Physiology Chapter: Baroreceptors and Blood Pressure

Course Information

• Course Title: Physiology (0603302)
• Course Chapter: Ch.3 Cardiac Physiology
• Semester: Summer 2023/2024
• Instructor: Dr. Mohammad A. Abedal-Majed
• Institution: The University of Jordan, School of Agriculture

Cardiac Physiology Resources

• YouTube video (327): How does human circulatory system work – 3D animation (in English)
• YouTube video (328): Human Heart Anatomy And Physiology | How Human Heart works? (3D Animation)
• YouTube video (335): Circulatory System and Pathway of Blood Through the Heart

Blood Flow

• Pulmonary circulation (low pressure): poorly oxygenated blood to the lungs, then oxygenated blood to the heart
• Systemic circulation (high pressure): oxygenated blood to the tissues, then deoxygenated blood to the heart
• Blood flows through the heart in a specific pathway, involving valves, to ensure proper flow direction.

Vascular System: Functional Components

• Pump (heart): responsible for blood circulation
• Distributing & collecting tubes (arterial & venous systems, respectively)
• Exchange system (capillary beds) where gas exchange takes place

The Pumps

• Right ventricle: pumps blood through the lungs; oxygen acquisition; carbon dioxide release
• Left ventricle: pumps blood through other tissues; delivers oxygen & nutrients to tissues; obtains waste products from tissues

Distribution & Collection

• Arterial system: branches of aorta & pulmonary arteries → smaller vessels → capillaries
• Venous system: empties into vena cava & pulmonary veins → progressively larger vessels → veins → capillaries → venules → veins

Capillary Beds (Exchange)

• Blood flow through tissue capillaries is reduced
• Venous blood differs from earlier state: lower oxygen content, higher carbon dioxide content.

Cardiac Output

• CO = SV x HR
• SV: volume of blood pumped per beat (average 70 ml/beat)
• HR: heart beats per minute (average 70 beats/min)
• Resting CO ≈ 5 L/min
• Exercise CO can increase to 20-25 L/min

Heart Valves

• Atrioventricular (AV) valves:
  • Left: mitral valve
  • Right: tricuspid valve
• Semilunar valves
  • Left: aortic valve
  • Right: pulmonic valve

Electrical Activity of Cardiac Muscle Cells

• Specialized muscle cells in the SA node spontaneously depolarize, generating an action potential
• Electrical coupling via gap junctions propagate the action potential throughout the heart
• Pacemaker cells depolarize without neural stimulation, setting the basal heart rate modified by the sympathetic & parasympathetic NS
• Motor neurons regulate the pace of depolarization.

Cardiac Action Potentials

• APs are relatively long (100-250 msec) compared to skeletal (1-2 msec) APs.
• Voltage-gated Ca2+ channels are crucial for extending APs.
• Phases of cardiac AP include depolarization (phase 0), initial repolarization (phase 1), repolarization plateau (phase 2), and final repolarization (phase 3), ultimately leading to resting membrane potential (phase 4).

Cardiac Cycle

• Contraction of cardiac muscle increments ventricular chamber pressure → opening of SL valves & closing of AV valves
• Relaxation of cardiac muscle lowers ventricular chamber pressure opening of AV valves & closing of SL valves
• end of diastole & beginning of systole via closing of AV valves
• end of systole & beginning of diastole via closing of semilunar valves

Blood Pressure

• Circulation is pressure-driven blood flow for transporting substances throughout the body.
• Pressures below the heart are additive.
• Pressures above the heart are subtractive.
• Blood pressure is sufficient to reach the brain.

Systemic Circulation Resistance

• Resistance is opposition to blood flow in a vascular bed.
• Arterioles have the greatest resistance, as well as the highest compliance.
• Veins are a storage site for blood volume and can accommodate large pressures.
• Vascular resistance is determined by arteriolar diameter.

Total Peripheral Resistance and Blood Pressure

• TPR = (aortic pressure - vena cava pressure)/cardiac output
• TPR ~ aortic pressure/cardiac output
• If TPR increases, aortic pressure and/or cardiac output also increase.
• arteriolar diameter influences vascular resistance significantly
• Blood flow depends on pressure gradient & vascular resistance

Clinical Applications of ECG

• ECG records electrical activity in the heart.
• P-wave: atrial depolarization
• QRS complex: ventricular depolarization.
• T-wave: ventricular repolarization.
• P-R interval: time for impulses to reach ventricles via AV nodes.
• QRS duration: ventricular depolarization duration -Q-T interval: ventricular depolarization & repolarization

Blood Pressure Regulation

• Sympathetic nervous system: increase in heart rate, contractile strength, vasoconstriction; Norepinephrine (NE) is the neurotransmitter.
• Parasympathetic nervous system: decrease in heart rate, vasoconstriction; Acetylcholine (ACh) is the neurotransmitter.
• Baroreceptors in the aortic arch and carotid sinus sense changes in blood pressure and send signals to the cardiovascular center.
• Atrial volume receptors indirectly sense changes in blood volume and signals to the CNS.

Baroreceptor & Volume Reflexes

• Baroreceptors sense changes, leading to adjustments in sympathetic & parasympathetic activity, ultimately regulating cardiac output & peripheral resistance.
• Atrial volume receptors regulate blood volume, stimulating thirst and altering kidney function when blood volume is low or high.

Blood Pressure Regulation with Kidneys

• Kidneys play significant roles in blood pressure regulation through releasing renin & activating the renin-angiotensin-aldosterone system (RAAS).
• Atrial natriuretic peptide (ANP), and Brain natriuretic peptides (BNP) are released by the heart in response to increased blood volume and pressure, directly lowering blood pressure.

Myocardial Ischemia

• Reduced blood flow to the heart (myocardium), commonly caused by plaques.
• Symptoms include chest discomfort, tightness, pain, heaviness, pressure, or aching.
• Diagnostic tool: blood troponin level measurements

Myocarditis

• Inflammation of the heart muscle (myocardium), impacting its ability to pump blood effectively.

Plasma Lipoproteins

• Five classes (based on density): chylomicrons, VLDL, IDL, LDL, HDL.
• Transport different lipids (cholesterol, triglycerides) throughout the body.

Dyslipidemia

• Abnormalities in lipoprotein metabolism.
• Hypercholesterolemia: high blood cholesterol levels, frequently resulting from abnormal LDL receptor expression, reduced LDL clearance, and excessive VLDL production.

Heart Failure

• Inability of the heart to pump blood efficiently, often due to weakened or thickened heart muscle.
• Types: systolic (contraction problems) & diastolic (relaxation problems).
• Treatment: Medications such as ACE Inhibitors.

Disorders of Cardiac Conduction System

• Irregularities in the initiation or conduction of electrical impulses within the heart can lead Arrhythmia (including bradycardia, tachycardia, atrial fibrillation).

Test your knowledge about the role of baroreceptors in blood pressure regulation and their reflexes. This quiz covers various aspects, including the adaptation to blood pressure changes, the dynamics of cardiac muscle contraction, and the relationship between atrial and ventricular pressures. Dive into the intricate physiology that keeps our circulatory system functioning smoothly.