20.2 Blood Flow, Blood Pressure, and Resistance

Blood Flow and Pressure

Blood flow, blood pressure, and resistance are the three variables that determine how blood moves through your cardiovascular system. Understanding how they interact explains everything from why your doctor checks your blood pressure to why standing up too fast can make you dizzy.

Types of Blood Pressure

Your blood pressure isn't a single number. It fluctuates with every heartbeat, so we describe it using four related measurements:

• Systolic pressure is the peak pressure in your arteries during ventricular contraction (systole). A normal value is around 120 mmHg.
• Diastolic pressure is the lowest pressure in your arteries during ventricular relaxation (diastole). A normal value is around 80 mmHg.
• Pulse pressure is the difference between systolic and diastolic pressure, normally about 40 mmHg. Two things determine pulse pressure: stroke volume (the amount of blood ejected per heartbeat) and arterial compliance (how well arteries can stretch and recoil). Higher stroke volume or stiffer arteries both increase pulse pressure.
• Mean arterial pressure (MAP) is the average pressure driving blood through the systemic circulation over one full cardiac cycle. It's calculated as:

$MAP = Diastolic\ Pressure + \frac{1}{3}(Pulse\ Pressure)$

With normal values, that gives you roughly 93 mmHg. Notice the formula weights diastolic pressure more heavily because the heart spends about twice as long in diastole as in systole. MAP is the single best indicator of whether tissues are receiving adequate blood flow.

Clinical Measurement of Cardiovascular Vitals

Pulse measurement is straightforward. You palpate a superficial artery (commonly the radial artery at the wrist or the carotid artery in the neck), count the beats over 15 seconds, and multiply by 4 to get beats per minute (bpm).

Blood pressure measurement uses a sphygmomanometer (blood pressure cuff) and a stethoscope. Here's the step-by-step process:

1. Place the cuff around the upper arm, about 2–3 cm above the antecubital fossa (the crease of the elbow).
2. Palpate the brachial artery on the medial side of the arm and position the stethoscope over it.
3. Inflate the cuff until the pulse is no longer heard, then go about 20–30 mmHg higher. At this point, the cuff has completely compressed the artery and no blood is flowing through.
4. Slowly deflate the cuff. The pressure at which you first hear a tapping sound is the systolic pressure. These sounds are called Korotkoff sounds, and they occur because blood is now forcing its way through the partially compressed artery in turbulent spurts.
5. Continue deflating. The pressure at which the Korotkoff sounds disappear is the diastolic pressure. At this point, the artery is fully open again and flow has returned to smooth (laminar) flow.

Factors Influencing Blood Flow and Pressure

Factors Affecting Arterial Circulation

Three major factors determine arterial blood pressure:

• Cardiac output (CO) is the volume of blood the heart pumps per minute (measured in L/min). It's the product of heart rate and stroke volume. When cardiac output increases, more blood enters the arteries per unit time, and arterial pressure rises.

• Peripheral resistance is the opposition to blood flow caused by friction between blood and vessel walls. Three variables control it:

  • Vessel diameter has the greatest effect. Vasoconstriction (narrowing) dramatically increases resistance, while vasodilation (widening) decreases it. Because resistance is inversely proportional to the fourth power of the radius, even small changes in diameter produce huge changes in resistance.
  • Blood viscosity (thickness) also matters. More viscous blood, such as in polycythemia (elevated red blood cell count), increases resistance.
  • Total vessel length contributes as well, though this doesn't change much in adults. These relationships are described by Poiseuille's Law, which states that flow is directly proportional to the pressure gradient and the fourth power of the vessel radius, and inversely proportional to viscosity and vessel length.

• Arterial compliance refers to how easily artery walls stretch and recoil. Healthy, elastic arteries absorb the surge of blood during systole and maintain pressure during diastole, smoothing out flow to tissues. When arteries stiffen (as in arteriosclerosis), they lose this buffering ability, and pulse pressure increases. This is why isolated systolic hypertension is common in older adults.

Determinants of Venous Blood Flow

Venous pressure is much lower than arterial pressure (around 10–15 mmHg or less), so veins rely on several mechanisms to return blood to the heart, especially from below the heart:

• Skeletal muscle pump: When skeletal muscles in the legs contract, they squeeze the veins running through them and push blood upward toward the heart. This is why prolonged standing without movement can cause blood to pool in the legs.
• Respiratory pump: During inhalation, pressure in the thoracic cavity drops while abdominal pressure rises. This pressure difference pulls blood from abdominal veins into the thorax and toward the heart. The effect reverses slightly during exhalation, but the net result favors venous return.
• Venous valves: One-way valves inside veins prevent blood from flowing backward. Without functioning valves, gravity would pull blood back down, which is exactly what happens in varicose veins.
• Sympathetic venoconstriction: The sympathetic nervous system can stimulate smooth muscle in vein walls to constrict. This reduces the volume of blood the veins hold (decreasing venous capacitance) and pushes more blood back to the heart, increasing venous return and therefore cardiac output.

Regulation of Blood Flow and Pressure

The body uses multiple mechanisms to keep blood pressure and tissue perfusion stable:

• The cardiac cycle of systole and diastole creates the pulsatile pressure that drives blood through the entire system. Systolic ejection generates the pressure wave; diastolic relaxation allows the ventricles to refill.
• Venous return directly affects cardiac output through the Frank-Starling mechanism: when more blood returns to the heart, the ventricles stretch more and contract more forcefully, increasing stroke volume.
• Baroreceptors are stretch-sensitive receptors located in the walls of the carotid sinuses and aortic arch. When blood pressure rises, baroreceptors fire more rapidly and signal the cardiovascular center in the medulla to decrease heart rate and cause vasodilation. When pressure drops, baroreceptor firing decreases, leading to increased heart rate and vasoconstriction. This reflex operates on a moment-to-moment basis.
• Hydrostatic pressure is the pressure exerted by a column of blood due to gravity. When you stand, hydrostatic pressure adds to blood pressure in your feet (increasing it) and subtracts from pressure above the heart. This is why dependent edema (swelling in the ankles and feet) develops after prolonged standing.
• Autoregulation is the ability of local tissues to adjust their own blood flow in response to changing metabolic needs, largely independent of systemic arterial pressure. For example, active skeletal muscle releases metabolic byproducts (like $CO_2$, lactic acid, and $H^+$) that cause local vasodilation, increasing blood flow to match demand. The brain and kidneys are especially good at autoregulating over a wide range of pressures.