CVS Physiology Lecture # 9 Study Notes: Blood Pressures

This lecture is about various blood pressures relating to the cardiovascular system and the different factors which affect them.

STROKE VOLUME (SV): Stroke volume is the amount of blood ejected through the left ventricle during one systole. Therefore stroke volume is the difference between the amount of blood in the ventricles prior to ejection (end-diastolic volume) and the amount of blood that remains in the ventricles after the ventricular contraction or systole (end-systolic volume). Normally the stroke volume in a healthy adult is roughly 70mL.

• Stroke volume (SV) = EDV – ESV

TOTAL PERIPHERAL RESISTANCE (TPR):Total resistance offered by systemic arteries to the blood flow across them is referred to as TPR. TPR is responsible for maintaining the diastolic blood pressure. The major contribution to the TPR is provided by the systemic arterioles.

By the time the blood reaches the systemic arterioles, its pressure has dropped to 50 mm of Hg in overcoming the vascular resistance encountered up till now. By the time blood flows across the arterioles, the blood pressure further drops to 20 mm of Hg. This means that a 30 mm of Hg of blood pressure is required to overcome the resistance of the systemic arteries in order for the blood to flow across them. The sympathetic and parasympathetic stimulation can decrease or increase the diameter of systemic arterioles and that will affect the resistance offered by these systemic arterioles.

Hence, it is safe to assume that systemic arterioles are the functional sphincters of the cardiovascular system and the primary contributor to the TPR.

COMPLIANCE refers to the distensibility of the blood vessels when it’s accommodating a blood volume. Elasticity of a blood vessel refers to the recoil of its wall which allows the forward flow of the blood. Both compliance and elasticity come as a result of the elastic fibers within the walls blood vessels (especially arteries). Compliance of a blood vessel reduces with age and some pathological conditions such as atherosclerosis.

Compliance of a blood vessel is defined as the blood volume which it can hold at a given pressure. Compliance can be calculated by the following equation:

• Compliance, C = V/P

V = Volume (mL)

P = Pressure (mm of Hg)

SYSTOLIC BLOOD PRESSURE (SBP): It’s defined as the peak pressure measured in a systemic artery during the cardiac cycle. This peak pressure coincides with ventricular systole phase of the cardiac cycle. In a normal healthy individual the SBP is measured to be 120 mm of Hg.

Stroke volume is directly proportional to the SBP and it’s also the biggest determinant of the SBP. An increase in the stroke volume causes greater distensibility of systemic arteries and hence greater recoil, thereby resulting in a greater pressure exerted on the walls of systemic arteries.

Heart rate has an inversely proportional relationship with the SBP. Reduced heart rate results in an increased filling time and hence a greater preload in the left ventricle. Increased preload places an increased stretch on the left ventricular myocardium. This increased myocardial stretch develops a greater force of left ventricular contraction and consequently the stroke volume is increased. Increased stroke volume as explained earlier, causes an increase in SBP.

Compliance refers to how easily a vessel tends to distend. If the vessel has a low distensibility, then more pressure is required for the blood to enter it and hence the SBP is measured to be high. On the other hand, reduced compliance results in a decreased recoil ability of the vessel. As a result, the vessel wall is unable to squeeze hard on the blood. This reduces the diastolic blood pressure. So a decrease in compliance (as in arteriosclerosis) results in an increased SBP and a decreased DBP.

DIASTOLIC BLOOD PRESSURE (DBP): It’s defined as the lowest pressure measured in a systemic artery during the cardiac cycle. This lowest pressure coincides with ventricular diastole phase of the cardiac cycle. In a normal healthy individual the DBP is measured to be 80 mm of Hg.

The most important factor which determines the DBP is the total peripheral resistance (TPR). DBP is increased if a greater volume of blood remains in the systemic arteries at the end of diastole. However, it is the TPR which resists flow of blood through a vessel; longer the blood stays within a vessel the more pressure it exerts on the vessel wall. So a decrease in the TPR allows more blood to flow across the vessel and the DBP is low. Since TPR is majorly contributed by the arterioles, it’s better to understand this concept at the arteriolar level. Dilation of the arterioles decreases TPR and DBP also decreases. Constriction of the arterioles increase TPR and as a result the DBP also increases.

PULSE PRESSURE (PP): It’s the difference between the SBP and the DBP. Various conditions may result in widening or narrowing of the PP:

A decrease in vessel compliance requires a greater SBP to allow for blood flow across the vessel. Decreased compliance also results in reduced recoil of the vessel which reduces the DBP. Overall decreased vessel compliance results in an increased gap between SBP and the DBP, so the PP widens. Such a situation can arise during arteriosclerosis which decreases the overall compliance of systemic arteries.

A reduced stroke volume causes a decrease in the SBP, however the DBP remains unchanged. This causes narrowing of the gap in between the SBP and DBP, effectively reducing the PP.

MEAN ARTERIAL PRESSURE (MAP): MAP is the average pressure within a blood vessel during one cardiac cycle. It’s not the arithmetic mean because the time spent in diastole and systole isn’t equal, so MAP averaged out over the time spent during each phase. One third of the time of a single cardiac cycle is spent in systole, while two thirds of the cardiac cycle is spent in diastole. MAP is used instead of SBP to determine whether there’s enough nourishing pressure to allow perfusion across the tissues. Normally if the MAP is less than 60% of the SBP, then the nourishment of the systemic tissues is compromised. MAP can be determined by the following equation:

• MAP = (CO x TPR) + CVP {CVP=0, so it can be ignored}

So, MAP = (CO x TPR)

In physiological conditions, the MAP needs to be maintained. Therefore, if there’s an increase in the cardiac output, then the TPR needs to be reduced to ensure the MAP doesn’t change. In contrast if the TPR increases the cardiac output needs to be reduced to maintain the MAP.

If the SBP and the DBP are known then the MAP can be determined by the following formulae:

• MAP = DBP + 1/3 (SBP – DBP) {For a normal healthy adult, DBP = 120 & SBP = 80}

So, MAP = 80 + 1/3 (120 – 80)

MAP = 80 + 1/3 (40)

MAP = 80 + 13.33

MAP = 93 mm of Hg (After rounding off)

Alternatively MAP can also be calculated by the following formula:

• MAP= 2/3 DBP + 1/3 SBP

MAP= (2/3 x 80) + (1/3 x 120)

MAP= 53.33 + 40 = 93 mm of Hg