Cardiac Cycle, ATP & Terms

Contractile Cell Refractory Period

• Lasts approximately 250 msec.
• Almost as long as the duration of contraction, which is 300 msec.

ATP Production in Cardiac Muscle

• Relies mainly on aerobic respiration at rest.
• Utilizes fatty acids (60%) and glucose (35%) for ATP production.
• Contains many mitochondria and obtains oxygen from the blood and myoglobin.
• Increases lactic acid usage from skeletal muscles during exercise.
• Generates ATP from creatine phosphate, similar to skeletal muscle.
• Elevated creatine kinase (CK) levels in the blood may indicate heart damage.

Key Cardiac Terms

• Diastole: relaxation phase of the heart.
• Systole: contraction phase of the heart.

Question 1: Cardiac Cycle Phases

• Correct Answer: b) atrial contraction
• Atrial contraction occurs when the atria are in systole and the ventricles are in diastole.

Cardiac Output (CO)

• Measures the amount of blood ejected per minute.
• Affected by preload, contractility, and afterload.
• Stroke volume at rest is 50-60% of the end-diastolic volume, 40-50% remains after each contraction.
• Preload is the degree of stretch on the heart before it contracts.
• Contractility refers to the forcefulness of ventricular muscle fiber contraction.
• Afterload: the pressure required to eject blood against arterial resistance.

Stroke Volume Regulation

• Preload: governed by the Frank-Starling Law, associated with length-tension relationship.
• Contractility: influenced by inotropic agents, example is the sympathetic nervous system, and intracellular Calcium(2+) concentrations.
• Afterload: pressure in the pulmonary trunk and aorta.

Preload Explained

• The Frank-Starling Law explains how contraction strength adapts to the volume of received blood.
• Optimal fiber stretch leads to stronger contractions and higher stroke volume; more EDV increases force.
• Excessive stretch reduces overlap, weakening contractions and stroke volume.
• Connective tissues prevent overstretching, ensuring optimal muscle operation

EDV Factors

• Filling time: duration of ventricular diastole.
• Venous return: volume of blood returning to the right ventricle.
• Increased venous return raises EDV.
• Faster heart rates cut filling time, lowering EDV and preventing adequate filling.
• If the heart rate is above 160 bpm stroke volume declines.
• Slow heart rates typically mean larger resting stroke volumes.
• The Frank-Starling Law ensures a good balance between pulmonary and systemic circulation.
• Venous return to the right ventricle increases if the left ventricle pumps more blood.

Question 2: EDV Influence

• Correct Answer: b) arterial blood pressure
• Arterial Blood Pressure has no influence on EDV

Inotropic Agents

• Affect contractility.
• Positive inotropic effect: increases contractility with a constant preload, raising stroke volume.
• Negative inotropic effect: decreases contractility with a constant preload, reducing stroke volume.

Norepinephrine Mechanism

• Sympathetic stimulation of the heart causes norepinephrine release.
• Activation of a G-protein signaling pathway increases contractility.
• Achieved by phosphorylating several target proteins.

Question 3: Heart Contractility Receptor

• Correct Answer: a) G protein-coupled receptor
• G protein-coupled receptors are involved in heart contractility.

Defining Afterload

• Blood ejection starts after the ventricular pressure exceeds 20 mmHg in the pulmonary trunk and 80 mmHg in the aorta.
• Afterload is the pressure needed to open semilunar valves.
• Reduced stroke volume and increased residual blood volume results from higher afterload from hypertension or atherosclerosis.
• Increased afterload decreases the velocity of shortening of ventricular muscle fibers.

Autonomic Heart Regulation

• Both sympathetic and parasympathetic divisions innervate heart.
• Cardiac output varies with heart rate and stroke volume to meet bodily needs.
• SA node at default sets about 100 bpm, which can be altered.
• Factors regulate heart rate: age, fitness, and temperature.
• Autonomic nervous system, hormones, and ions impact baseline levels.
• Chronotropic agents affect heart rate.

Sympathetic Influences

• Increases the rate of spontaneous depolarization within the SA node.
• Causes nodal cells to generate action potentials more quickly.
• Increases rate of action potential conduction from the atria to the ventricles.
• Increases contractility.

Parasympathetic Influences

• Stimulation decreases the rate of spontaneous depolarization in the SA node.
• The body generates action potentials more slowly and heart rate decreases.

Impact of Cardiac Output

• Filling time, Venous return, Sympathetic nervous system, Hormones, Ions & Drugs all have an impact on it.

