Systemic and Pulmonary Circulations, Mean Arterial Pressure and the Measurement of Blood Pressure.

In this post, we'll be talking about systemic and pulmonary circulations, mean arterial pressure and the measurement of blood pressure. I'll explain the difference between serial and parallel blood flow, the anatomy of blood vessels, and how arterioles work to vary vascular resistance. I'll also discuss mean arterial pressure and how it is calculated. In addition, I'll talk about how blood pressure can be measured in a clinical setting as well as how various factors can influence blood pressure.

Series and Parallel Flow

The heart is effectively two pumps working is series, this forms the pulmonary and systemic circuits. Many of the smaller, more branching pathways of the systemic circuit are arranged in parallel. For blood vessels arranged in series, the total resistance is the sum of the individual resistances (known as a relative resistance) of each blood vessel. The blood vessel which has the highest relative resistance has the greatest effect on total resistance when it is changed. In the body, the arterioles have the highest relative resistance and so have the greatest influence on total resistance. This website has a helpful explanation of series blood flow.

A parallel arrangement of vessels results in less resistance, this is due to how the total resistance for a parallel arrangement is calculated. This website shows how it is done and provides a good explanation of the concept. 

Structure of Blood Vessels

• Arteries: these have a relatively wide diameter (~4mm) and thick walls (~1mm), they are muscular and highly elastic.
• Arterioles: these have an average diameter of 0.03mm and a wall thickness of 0.006mm, they are muscular and well innervated. This means that their diameter can be altered which will have a great effect on total resistance in the body.
• Capillaries: these are the smallest blood vessels which are thin walled and highly permeable.
• Venules: they have an average diameter of 0.02 mm and a wall thickness of 0.001mm, they are thin walled and have some smooth muscle. 
• Veins: these are relatively large blood vessels (diameter: ~5mm) that are thin walled (~0.5mm in thickness), fairly muscular and highly distensible.

Because the capillaries have the smallest diameter, one would expect that they would produce the most resistance. However, this is not the case as capillaries are usually arranged in parallel which results in lower resistance. Instead, the arterioles produce the most resistance due to their small diameter, and the fact that they carry freshly oxygenated blood from the heart. Venules do not create much resistance because their perfusion pressure is low, this follows the equation:

Flow = the change in pressure / Resistance

Where the change in pressure is termed the perfusion pressure.  

Total Peripheral Resistance 

Total Peripheral Resistance (TPR) = (mean aortic pressure - vena caval pressure)/cardiac output.

However, for a dog at rest, the vena caval pressure is close to zero and the equation can be arranged to:

Mean aortic pressure = Cardiac Output x Total peripheral resistance

Mean Arterial Pressure

Now, the mean arterial pressure (MAP) is the average pressure within an artery over a complete cycle of one heartbeat. It is calculated according to the equation:

Mean arterial blood pressure = CO x TPR

Where CO is the cardiac output and TPR is the total peripheral resistance.

Arterial pressures are pulsatile, so systolic and diastolic pressures exist. The MAP is not simply the average of the systolic (SP) and diastolic pressures (DP). This is because for most of the cardiac cycle, the pressure in the distal arteries is closer to the diastolic pressure and reaches peak pressure only briefly, the pressure waveforms in arteries are not symmetric, so the MAP is slightly above the diastolic pressure. The rule for calculating MAP is:

MAP= (SP + 2 x DP)/3
or
MAP = DP + 1/3 Pulse Pressure

Pulmonary and Systemic Circuits:

The pulmonary circuit is a low pressure, low resistance circulation. Pulmonary resistance is much lower (<10%) than the systemic circuit. When an animal exercises, the pulmonary resistance decreases to allow an increased flow without a large increase in the pressure of the pulmonary artery.

The pulmonary blood vessels are highly compliant and thus gravity has an effect on pulmonary blood flow. Gravity increases the pressure in the more ventral blood vessels of the lungs. This increase in pressure distends the vessels, decreasing the resistance and increasing flow. This can cause an imbalance between the alveolar blood flow and ventilation as blood flow tends to be excessive in the more ventral areas of the lung. This is called a ventilation-perfusion mismatch and is particularly a problem in large animals.

Blood Pressure

Blood pressure can be measured directly and indirectly in animals. With direct measurement, a thin tube is placed in an artery to allow blood to flow through a sterile fluid filled system which is connected to an electronic monitor. This allows blood pressure to be measured beat by beat. With indirect measurement a sensor is placed on the animals artery in the paw and a blood pressure cuff is placed above the sensor. The sensor amplifies the sound of the blood moving through the artery and allows the veterinarian to determine the systolic and diastolic pressures using the blood pressure cuff.   

Factors Affecting Pulse Pressure

• increased stroke volume increases cardiac output, pulse pressure and mean arterial pressure.
• decreased HR increases the time for the blood to run into the systemic circulation, increasing pulse pressure but decreasing cardiac output (because CO=HR x SV) and mean arterial pressure. 
• With increased stroke volume and decreased heart rate, cardiac output and mean arterial pressure remain unchanged put the pulse pressure increases. This occurs with aerobic conditioning.

That's it for this post :) If you have any questions please feel free to ask in the comments section below.