Physiology of Blood Pressure

Physiology and Anatomy of Systems

Blood Pressure - A Brief Review

• Blood pressure (BP) refers to the pressure in the large arteries, oscillating with the cardiac cycle
• BP is typically measured at the level of the heart in one arm using a sphygmomanometer
• Normal systolic and diastolic pressures are approximately 120/80 mmHg
• Mean BP is calculated as the time-weighted average of the systolic and diastolic blood pressures or 2/3 x DBP + 1/3 x SBP

Mechanisms Governing Pressure and Flow Waves in the Arteries

• Blood from the heart hits the aorta, causing pressure and flow waves propagated down the vascular system
• The pressure wave becomes larger as it moves down the arterial tree due to greater arterial stiffness
• Flow is progressively smoothed out as blood moves into the arterioles and the microcirculation
• Blood flow is pulsatile in the aorta and arteries, with pulse and pressure waves moving at approximately 5 m/s
• Blood cells move at approximately 32 cm/s, with an average aortic length of approximately 33 cm

Acute Regulation of Blood Pressure: Baroreceptor Reflex

• Baroreceptors contain fine nerve endings sensitive to stretch (mechanoreceptors)
• Decreased pressure causes decreased firing, most sensitive when mean BP is between 80-150 mmHg
• Sensitivity also increased by a large pulse pressure, with receptors showing adaptation
• Baroreceptors respond rapidly to changes in MAP and pulse pressure in the short-term
• The baroreflex responds rapidly to changes in MAP and pulse pressure, communicating via sympathetic/parasympathetic NS

Determinants of Mean Arterial Blood Pressure

• Baroreceptor reflex
• Arterial tone
• Total peripheral resistance
• Cardiac output (CO x TPR = BP)
• Heart rate and contractility (i.e., heart)
• Venous return (i.e., veins)
• Blood volume (i.e., kidneys)

Renin-Angiotensin-Aldosterone System (RAAS)

• RAAS activates vasoconstriction, increasing total peripheral resistance
• RAAS increases blood volume through sodium retention and potassium excretion
• RAAS also stimulates the release of aldosterone, promoting sodium reabsorption and potassium excretion in the kidneys

Regulation of Mean Arterial Blood Pressure by RAAS

• Various stimuli activate the RAAS, leading to increased renin release from juxtaglomerular cells
• Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE)
• Angiotensin II stimulates vasoconstriction, aldosterone release, and sodium reabsorption in the kidneys

Long-term Determinants of Mean Arterial Blood Pressure

• Stable body sodium content
• Stable extracellular fluid volume
• Stable plasma/blood volume
• Pressure natriuresis

Pressure Natriuresis

• Increased renal perfusion and medullary blood flow decrease angiotensin II production
• Increased nitric oxide, prostaglandins, and renal kinins stimulate natriuresis and diuresis
• Pressure natriuresis increases renal interstitial hydrostatic pressure, decreasing tubular sodium reabsorption and increasing sodium excretion

Long-term Regulation of Arterial Blood Pressure

• Stabilization of BP in the long term is mainly due to maintenance of a constant ECF volume
• ECF volume is controlled by the sodium concentration of the ECF
• Diseases of civilization, such as hypertension and type 2 diabetes, may be explained by the thrifty genotype hypothesis
• Human evolution has programmed us to crave and conserve nutritional resources, contributing to the development of hypertension and type 2 diabetes.