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Blood Pressure Fluctuations

For medical students2 min readUpdated 2026-10-10

Blood pressure in the aorta and large arteries is not constant. Direct recording reveals complex oscillations of three orders, which depend on the phases of the cardiac cycle, respiration, and autonomic nervous system activity.

1st order wavesHigh-frequency pulse oscillations dependent on the rhythmic activity of the heart

2nd order wavesSynchronized with respiratory movements and intrapleural pressure changes

3rd order wavesSlow oscillations reflecting vasomotor center tone

Pulse pressureDifference between systolic and diastolic BP; normal range is 35–55 mmHg

Mean arterial pressureIndicator of the average energy of continuous blood flow

Types of Blood Pressure Waves

When blood pressure is recorded directly, the curve has a complex shape. It features periodic rises and falls, classified into three orders.

1. First-order waves (pulse waves) These are the most frequent oscillations, strictly tied to the cardiac cycle.

Pressure rise: Occurs during the rapid ventricular ejection phase. At this moment, blood inflow into the aorta significantly exceeds peripheral outflow, raising pressure from 80 to 120 mmHg.
Pressure drop: Occurs as blood actively flows out of the aorta into smaller arteries and the microvasculature.

2. Second-order waves (respiratory waves) These are peripheral fluctuations in both systolic and diastolic pressure. They have a lower frequency and longer duration than pulse waves because they are synchronized with the respiratory rate. Their mechanism is related to thoracic mechanics:

During inspiration, the thoracic volume increases, and pleural pressure drops to -6 mmHg. This reduces external pressure on systemic large vessels (aorta and arteries), changing their configuration, decreasing vascular resistance, and slightly lowering arterial pressure.
During expiration, pleural pressure rises to -3 mmHg. External mechanical pressure on the vessels increases, leading to an elevation of intravascular pressure.

3. Third-order waves (central waves) These are the slowest oscillations, with a frequency of only 1–3 waves per minute. On the graph, they appear as a smooth shift in the baseline upon which faster pulse and respiratory fluctuations are superimposed. These waves result from periodic changes in the tone of the vasomotor and respiratory control centers.

Key Blood Pressure Parameters

Several pressure indices are used to assess hemodynamics:

Systolic Blood Pressure (SBP): The maximum pressure recorded in the arteries during ventricular systole. When measured at the brachial artery, the normal range is 110–130 mmHg.
Diastolic Blood Pressure (DBP): The minimum value during diastole. The normal range is 70–80 mmHg.
Pulse Pressure (PP): The difference between systolic and diastolic values. On average, it equals 40–50 mmHg (acceptable normal range is 35–55 mmHg).

Physiologically, pulse pressure represents the excess of systolic pressure over diastolic pressure, which is critical for opening the semilunar aortic valves. PP is viewed as the arterial volume increment. Its magnitude equals the difference between the stroke volume ejected during rapid ejection and the volume that has drained into capillaries during the same phase.

Mean Arterial Pressure (MAP)

This pressure reflects the average energy that ensures continuous blood flow during both systole and diastole. This parameter is relatively constant for a specific vessel and the organism as a whole, reflecting the elasticity of the arterial wall.

Mean pressure depends on four main factors:

Pumping action of the heart.
Total circulating blood volume (CBV).
Blood viscosity.
Total peripheral vascular resistance (TPVR).

Calculation Features It is important to understand that MAP is not a simple arithmetic mean between systolic and diastolic pressure. To determine it accurately, one must measure the area under the BP curve and divide it by the length of this curve.

Since the duration of the systolic pressure phase is shorter than the diastolic phase in the cardiac cycle, the MAP value is always shifted closer to the diastolic pressure.

For rough estimation, the formulas are: PP = SBP - DBP MAP = DBP + (PP / 3)

Mnemonic

To remember the order of BP waves, recall the frequency of physiological processes: the heart beats most frequently (1st order waves), we breathe less often (2nd order waves), and autonomic center tone changes very slowly (3rd order waves).

Frequently asked questions

Which hemodynamic factors determine the isolated values of systolic and diastolic pressure?

Sources indicate the following hemodynamic factors influencing blood pressure:

Systolic pressure increases with rising cardiac output; its value is directly proportional to stroke volume.
Diastolic pressure depends on vascular resistance: it decreases when total peripheral vascular resistance drops and vasodilation occurs.
Pressure magnitude is inversely proportional to the compliance of the arterial bed.

What direct and indirect methods are used to measure human blood pressure?

There are invasive (direct) and non-invasive (indirect) methods for determining blood pressure.

Direct method — insertion of a needle with a manometer directly into a vessel (the most accurate).
Indirect methods — require a sphygmomanometer:
Riva-Rocci method — based on pulse palpation during cuff deflation (determines systolic pressure only).
Korotkoff method — based on arterial auscultation; listening to Korotkoff sounds allows determination of both systolic and diastolic pressure.

Why is mean arterial pressure not equal to the arithmetic mean of systolic and diastolic pressure?

Because systole lasts shorter than diastole. Consequently, blood remains under lower (diastolic) pressure longer, shifting the final mean pressure value toward the diastolic pressure.

What causes second-order blood pressure waves?

They are linked to the act of respiration. During inspiration, intrathoracic pressure drops and vascular resistance decreases. During expiration, pressure rises, vessel walls experience external compression, and arterial pressure increases.

What does pulse pressure reflect?

It reflects the arterial volume increment during the ejection phase. It is the difference between the blood volume ejected by the heart into the aorta and the volume drained into the microvasculature.

Go deeper

Mechanisms of vasomotor center regulation
Hemodynamics during the rapid ejection phase of the cardiac cycle
Effect of negative intrapleural pressure on circulation
Factors determining vascular resistance and blood viscosity
Direct methods for blood pressure recording