Exercise and Heart Health

• Cardiovascular fitness is improved at any age through aerobics.
• Any exercise works a big muscle group for greater than or equal to 20 minutes.
• Maximal cardiac output increases after weeks of training.
• Training can lead to hypertrophy of the heart.
• Well trained individuals see 40-60 bpm.

Pulmonary Circulation

• The pulmonary circuit involves blood flow between the heart and lungs for gas exchange.

Systemic Circulation

• Represents a parallel arrangement of blood vessels.

The Heart

• The fist-sized hollow organ primarily resides on the side of the left thoracic cavity.

The Pericardium

• Serves to protect and secure the heart.

Basic Heart Anatomy

• Contains four chambers and two pumps each dividing into left and right.
• Right side manages the pulmonary circuit.
• Left side manages the systemic circuit.

Atria

• Vessels transportating blood to the atria:
• Right side: superior and inferior vena cava.
• Left side: pulmonary veins.

Ventricles

• Vessels transporting blood away from the ventricles:
• Right side: pulmonary trunk.
• Left side: aorta.

Valves

• Ensure one-way flow of blood.
• Right AV valve: tricuspid.
• Right semilunar valve: pulmonary.
• Left AV valve: bicuspid/mitral.
• Left semilunar valve: aortic.
• Valves connect to a fibrous skeleton.

Blood vessels

• The thickness of the walls gives the function of the vessel
• There are 3 types
• Endothelium, Smooth Muscle, Connective Tissue

Blood Flow

• The flow rate of a liquid is related to the pressure difference
• Is proportional to the pressure difference between the two ends, and inversly proportional to the resistance
• The pressure gradient pushes liquid through a pipe, flows from high to low.

Mean Arterial Pressure (MAP)

• In the systemic circuit, the average of MAP is 85mmHG average pressure.
• Venous Pressure is approximately 2-8 mm Hg
• The pressure drives blood flow in the systemic circuit.

Pressure Gradient

• Driving blood flow through the systemic circuit matches the arterial pressure.

Flow and Resistance

• Greater the resistance, lower the flow.
• Lower the resistance, greater the flow.
• Resistance depends on:
  • Vessel radius
  • Vessel length -Blood Viscosity

Pressure Reservoirs

• The arteries are able to expand and contract with blood pressure because they contain elastic tissue.
• They capture pressure in order to ensure the blood is continuously flowing.
• Arteries have to have a low compliance in order to be the pressure reservoir.

Low Compliance

• Compliance measures the relationship between pressure and volume
• Small volume increase equals a large increase of blood pressure
• Artery will expand, and elasticity of the vessel walls ensures low resistance.

Arterial Blood Pressure

• Ventricle ejects blood into the aorta, and the pressure increases.
• Blood flow declines in arterial blood pressure before the next systole.
• The ventricular pressure decreases to 0 mm HG
• The arteries walls recoil increasing pressure.
• During systole, there is maximum pressure with is called the systolic pressure

Key points of Aterioles

• Deliver blood to the capillaries.
• Known as resistance vessels and controlled by resistance
• Two functions
  • Controlling blood flow to capillary beds.
  • regulating MAP

Capallaries

• Sphcinter relaxed means blood flows through
• Sphincters contrated means blood flows through only merteriole

Blood Distrbution

• Organs regulate it's blood flow through local control.
• Changes in the distribution of blood flow to organs-- that are primarily to changes in the vascular resistance.
• Organs altered in arterioles during contraction or relaxation.
• Intrinsic control mechanism regulates to the heart, brain, and skeletal muscles

The Adequate Blood Flow

• Through sensors in organs and tissues, these sensors sense blood flow themselves through vascular smooth muscle.
• Conditions in the extracellular fluids such as oxygen, carbon dioxide, potassium ions will change blood flow.

Capillaries

• Sites of waste exchange and nutrient.
• Walls are very thin and contain pores.
• They form beds at tissue, and controlled by two ways
  • Metarteriole and Precapillary sphincters

Starling Forces

• Involved in filtration (fluid moving out of the capillary) and reabsorption (fluid moving into the capillary).
• Net Filtration Pressure (NFP) is determined by hydrostatic and osmotic pressures.
• Arterial end: filtration dominates, moving fluid and solutes out of the capillary.
• Venous end: reabsorption dominates, drawing fluid and waste products back into the capillary.

Venules and Veins

• Function as blood reservoirs
• Various factors like muscle pump activity, respiratory pump activity increase venous return and affect blood volume.
• Compliance in vein, pressure return and even atrial diastolic pressure are responsible for venous return