Blood pressure and flow are vital for our body's health. Systolic, diastolic, pulse, and mean arterial pressures all play key roles. Doctors measure these using a blood pressure cuff and stethoscope, listening for specific sounds to determine readings.

Many factors influence blood flow and pressure in our arteries and veins. Cardiac output, peripheral resistance, and vessel elasticity affect arterial circulation. For veins, muscle pumps, breathing, and valves help blood return to the heart against gravity.

Blood Flow and Pressure

Types of blood pressure

• Systolic pressure represents the highest pressure in the arteries during ventricular contraction (systole) and is normally around 120 mmHg
• Diastolic pressure represents the lowest pressure in the arteries during ventricular relaxation (diastole) and is normally around 80 mmHg
• Pulse pressure is the difference between systolic and diastolic pressures, normally around 40 mmHg, and is influenced by stroke volume (amount of blood ejected per heartbeat) and arterial compliance (ability of arteries to expand and recoil)
• Mean arterial pressure (MAP) is the average pressure in the arteries over one cardiac cycle, calculated as $MAP = Diastolic Pressure + \frac{1}{3}Pulse Pressure$, normally around 93 mmHg, and determines the rate of blood flow through the systemic circulation (blood vessels supplying the body)

Clinical measurement of cardiovascular vitals

• Pulse measurement involves palpating the radial artery at the wrist or carotid artery in the neck, counting the number of beats in 15 seconds, and multiplying by 4 to obtain beats per minute (bpm)
• Blood pressure measurement using a sphygmomanometer (blood pressure cuff) and stethoscope involves:
  1. Placing the cuff around the upper arm, 2-3 cm above the antecubital fossa (elbow pit)
  2. Palpating the brachial artery and placing the stethoscope over it
  3. Inflating the cuff until the pulse is no longer heard (20-30 mmHg above systolic pressure)
  4. Slowly deflating the cuff, noting the pressure at which the first Korotkoff sound (blood flow turbulence) is heard (systolic pressure)
  5. Continuing to deflate until the sounds disappear (diastolic pressure)

Factors Influencing Blood Flow and Pressure

Factors affecting arterial circulation

• Cardiac output, the volume of blood pumped by the heart per minute (L/min), directly influences arterial pressure; increased cardiac output leads to increased arterial pressure
• Peripheral resistance opposes blood flow due to friction between blood and vessel walls and is influenced by vessel diameter (vasoconstriction/vasodilation), blood viscosity (thickness), and total vessel length; increased resistance leads to increased arterial pressure
  • Poiseuille's law describes how these factors affect blood flow in vessels
• Elasticity and compliance of arteries refer to their ability to expand and recoil with each heartbeat, helping maintain a steady flow of blood to tissues; decreased compliance (arterial stiffening) leads to increased pulse pressure

Determinants of venous blood flow

• The skeletal muscle pump involves contraction of skeletal muscles surrounding veins, compressing them and helping push blood back to the heart against gravity (important in the legs)
• The respiratory pump takes advantage of decreased intrathoracic pressure during inhalation to facilitate venous return and increased intra-abdominal pressure during exhalation to aid venous return
• Venous valves prevent backflow of blood in veins, ensuring unidirectional flow towards the heart
• Sympathetic venoconstriction occurs when sympathetic nervous system stimulation causes veins to constrict, decreasing venous capacity (blood volume) and increasing venous return to the heart

Regulation of Blood Flow and Pressure

• The cardiac cycle, consisting of systole and diastole, influences blood flow and pressure throughout the cardiovascular system
• Venous return, the volume of blood returning to the heart, plays a crucial role in maintaining cardiac output
• Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure and initiate reflexes to maintain homeostasis
• Hydrostatic pressure affects blood flow, particularly in the lower extremities, and can contribute to edema
• Autoregulation allows tissues to maintain relatively constant blood flow despite changes in arterial pressure, ensuring adequate perfusion