Systemic Blood Pressure (SBP)

Definition and formula

• Systemic blood pressure is the pressure exerted by circulating blood on the walls of the systemic arterial circulation.
• From the hydraulic analogue of Ohm law: Pa − Pv = Q × R.

For the systemic circulation: MAP − CVP = CO × SVR.

As CVP is usually small: MAP ≈ CO × SVR.

Determinants of systemic blood pressure

1. Cardiac output

CO = HR × SV

• Heart rate: increasing HR generally increases CO and MAP; excessive tachycardia reduces diastolic filling and may reduce SV and CO.
• Stroke volume: determined by preload, contractility and afterload.
  • Increased preload → Frank-Starling mechanism → increased SV → increased CO → increased BP.
  • Increased contractility → decreased ESV → increased SV → increased CO → increased BP.
  • Acute increased afterload tends to decrease SV and CO.

2. Arterial blood volume

• For a given arterial compliance, increased arterial blood volume increases arterial pressure.
• Decreased arterial blood volume, e.g. haemorrhage, decreases arterial pressure.
• Arterial volume reflects total circulating volume, cardiac inflow into the arterial compartment and runoff into the peripheral circulation.

3. Arterial elastance / compliance

C = ΔV / ΔP

• Decreased arterial compliance / increased elastance means a given stroke volume produces a larger pressure rise.
• Increased compliance buffers systolic ejection and reduces the pressure rise.
• Particularly important in determining systolic and pulse pressure.

4. Peripheral arterial resistance

R = 8ηL / πr⁴

• Most variable resistance resides in small arteries and arterioles.
• Radius: most important determinant; decreased radius markedly increases resistance and MAP, while vasodilation lowers resistance and MAP.
• Blood viscosity: increased viscosity increases resistance.
• Vessel length: increased length increases resistance; relatively fixed acutely.
• Arteriolar radius is influenced by sympathetic tone, circulating hormones and local metabolic/autoregulatory factors.

5. Systolic, diastolic and pulse pressure

• Systolic pressure: influenced predominantly by stroke volume, rate of ventricular ejection and large-artery compliance.
• Diastolic pressure: influenced predominantly by SVR, heart rate / duration of diastole and elastic recoil of large arteries.
• Pulse pressure: increases with increased stroke volume or decreased arterial compliance.

Summary: systemic arterial pressure reflects the interaction between cardiac output, arterial blood volume, arterial elastance/compliance and peripheral arterial resistance.