressure and an increase in aortic pressure is seen

• Isovolumetric Ventricular Relaxation

  • The volume in the aorta is exiting at the same time as the ventricles and when the pressure of the aorta is greater than the pressure of the ventricles, the aortic and pulmonary valves will be shut closed

    • Second heart sound, T wave, end of systole

• The pressure in the ventricles will continue to drop as blood exits, and when it becomes less than the pressure in the atrium, the atrio-ventricular valves will open for ventricular filling, where the cycle will repeat.