Cardiac Physiology and Rhythmicity Quiz

Cardiovascular Physiology

• The cardiovascular system comprises the heart and a closed system of blood vessels filled with blood.
• The heart's pumping action facilitates continuous blood circulation and exchange between blood and interstitial fluid.

Four Major Functions of the Cardiovascular System

• Transporting nutrients, gases, and waste products throughout the body
• Protecting the body from blood loss
• Maintaining a constant body temperature (thermoregulation)
• Maintaining fluid balance within the body

Properties of Cardiac Muscle

• Rhythmicity: The ability of cardiac muscle to beat regularly and spontaneously. This is an inherent property, not reliant on the nervous system.
• Contractility: The ability of the cardiac muscle to contract and pump blood. This is an all-or-none response; the whole heart contracts maximally or not at all.
• Excitability: The capacity of the cardiac muscle to respond to stimuli. This involves the process of depolarizing membranes. There are different phases that influence excitability, including the Absolute Refractory Period (ARP), Relative Refractory Period (RRP), and Supernormal phase.
• Conductivity: The ability of the cardiac muscle to transmit impulses from one part to another. Conduction pathways include the Sinoatrial (SA) node, intermodal pathways, Atrioventricular (AV) node, AV bundle, and the bundle branches, and Purkinje fibers.

Rhythmicity of Different Heart Parts

• Sinoatrial (SA) node: 120 beats/minute
• Atrioventricular (AV) node: 100 beats/minute
• Ventricular: 35-40 beats/minute

Factors Affecting Rhythmicity

• Nervous factors:
  • Sympathetic system: secretes adrenaline increasing rhythmicity
  • Parasympathetic system: secretes acetylcholine decreasing rhythmicity
• Ions and pH:
  • Excess Na ions decrease rhythmicity
  • Excess K favors diastole and stops the heart in diastole
  • Excess Ca favors systole and stops the heart in systole
  • Acidic conditions favor diastole
  • Alkaline conditions favor systole
• Oxygen supply: Absence of oxygen (O2) stops the heart after a few beats.
• Drugs: Adrenaline increases rhythmicity, while acetylcholine decreases it.
• Temperature: Moderate temperature increase increases rhythmicity. Moderate temperature decrease decreases rhythmicity. Extreme warming/cooling stops rhythmicity.

Contractility

• The ability of the cardiac muscle to contract and push blood into the circulation.
• The heart contracts fully or not at all.

Factors Affecting Contractility

• Same factors influencing rhythmicity
• Starling's Law: The force of cardiac contraction is directly proportional to the initial length of the cardiac muscle, providing other factors remain constant. This allows the heart to adjust its pumping capability to accommodate different blood volumes entering the heart.

Excitability:

• The period during which the excitability is lost (Absolute Refractory Period (ARP)) and the heart cannot respond to stimuli. The membrane is depolarized in this period.

Relative Refractory Period (RRP):

• Period of recovery before the heart is excitable again
• Strong stimulation can produce weak contractions and is known as extrasystole

Supernormal Phase:

• The ability of the cardiac muscle is greater than normal.
• Weak stimuli can produce strong contractions.

Conductivity

• The ability of the heart to transmit electrical impulses through various structures. Conduction structures include the SA node, intermodal pathways, AV node, AV bundle, bundle branches, and Purkinje fibers.

Cardiac Innervations

• The heart's activity is regulated to meet the body's demands.
• The autonomic nervous system is vitally important.
  • Sympathetic system: secretes adrenaline, increasing rhythmicity.
  • Parasympathetic system: secretes acetylcholine, decreasing rhythmicity.

Heart Rate

• Normal heart rate ranges from 60-100 beats per minute.
• Average heart rate is 70 beats per minute in resting adult males.
• Variation exists with age (faster in infancy), sex (faster in females), and physical training (slower in athletes).

Factors Regulating Heart Rate

• Nervous Regulation:

  • Impulses from the circulatory system (baroreceptors for example). Mary's Law: heart rate inversely proportional to arterial pressure
  • Impulses from respiratory center: Heart rate increases during inspiration and slows during expiration. This is called respiratory sinus arrhythmia.
  • Impulses from higher centers (cerebral cortex, hypothalamus): Conditioned reflexes, emotions

• Humoral Regulation:

  • Adrenaline: increases heart rate
  • Noradrenaline: decreases heart rate (generalized vasoconstriction and elevation in arterial blood pressure).
  • Thyroxine: accelerates heart rate and increases metabolism in the heart

• Physical Regulation: -Body temperature: A rise in temperature increases heart rate, while a drop decreases it. An increase of 1°C in temperature leads to approximately 15 more beats per minute. Extreme temperatures can decrease heart rate.

