Study Guide: Cardiac Physiology and Blood – Structure and Function

Cardiac Physiology

Ion Movement and Membrane Potentials in Cardiac Cells

The membrane potential of cardiac cells is crucial for the initiation and propagation of action potentials that drive the heartbeat. Cardiac cells are classified as either conductive (pacemaker) or contractile cells, each with distinct ion channel dynamics.

• Depolarization: Conductive cells depolarize to approximately +35 mV, while contractile cells reach about +65 mV.

• Repolarization Thresholds: The membrane potential must return to about -40 mV in conductive cells and between -80 mV (atrial) and -90 mV (ventricular) in contractile cells for repolarization.

• Ion Channel Activity: Closure of calcium channels and opening of potassium channels lead to repolarization in both cell types.

• Sodium Ions: Sodium influx causes rapid depolarization in contractile cells but slower depolarization in conductive cells.

Example: During the cardiac action potential, the plateau phase is maintained by slow calcium influx, which is essential for proper contraction timing.

Plateau Phase in Cardiac Action Potentials

The plateau phase is a unique feature of cardiac muscle action potentials, especially in contractile cells, and is responsible for the heart's prolonged contraction and refractory period.

• Slow Calcium Channels: Responsible for the prolonged depolarization plateau and the absolute refractory period.

• Prevention of Summation: The plateau phase prevents summation and premature contractions, ensuring the heart functions efficiently as a pump.

• Potassium Channels: Fast potassium channels are not responsible for the plateau phase; they contribute to repolarization.

• Cardiac Cycle Coordination: The plateau phase allows the cardiac cycle to synchronize and prevents contractions from occurring too rapidly.

Equation:

$\text{Action Potential Duration} = \text{Depolarization Phase} + \text{Plateau Phase} + \text{Repolarization Phase}$

The Pericardium

The pericardium is a double-walled sac that surrounds and protects the heart.

• Structure: Consists of a tough outer fibrous pericardium and an inner serous pericardium.

• Function: The fibrous pericardium anchors the heart and maintains its position in the thorax; it is made of dense connective tissue.

• Not a Source of Autorhythmicity: The pericardium does not generate electrical impulses.

Cardiac Skeleton

The cardiac skeleton is a dense connective tissue structure that supports the heart's valves and electrical conduction system.

• Boundary: Acts as a boundary for the heart's electrical conduction system.

• Reinforcement: Heavily reinforced with dense connective tissue.

• Attachment: Serves as the point of attachment for the heart valves.

• Openings: Surrounds the openings between the atria and ventricles, and the pulmonary trunk and aorta.

Autonomic Innervation of the Heart

The heart is innervated by both sympathetic and parasympathetic nervous systems, which regulate heart rate and contractility.

• Sympathetic Stimulation: Releases norepinephrine, shortens the repolarization period, increases the rate of depolarization and contraction, and increases heart rate.

• Parasympathetic Stimulation: Releases acetylcholine, opens potassium channels, slows depolarization, extends repolarization, and decreases heart rate.

Cardiac Muscle Metabolism

Cardiac muscle cells rely on aerobic metabolism to meet their energy needs.

• Oxygen Storage: Myoglobin stores oxygen within cardiac muscle cells.

• Energy Substrates: Fatty acids and glycogen are stored and metabolized in mitochondria to produce ATP.

• Aerobic Metabolism: Cardiac muscle metabolism is normally entirely aerobic.

Fibrous Skeleton of the Heart

The fibrous skeleton provides structural and electrical support for the heart.

• Anchors Heart Valves: Provides attachment points for the valves.

• Separates Atria and Ventricles: Maintains structural separation.

• Electrical Insulation: Prevents electrical impulses from passing directly between atria and ventricles.

• Framework for Myocardium: Supports the attachment of cardiac muscle fibers.

Cardiac Cycle and Valve Function

The cardiac cycle describes the sequence of events in one heartbeat, including chamber contraction and valve operation.

• Chamber Contraction: Typically, only two of the four chambers contract at any one time (either both atria or both ventricles).

• Valve Operation During Ventricular Contraction: The semilunar valves open and the atrioventricular (AV) valves close to prevent backflow.

Heart Valves

Heart valves ensure unidirectional blood flow through the heart.

• Semilunar Valves: Have half-moon shaped, pocketlike cusps; each has three cusps.

• AV Valves: Supported by chordae tendineae (tendinous cords) to prevent prolapse; tricuspid valve has three cusps, mitral (bicuspid) valve has two.

Intercalated Discs and Cell-to-Cell Contacts

Cardiac muscle fibers are connected by specialized structures called intercalated discs, which facilitate coordinated contraction.

• Desmosomes: Prevent cardiac muscle fibers from pulling apart during contraction.

• Gap Junctions: Allow rapid cell-to-cell communication and electrical coupling.

Processes Driven by Cardiac Cycle Pressure Changes

Pressure changes within the heart drive essential processes for effective circulation.

• Distribution of Blood: Ensures blood is delivered to heart muscle fibers.

• Unidirectional Flow: Maintains one-way movement of blood through the chambers.

• Valve Operation: Opening and closing of valves prevent backflow.

Heart Anatomy: Papillary Muscles and Internal Structures

• Papillary Muscles: Attach to the cusps of the AV valves via chordae tendineae (tendinous cords).

• Trabeculae Carneae: Large, irregular muscular ridges on the internal surface of the ventricles.

Pericardial Cavity and Serous Fluid

The pericardial cavity contains serous fluid that lubricates the membranes of the pericardium, reducing friction during heartbeats.

Coronary Arteries

Coronary arteries supply the heart muscle with oxygenated blood, nutrients, and remove waste products.

• Function: Distribute blood containing nutrients and oxygen to the heart muscle cells.

• Unidirectional Flow: Ensure there is no backflow of blood from the aorta to the right ventricle.

Blood: Structure and Function

Key Learning Objectives

• Identify the characteristics and functions of blood and the significance of each.

• Define and state the normal hematocrit values.

• Compare the relative abundance of the types of formed elements in blood.

• Identify the various types of plasma proteins.

• List dissolved substances in plasma.

• Identify how hemopoiesis changes with age.

• Identify the growth factors required for erythrocyte development.

• Identify the structure, shape, and function of erythrocytes.

• Describe the origin of carbon dioxide and how it is transported in blood.

• Define hypoxia.

• Describe hemoglobin's structure and function.

• List the nutrients required for erythrocyte development.

• Describe the process by which erythrocyte components are recycled.

• Define diapedesis and positive chemotaxis.

• Distinguish between the various types of granular and agranular leukocytes.

• List the main functions of leukocytes.

• Name the main characteristics of platelets, granular and agranular leukocytes.

• Describe the phases of hemostasis.

• List the characteristics of the different Rh and ABO blood types.

• Identify what causes hemolytic disease of the newborn.

• Identify the importance of blood type when transfusing blood.

Table: Comparison of Cardiac Cell Types

Feature	Conductive Cells	Contractile Cells
Depolarization Peak	+35 mV	+65 mV
Repolarization Threshold	-40 mV	-80 to -90 mV
Depolarization Speed	Slow	Rapid
Plateau Phase	Present (shorter)	Present (longer)

Additional info: The above table summarizes key differences between conductive (pacemaker) and contractile cardiac cells, which is essential for understanding cardiac electrophysiology.