Cardiovascular System Dynamics

• Blood pressure is generated by the contraction (systole) of the heart's ventricles and is essential for pushing blood around the body
• Blood pressure decreases progressively as blood flows through arteries, arterioles, capillaries, venules, and veins
• Pressure in the arteries varies during the cardiac cycle
• Ventricles contract (systole), pushing blood into the arterial system, then relax to fill with blood again
• Arterial blood pressure is highest immediately after the ventricle contracts (systolic pressure) and lowest when the ventricle relaxes (diastolic pressure)

Pulse Pressure

• Difference between systolic and diastolic pressure in the arteries
• Example: 120 mmHg systolic – 80 mmHg diastolic = 40 mmHg pulse pressure
• The difference between systolic and diastolic pressures decreases as vessel size decreases
• Small arterioles and capillaries lack systolic or diastolic pressure, having just one blood pressure
• Left ventricular systolic pressure is higher than systolic arterial pressure
• Ventricular diastolic pressure is almost zero (0 mmHg)
• Aortic valve shuts and traps blood under pressure in the aorta and arteries, preventing diastolic pressure in the arteries from dropping to zero

Historical Determination of Blood Pressure

• Systolic and diastolic pressures could be measured by inserting a small catheter into an artery connected to a pressure gauge, but this direct measurement method is invasive, inconvenient, and impractical
• Rev. Stephen Hales used this method to measure blood pressure in a horse in 1714
• The direct method helps in understanding blood pressure and its measurement units
• Pressure can be measured as the height of liquid it can push against gravity

Blood Pressure Units

• Blood and water have similar densities, so blood pressure can be measured in units of centimeters of water, inches of water, or feet of water
• Horse's systolic arterial blood pressure: ~180 cm (6 feet or 72 inches) of water
• Height blood would spurt from a severed carotid artery without being caught in a glass tube
• In humans, blood would spurt to ~163 cm (5 feet 4 inches) during systole and fall to ~109 cm (3 feet 7 inches) during diastole

The Use of Mercury

• Mercury is 13.5 times heavier than water or blood for the same volume
• If Hales had filled his tube with mercury, the blood pressure of the horse would only have pushed the mercury up the tube to one thirteenth of the height: 12 cmHg or 120 mmHg (Hg is the chemical symbol for mercury)
• Pressure units in mmHg avoid using a water-filled sphygmomanometer 13.5 times taller

Blood Pressure Measurements Today

• Arterial blood pressure is usually measured indirectly
• Venous blood pressure can be directly measured in intensive care patients via a plastic tube filled with saline connected to the vein, called central venous pressure (CVP)
• Normal CVP: around 8-15 cmH2O or 6–11 mmHg
• Arterial pressure can be estimated with good accuracy using noninvasive, indirect methods
• Auscultatory method: Using a stethoscope to listen to heart sounds with a blood pressure cuff connected to a mercury sphygmomanometer to measure cuff pressure
• Cuff pressure that stops blood flow is the systolic pressure in the artery

Indirect Blood Pressure Measurements:

• Cuff is placed on the upper arm and inflated to stop arterial blood flow from the brachial artery, collapsing the artery when the cuff pressure exceeds the artery's pressure
• The disappearance of the pulse below the cuff helps approximate the systolic pressure by measuring the pressure in the cuff
• Cuff pressure is slowly released
• When the pressure begins to fall below the systolic pressure, blood flows only intermittently into the arm; this turbulent flow creates tapping sounds called Korotkoff sounds that can be heard through the stethoscope, approximating systolic pressure
• The cuff pressure continues to decrease and the artery regains its normal diameter, flow streamlined, and sounds muffled then disappear
• Cuff pressure at the point of the muffling sound approximates diastolic pressure, but the disappearance of sound is easier to detect
• The disappearance of sound commonly determines diastolic pressure
• Korotkoff sounds can be heard at pressures below the true diastolic pressure in some healthy people, making it impossible to accurately define diastolic pressure for them

Describing Blood Pressure

• Blood pressure is conventionally measured at heart level
• Blood pressure is measured in the brachial artery with the upper arm by the side
• Peak systolic pressure and minimum diastolic pressure are written as systolic/diastolic (e.g., 120/80) with the units mmHg to reflect use early sphygmomanometers

Normal Blood Pressure Values

• There is no "normal" value for blood pressure, with a normal distribution of blood pressures
• Most healthy people's pressures range between 100/60–140/90 mmHg in a "one-off" measurement
• Individuals can have differences of 5–10 mmHg between the pressures in the two arms

Blood Pressure and Blood Flow Relationship

• Each heartbeat ejects blood at a sufficient pressure to provide oxygen and nutrients to tissues, and remove waste products
• Blood flow is proportional to blood pressure
• Narrower tubes provide more resistance to flow
• Blood flows through arteries, arterioles, capillaries, and then back to the heart through the venules and veins; these vessels provide resistance to the flow
• Arterioles contribute the most to vascular peripheral resistance
• Resistance can be increased by vasoconstriction and lowered by vasodilatation

