BLOOD PRESSURE & IT’S REGULATION

Prof. Dr. Ayisha Qureshi MBBS, MPhil

Department of Physiology

Learning Objectives

By the end of the lecture, you should be able to: • Define Blood Pressure (BP). • Differentiate between systolic blood pressure (SBP) and diastolic blood

pressure (DBP). • Define mean arterial pressure (MAP) and pulse pressure (PP) and give their

significance.• Calculate the MAP and PP from information provided. • Enlist the factors essential for maintenance of BP. • Explain the role of Total Peripheral resistance in BP maintenance.

Blood Pressure/ Arterial Blood Pressure Arterial blood pressure is defined as the lateral pressure exerted by the column of blood on the walls of arteries. This pressure is exerted when blood flows through the arteries. (Generally, the term ‘blood pressure’ refers to arterial blood pressure.)The unit of measurement is mmHg. Occasionally, it is measured in centimeters of water (cm H2o). (1 mmHg equals 1.36 cm H2o). Normal Blood Pressure is 120/80 mmHg (where 120 is systolic and 80 is diastolic pressure).Systolic BP develops at the peak of the left ventricular systole while Diastolic BP is manifested during left ventricular diastole.

A quick review of all the relevant concepts for Blood Pressure:

Water Faucet Analogy to understand Blood Pressure (Credits: Prof. Fink)

Water hose analogy to understand what happens to BP as you move further & further away from the faucet!

Pulse pressure is the difference between the systolic

pressure and diastolic pressure.Normal: 40 mm Hg (120 – 80 = 40).

Diastolic blood pressure is defined as the minimum pressure exerted

in the arteries during diastole of heart.Normal: 80 mmHg (Range: 60 – 80 mmHg)

Mean Arterial Pressureis the average pressure existing in the

arteries. (Remember: It is not the arithmetic mean of the systolic and diastolic pressure)

Systolic blood pressureis defined as the maximum pressure exerted

in the arteries during systole of heart.Normal: 120 mmHg (Range: 110-140 mmHg)

Arterial Blood Pressure

PULSE PRESSURE (PP)To understand the importance of Pulse Pressure, answer this:

Let’s say the blood pressure is 120/75 mmHg. It means that the pressure in the aorta was raised from a diastolic low of 75 mmHg to a systolic high of 120 mmHg. How much did your ventricles raise the aortic pressure by?

45 mmHg.

Thus, the PP represents the force created by your ventricles. And is an indication of the power of the ventricles.

In patients when the systolic pressure goes up and the diastolic pressure goes down, the PP actually increases. This is called WIDENING of the PP. E.g. 160/65

Differentiate between Pulse Pressure and Pulse.

Two major factors influence pulse pressure: 1. Stroke volume 2. Vascular compliance - Stroke volume has already been discussed. - The compliance of blood vessels decreases as arteries age. Thus, the arteries in older

people become less elastic (or arteriosclerotic). This causes the pressure in the aorta to rise more rapidly and to a greater degree during systole and to fall more rapidly to a lower diastolic value.

Thus, the most important cause of elevated pulse pressure is stiffness of the aorta, the largest artery in the body. The

stiffness may be due to fracture of the elastic fibers due to aging, leaving them less elastic (atherosclerosis) as they get

replaced by collagen fibers.

Thus, the higher the Pulse Pressure, the more stiff and damaged the vessels are thought to be, and the higher are the chances of stroke (or Cardiovascular

accident).

An increase in Pulse Pressure will result when the systolic BP rises or the diastolic BP falls!

Mean arterial Pressure (MAP)It is the average blood pressure in a person’s blood vessels during a single cardiac cycle. It is important because It has a value of app. 90 mmHg.

Normally an average is calculated by adding the 2 numbers and dividing them by 2. Not in this case.

WHY?

MAP in all ages is nearer to the Diastolic BP than to systolic b/c BP spends twice as long going down (Diastole) as it does going up

(Systole).

However, with an increase in HR, diastole comprises a smaller fraction of the cardiac cycle & MAP is more closely approximated as an average

of systolic and diastolic BP.

Why should we calculate MAP, in addition to PP?MAP is extremely important because the function of the cardiovascular center

in the medulla oblongata is to maintain the average pressure. It does not regulate the systolic high or the diastolic low.

Why is it important to regulate the MAP?

MAP is considered to be the perfusion pressure of the organs. MAP that is ≥ 60 mmHg is enough to sustain the organs of the average person. If MAP is ≤ 60 mmHg, then the organs are not being adequately perfused and

they will become ischemic.

To understand BP, we are going to study BP under the following headings:

Variations in BP

Factors maintaining BP

Factors regulating BP

Variations in Blood Pressure- Systolic pressure (under physiological variations) is subjected for variations easily and quickly and its variation occurs in a wider range. - Diastolic pressure is not subjected to easy and quick variations and its variation occurs in a narrow range.- This is because systolic pressure is regulated by the cardiac output and the diastolic pressure by the total peripheral resistance.

Normal Blood Pressure is taken as 120/80 mmHg. However, there is an acceptable range:

90/60 140/90Hypotension Hypertension

PHYSIOLOGICAL VARIATION PATHOLOGICAL VARIATION

• Hypertension• Hypotension

• Age: BP ↑ with ↑ in age.• Gender: Slightly less (about 5

mmHg) in females than in males.• Body Built: More in obese than in

lean persons.• Diurnal variation: morning to noon

to night.• Post Meals• During sleep • Emotional conditions • Exercise

There is a formula for relating the flow, pressure and resistance, thru any system of pipes:

F= ∆P/ R (Ohm’s Law)∆P = F X R

Art. BP = Cardiac Output X Total Peripheral Resistance

FACTORS MAINTAINING BLOOD PRESSURE: These factors are called the determinants of Blood Pressure. They include: 1. Cardiac Output (and thus, factors like HR and SV also effect HR)2. Total Peripheral Resistance 3. Blood Volume 4. Venous Return5. Elasticity of blood vessels6. Velocity of blood flow 7. Diameter of Blood Vessels8. Viscosity of Blood

Of all the factors mentioned, there are two main factors that maintain BP at any given time. These are:1. Cardiac Output, and 2. Total Peripheral Resistance to arterial blood flow. Thus, it is said that Blood Pressure is equal to Cardiac Output X TPR.

Water Faucet Analogy to understand CO & TPR

What factors affect Cardiac Output?

For factors maintaining Blood Pressure, we are already aware of Cardiac Output (HR X SV). What about TPR?

TPR depends on 2 factors (among many):1. Arteriolar Vasoconstriction/ vasodilation

2. Vascular stenosis (due to atherosclerotic plaque) leading to narrowing of the vessel lumen.