U8 SS Cardiac

Structure and Function of the Cardiovascular System

Overview of Cardiovascular System

Must Knows: JVD, stenosis and murmurs, diastolic v systolic, blood vessel distension and compliance, vagus nerves and hr

  • Main Functions:

    • Transportation:

    • Delivering oxygen and nutrients to tissues necessary for metabolic processes.

    • Carrying waste products from tissues to kidneys and other excretory organs for elimination.

    • Other Functions:

    • Circulating electrolytes and hormones to regulate body function.

    • Regulating body temperature by transporting core heat to the periphery.

    • Reference: Levine, 2024, p. 707.

The Heart as a Pump

Functional Anatomy of the Heart

  • General Description:

    • The heart is a 4-chambered muscular pump, approximately the size of an adult male's fist.

    • Daily moves more than 1800 gallons of blood throughout the body.

  • Location:

    • Between lungs in the mediastinal space of the intrathoracic cavity in a pericardial sac.

    • Suspended by great vessels, with the base facing upward and the apex pointing downward, forward, and to the left.

    • Positioned obliquely, with the right side mostly in front of the left side, only a small portion of the lateral left ventricle on the frontal plane.

  • Point of Maximum Impulse (PMI):

    • The main impact of the heart's contraction is felt between the 5th and 6th ribs, a little below the nipple, and approximately 3 inches to the left of the midline.

Heart Wall Structure

  • Composed of:

    • Epicardium: Outer layer lining the pericardial cavity.

    • Myocardium: Muscle layer responsible for the heart’s contractions.

    • Endocardium: Smooth inner surface lining the chambers of the heart.

    • Fibrous Skeleton: Supports the valvular structures of the heart.

    • Interatrial and Interventricular Septa: Divides the heart into right and left pumps, each with thin-walled atria (reservoirs) and thick-walled ventricles (pumping chambers).

    • The left ventricular wall is thicker due to additional work needed to pump oxygenated blood to extremities.

Pericardium

  • Definition: Forms a fibrous covering around the heart.

  • Function:

    • Holds the heart fixed in place in the thorax.

    • Provides physical protection and acts as a barrier against infection.

  • Structure:

    • Tri-layer sac:

    • Outer fibrous layer: tough, resistant to distention, attached to great vessels, the sternum, and diaphragm.

    • Inner serous layer: composed of visceral and parietal layers.

      • Visceral Layer: Covers the heart and great vessels.

      • Parietal Layer: Lines the fibrous epicardium.

  • Pericardial Cavity: Lies between visceral and parietal layers, holds 30-50 mL of serous fluid, which acts as a lubricant during heart contractions and relaxations.

    • Reference: Levine, 2024, p. 707.

Myocardium Structure and Function

  • Definition: The muscular portion of the heart that forms walls of the atria and ventricles.

  • Properties:

    • Cardiac muscle cells are striated like skeletal muscle, comprising sarcomeres with actin and myosin filaments.

    • Smaller, more compact sarcomeres than skeletal muscle; contain large numbers of mitochondria for energy.

    • Contractility is similar to skeletal muscle, but cardiamuscle contractions are involuntary and longer-lasting.

  • Cell Arrangement: Cardiac muscle cells form an interconnecting latticework which allows simultaneous contraction and functional syncytium behavior.

    • Syncytia:

    • Atrial Syncytium: Constitutes walls of atria.

    • Ventricular Syncytium: Constitutes walls of ventricles, allowing atria to contract ahead of ventricles for efficient heart pumping.

    • Reference: Levine, 2024, pp. 707-708.

Heart Valves and Fibrous Skeleton

  • Function: Ensures unidirectional blood flow through the heart chambers, essential for efficiency.

  • Valves:

    • Atrioventricular (AV) Valves: include the tricuspid (right) and bicuspid (mitral) (left).

    • Semilunar Valves: include the pulmonary and aortic valves.

  • Mechanism:

    • AV valves close to prevent backflow into the atria during ventricular contraction.

    • Fluttering of papillary muscles and chordae tendineae ensures valves close tightly and prevent inversion during contraction.

  • Importance of Fibrous Skeleton: Acts as rigid support for valve attachment, providing proper function and separation for atrial and ventricular contractions.

Cardiac Cycle

  • Definition: Describes the rhythmic pumping action of the heart, consisting of two main phases:

    • Systole Phase: Ventricles contract.

    • Diastole Phase: Ventricles relax and fill with blood.

  • Additional Events:

    • Changes occur in left atrial pressure, left ventricular pressure, aortic pressure, ventricular volume, the ECG, and heart sounds during cardiac cycle.

    • Electrical activity precedes mechanical events, with specific waves representing nodes and muscle activity on ECG (P wave, QRS complex, T wave).

    • Reference: Levine, 2024, pp. 711-712.

Atrial Filling and Contraction

  • Atrial pressure waves during the cardiac cycle:

    • A wave: Last part of diastole caused by atrial contraction.

    • C wave: Ventricles contract, causing AV valves to bulge into atria.

    • V wave: Slow buildup of blood in the atria before AV valves open.

  • Atria aid ventricular filling, contributing approximately 20% during increased demands.

    • Reference: Levine, 2024, p. 713.

Regulation of Cardiac Performance

  • Cardiac Output (CO): Amount of blood pumped by the heart each minute calculated via the formula: CO=SVimesHRCO = SV imes HR

    • Average resting cardiac output: 4-6 L/min.

  • Factors Influencing Cardiac Output:

    • Preload: End-diastolic pressure and volume of blood in the heart before contraction.

    • Afterload: Pressure against which the heart must work to eject blood.

    • Contractility: Heart's ability to contract.

    • Heart Rate (HR): Frequency of contractions, influencing overall output.

    • Reference: Levine, 2024, pp. 713-714.

Principles of Blood Flow

  • Hemodynamics: Refers to the underlying physics governing blood flow, influenced by pressure, resistance, and flow.

    • Blood's flow is directly impacted by the pressure gradient and resistance throughout the vessels.

    • Key influences include viscosity, vessel diameter, and overall integrity of blood vessels.

The Microcirculation and Lymphatic System

Structure and Function of the Microcirculation

  • Microcirculation: Consists of arterioles, capillaries, and venules, facilitating nutrient transfer and waste removal.

    • Blood flows from arterioles into capillaries and exits through venules.

    • Precapillary sphincters control blood flow into capillary networks.

Capillary Structure and Function

  • Capillaries are one-cell thick, allowing efficient diffusion of gases and nutrients.

    • Movement occurs through capillary pores and intercellular junctions, enabling fluid exchange and interaction with surrounding interstitia (tissue fluid).

Lymphatic System Function

  • Functions of the Lymphatic System: Serves as an accessory route for fluid transport back into systemic circulation.

    • Collects interstitial fluid, proteins, and other larger particles that are too big to return via capillaries.

    • Filters out bacteria and foreign substances at lymph nodes.

    • A pumping mechanism facilitates lymph movement, supported by smooth muscle contraction and external pressures.

Neural Control of Circulatory Function

  • Neural Control Mechanism:

    • Medulla oblongata centers integrate and modulate cardiovascular function primarily through sympathetic and parasympathetic mechanisms.

  • Sympathetic vs. Parasympathetic Roles:

    • Sympathetic Nervous System (SNS): Influences heart rate and vasomotion positively.

    • Parasympathetic Nervous System (PNS): Primarily affects heart rate negatively and regulates relaxation of the heart chambers.