Understanding Arterial Blood Pressure

• Arterial blood pressure is the force that pushes blood through the circulation to ensure adequate tissue perfusion.
• Blood pressure is the pressure of circulating blood against blood vessel walls.
• A majority of this pressure comes from the heart pumping blood.
• Blood pressure usually refers to the pressure in large arteries and is expressed as systolic over diastolic pressure.
• Systolic blood pressure is the highest pressure during the cardiac cycle and typically ranges from 100-140 mmHg.
• Diastolic blood pressure is the lowest pressure during the cardiac cycle and ranges from 60-90 mmHg.
• Pulse pressure is the difference between systolic and diastolic pressure, and is usually 40 mmHg.
• Blood pressure can be measured with a sphygmomanometer.
• Blood pressure is also measured using a stethoscope.
• The mean arterial blood pressure (ABP) is the average arterial pressure throughout the cardiac cycle.
• In the aorta and central artery, mean ABP is about 100 mmHg.
• In peripheral arteries, mean ABP is about 93 mmHg.
• Calculation for Mean BP: Diastolic + 1/3 pulse pressure= 80 + 13= 93 mmHg.

Physiological Variations in Arterial Blood Pressure

• Arterial blood pressure increases with age.
• Arterial blood pressure is higher in males than in females.
• Arterial blood pressure is higher in obese individuals compared to leaner individuals.
• Sleep decreases arterial blood pressure.
• Emotions and exercise both increase arterial blood pressure.

Determinants of Arterial Blood Pressure

• Increased cardiac output increases arterial blood pressure.
• Total Peripheral Resistance (TPR) is the resistance blood encounters in blood vessels.
• Increased peripheral resistance increases arterial blood pressure, and vice versa.
• Factors determining peripheral resistance include blood viscosity, arteriole diameter, and blood vessel length.
• Arteriole diameter is the most important mechanism of ABP regulation.

Short-Term Regulation of Blood Pressure

• Short term includes nervous mechanisms and rapidly acting hormones to regulate blood pressure.
• Nervous mechanisms include baroreceptors and chemoreceptors.
• The CNS Ischemic response regulates blood pressure in the short term.

Baroreceptors

• Baroreceptors’ stimulus is an increase in ABP.
• Arterial baroreceptors are mechanoreceptors sensitive to stretch.
• They are located in the walls of the carotid sinus and aortic arch.
• Afferent nerves include glossopharyngeal and vagus nerves.
• The cardiovascular center in the medulla oblongata is the center.
• It includes cardiac accelerator and inhibitory centers.
• Efferent signals travel through sympathetic and parasympathetic fibers.
• Inhibition of VMC leads to inhibition of sympathetic discharge, causing vasodilation of arterioles and decreased peripheral resistance.
• Venodilation reduces venous return and stroke volume.
• A decrease in rhythmicity, conductivity, excitability, and contractility of the heart can occur; this also decreases CO and HR.
• Stimulation of CIC increases vagal discharge and decreases heart rate and CO.

Arterial Chemoreflex

• The stimulus for arterial chemoreflex is decreased ABP.
• Hypotension leads to decreased PO2, increased PCO2, and increased H+ (decreased pH).
• Chemoreceptors are located in the carotid and aortic bodies.
• Afferent nerves are the IX and X cranial nerves (buffer nerves).
• The center is the cardiovascular center (VMC & CIC).
• Vagus and sympathetic nerves are efferent.
• Effector organs include blood vessels and the heart.
• The response to chemoreflex is stimulation of VMC and inhibition of CIC, resulting in arteriolar constriction, venoconstriction, and increased heart rate and contractility.
• Chemoreflex also stimulates respiration.

CNS Ischemic Response

• Stimulus for CNS ischemic response is decreased ABP.
• VMC and sympathetic activation occur in the center.
• Effector organs include the heart and blood vessels.
• The response is increased ABP.
• A decrease in ABP leads to a decrease in cerebral blood flow, causing O2 lack and CO2 accumulation.
• In severe ABP decrease the CNS ischemic response is the last line of defense against death of neurons and severe brain damage.

Rapidly Acting Hormones

• Adrenaline and noradrenaline are rapidly acting hormones.
• Vasopressin is another rapidly acting hormone.
• Renin-angiotensin system.

Adrenal Medullary Hormones (Catecholamines)

• Catecholamines are epinephrine and norepinephrine.
• They are secreted by the adrenal medulla in response to decreased ABP.
• Action is increased heart rate and cardiac contractility.
• Vasoconstriction occurs, increasing peripheral resistance.

Antidiuretic Hormone (Vasopressin)

• Secreted by the posterior lobe of the pituitary gland in response to decreased blood volume.
• Vasoconstriction occurs.
• Water reabsorption in renal tubes increases blood volume.

Renin-Angiotensin System

• Renin is secreted from the kidney in response to hypotension.

Long-Term Regulation of Blood Pressure

• Pressure diuresis and natriuresis are included in long term.
• Includes slowly acting hormones, such as: Renin-angiotensin, ANP, ADH, & Aldosterone.

Pressure Diuresis & Natriuresis

• When blood pressure rises, the kidneys excrete large quantities of sodium and water because of pressure natriuresis and pressure diuresis.
• This decreases the blood volume and consequently the arterial blood pressure.
• When blood pressure falls, the kidneys reduce the rate of sodium and water excretion, increasing the blood volume/arterial blood pressure.

Atrial Natriuretic Peptide

• ANP is secreted by the atria in response to increased ECF volume and atrial pressure.
• Relaxation of vascular smooth muscle occurs, causing vasodilation.
• TPR and ABP decrease.
• The kidney increases Na+ and water excretion.

Blood Pressure Stages

• Normal blood pressure is less than 120 systolic and less than 80 diastolic.
• Elevated blood pressure is 120-129 systolic and less than 80 diastolic.
• High blood pressure (hypertension stage 1) is 130-139 systolic OR 80-89 diastolic.
• High blood pressure (hypertension stage 2) is 140 or higher systolic OR 90 or higher diastolic.
• Hypertensive crisis is higher than 180 systolic AND/OR higher than 120 diastolic.
• Requires immediate care.
• Hypertension symptoms may include diziness, headache, heart palpitation, nosebleed, shortness of breath, anger, red face, visual problems, fatigue, insomnia, & a sore knee.
• Hypotension symptoms may include dizziness, fainting, sleepiness, palpitation, confusion, blurred vision, nausea, clammy skin, shallow breathing, fatigue & depression.