Cardiac Output

• Stroke volume (SV): The amount of blood pumped out by each ventricle per beat (70 ml/beat).
• Cardiac output (CO): The amount of blood pumped out by each ventricle per minute. Usually equal in both ventricles (70 mL/beat x 70 beats/minute = 4900 ml/min).

Factors Controlling Cardiac Output

• Factors determining Stroke Volume (SV)
  • Venous return: blood coming back into the heart
  • Arterial blood pressure
  • Contractility

Factors Affecting Venous Return

• Pressure gradient: Pressure difference between capillaries (15 mmHg) and veins (0 mmHg) drives blood flow toward the heart

  • Respiratory Muscle: Inspiration increases venous return owing to increased negative intrapleural pressure
  • Skeletal Muscle Pump: Contraction of skeletal muscles compresses veins, increasing venous return.
  • Arterial Diameter: Dilation of arterioles increases blood flow to veins, enhancing venous return and Cardiac Output
  • Capillary tone: Widely dilated capillaries offer more volume to blood, decreasing venous return and cardiac output
  • Venous tone: Excess vasodilation results in pooling of blood in the venous system, decreasing Venous Return.

• Arterial pulsation: Vein running parallel with artery helps blood flow toward the heart

• Spleen and blood reservoirs: Contraction of the spleen increases venous return.

• Contractility: Increase in contraction force increases the Cardiac output

• Arterial Blood Pressure (ABP): Does not influence cardiac output in healthy hearts during moderate changes

• Heart Rate (HR): Moderate changes don't significantly affect cardiac output (CO) because stroke volume (SV) often compensates. However, excessive changes (abnormal/pathological) reduce cardiac output.

Arterial Blood Pressure (ABP)

• Definition: The pressure exerted by blood on the arterial walls.
• Systolic pressure: The highest pressure during cardiac contraction.
• Diastolic pressure: The lowest pressure during cardiac relaxation. Normal blood pressure in healthy adults is typically 120/80 mmHg (systolic/diastolic).

Physiological Variations Affecting ABP

• Age: ABP is low at birth (50/30 mmHg), increases gradually (e.g. 90/60 and 120/80), and slightly rises further.

• Sex: In children, ABP is similar between genders, and in adulthood, females tend to have slightly lower ABP than males. At menopause, ABP usually increases in females.

• Body build: Obese individuals generally have higher ABP than those with a normal build.

• Gravity: Pressure is higher in vessels below the heart and lower above the heart.

• Meals: Digestion increases ABP by 5-10 mmHg due to splenic contraction.

• Circadian rhythm: ABP exhibits cyclical changes throughout the day, lowest in the early morning and highest in the afternoon.

• Emotions and muscular exercise: Emotions and vigorous exercise significantly elevate ABP (e.g. up to 180 mmHg during exertion).

• Sleep: Deep sleep typically lowers ABP.

• Pregnancy: ABP typically drops during normal pregnancy.

• Physical factors: Cold exposure constricts blood vessels increasing ABP, while heat exposure dilates vessels decreasing ABP. Respiratory phases alter ABP slightly.

Factors Maintaining Arterial Blood Pressure

• Peripheral Resistance (PR): The resistance the blood encounters in arteries and capillaries (influenced by vascular diameter & blood viscosity).
• Cardiac output: Directly proportional to the Arterial Blood Pressure (ABP)
• Arterial elasticity: Normal arterial elasticity prevents extreme fluctuations in blood pressure during systole and diastole.
• Blood volume: Moderate changes in blood volume are compensated for by changes in heart rate (HR) and capillary capacity, but severe changes (e.g. hemorrhage) significantly impact ABP.

Hemorrhage

• Definition: Loss of blood from the cardiovascular system.

• Effects: Depends on the amount and speed of blood loss.

  • A loss of less than 30% of blood volume usually allows the body to compensate, but severe/rapid loss can trigger death without prompt intervention (e.g., transfusion).

• Compensatory mechanisms: Occur immediately in response to blood loss and aim to restore blood pressure to within normal levels and include mechanisms like blood clotting and cardiovascular adjustments.

• Long-term mechanisms: Begin after 12-72 hours to restore blood volume and plasma proteins, including water/electrolyte reabsorption, new protein synthesis, and red blood cell restoration.

Test your knowledge on cardiac muscle properties and rhythmicity. This quiz covers various factors affecting heart function, including the role of the sympathetic nervous system and the effects of ions and temperature on cardiac rhythms. Perfect for students studying cardiac physiology.