Peripheral Circulation

• Blood leaves the arterial system continuously through the capillaries but enters intermittently from the heart
• Ventricles contract during systole, the semilunar valves open, and blood flows into the arterial system, stretching the arteries and increasing the blood pressure
• "Systolic pressure" is defined as the peak pressure reached during the cardiac cycle
• The period of relaxation of the ventricles is called "diastole," during which the ventricles fill with blood returning from the veins and blood continues to flow out of the arterial system into the capillaries from recoil of major arteries
• "Diastolic pressure" occurs when the arterial blood pressure decreases to its lowest, immediately before the contracting ventricle pushes blood into the arteries again.

Arterial Blood Pressure Determinants

• Arterial blood pressure is the product of cardiac output (CO) and peripheral resistance (PR)
• BP = CO × PR
• CO is determined by heart rate (HR) and stroke volume (SV)
• BP = HR × SV × PR
• Any alterations may affect arterial blood pressure

Regulation of Arterial Blood Pressure

• The heart and blood vessels regulate blood pressure by altering cardiac output and peripheral resistance
• Arterial blood pressure is monitored by pressure receptors in the aortic arch and the carotid sinus which detect the degree of stretch of the arterial wall
• Acute changes in arterial blood pressure result in compensatory changes to return blood pressure to the normal range
• Acute blood loss decreases arterial blood pressure, detected by baroreceptors
• Baroreceptors activate the cardiovascular control centers in the brain, stimulating the autonomic nerves to vasoconstrict peripheral blood vessels and increase cardiac output to maintain blood flow to the brain and vital organs
• An acute increase in blood volume will increase arterial blood pressure, causing peripheral vasodilation and a decreased heart rate.

Position Effects on Measuring Arterial Blood Pressure

• Convention is to reference all arterial blood pressure measurements to the position of the heart
• Measuring the pressure in an artery that is below heart level increases pressure due to the effect of gravity on the blood column in the vessels, contributing to hydrostatic pressure
• Measuring blood pressure in a femoral artery in the thigh with the person lying down, has no extra pressure contributed by gravity
• Height of blood below the heat contributes around an additional 50 mmHg to the pressure when measuring blood pressure in the femoral artery with the person sitting or standing
• Hydrostatic pressure affects all fluids at the same level, keeping the pressure difference between interstitial fluid and adjacent capillaries the same whether a person is standing or lying down
• Increased pressure causes blood to pool in the veins, decreasing venous return, which is counteracted by valves in the veins and skeletal muscle contraction

Effect of Cuff Size on Arterial Blood Pressure.

• Ssphygmomanometer measurements require artery compressed and pressure in transferred accurately to wall.
• Essential use right sized cuff
• Values high if cuff too narrow
• Values too low if cuff too wide

Adult Cuff Sizes:

• Arm circumference 22 to 26 cm use "small adult" cuff, 12 x 22 cm
• Arm circumference 27 to 34 cm use "adult" cuff, 16 x 30 cm
• Arm circumference 35 to 44 cm use "large adult" cuff, 16 x 36 cm
• Arm circumference 45 to 52 cm use "adult thigh" cuff, 16 x 42 cm
• Smaller cuffs are required for measuring blood pressure in children

Clinical Significance

• It is essential to know blood pressure for correct care and it is routinely measured as apart of a physical
• Transfusions should be done with rising pressure.
• Management effective for hyperextention
• Health pros need to be accurate.

Hypertension

• Increased arterial is often clinical especially if 140/90 mmHg is chronic condition
• 30% adults get
• Take repeat measurements and measure over 24 hours better.
• Depends on rest, position, cuff size
• Country viewpoints different
• Someone with 141/91 mmHg similar to 139/89 mmHg

Essential vs Secondary Type

• 90% adults have it with no cause, comes with age
• Essential type often with lack of exercise, obesity, family etc
• 10% people identify cause secondary hypertension
• Chornically raised blood pressure with steroid production like Cushing's syndrome

Adrenal Steroids

• Affect glucose protein and salt excretion
• Increased production of cortical steroids cause reabsorption by kidneys
• This leads to fluid retention, and raises pressure, which also raises stroke volume.

Epinephrine (Adrenaline)

• Increases pressure via vasoconstriction
• Increases rate and cardiac output

Chronic Renal Disease

• Poor prefusion and retention of fluids often result
• Increases angiotensin II levels
• Renin is enzyme secretes and breaks down angiotensinogen to Angiotensin I
• I is converted to II by enzyme
• These inhibitors and antagonists treat hypertension

Explore the factors influencing blood pressure, flow, and resistance in the circulatory system. Understand systolic vs. diastolic pressure, and regulation of oxygen and nutrient delivery to tissues. Learn about blood pressure dynamics.