APK chapter 13

Overview of the Cardiovascular System

The cardiovascular system comprises the heart, blood, and blood vessels. It functions as an organ system, integrated to ensure efficient circulation and facilitate various physiological processes.

Heart

  • The heart serves as the pump made of cardiac muscle, moving blood through blood vessels.
  • It has both sensory and endocrine functions, primarily focusing on regulating blood pressure and volume.

Blood Vessels

  • Blood vessels are tubular conduits that circulate blood throughout the body. They possess sensory capabilities and function as an effector organ regulating blood distribution and pressure.

Blood

  • Blood is a connective tissue fluid, transporting nutrients and wastes to and from cells, and facilitating communication between organs via hormone transport.
  • The system is classified as a closed system, meaning it operates internally without exposure to the external environment. Its main role is to coordinate the delivery and removal of molecules to meet body demands.

Importance of Circulation

  • Essential for the transfer of cellular materials between internal and external environments (e.g., oxygen intake and waste disposal).
  • Diffusion only works over short distances, necessitating circulation via blood vessels for nutrient and gas exchange.

Basic Terminology

Understanding key terms is vital:

  • Artery: A blood vessel always carrying blood away from the heart.
  • Vein: A blood vessel that transports blood to the heart.
  • Atrium: A heart chamber that receives blood.
  • Ventricle: A heart chamber that expels blood.

Anatomy of the Heart

The heart's structure resembles an upside-down, backward-leaning cone covered by epithelial tissue, with its apex pointing down and to the left. Key points include:

Shape and Position

  • The left side location is due to the heart's apex positioning; the bottom is connected to the diaphragm via the pericardium.

Pericardium

  • A double-layered serous membrane encases the heart, composed of an outer fibrous layer and an inner visceral layer with serous fluid that lubricates the heart, reducing friction against surrounding organs, such as the lungs.

Chambers of the Heart

  • The heart consists of four chambers: two atria and two ventricles. The atria receive blood from the body, and the ventricles pump blood away from the heart. Notably, ventricular walls are thicker than atrial walls due to their pumping workload.

Heart Valves

  • Atrioventricular (AV) Valves: Flaps of connective tissue separating atria from ventricles, opening and closing based on pressure changes. Major AV valves include:
    • Bicuspid (Mitral) Valve: Left AV valve with two flaps.
    • Tricuspid Valve: Right AV valve with three flaps.
  • Semilunar Valves: Separate ventricles from arteries, including:
    • Aortic Semilunar Valve: Between the aorta and left ventricle.
    • Pulmonary Semilunar Valve: Between the right ventricle and pulmonary trunk.

Heart Wall Layers

  • Myocardium: Thick, muscular layer responsible for contraction, with cardiac myocytes as its primary cellular composition.
  • Endocardium: Inner lining made of simple squamous epithelium, continuous with endothelial lining of blood vessels.
  • Epicardium: Outer layer or visceral pericardium, also simple squamous epithelium.

Anatomy of Blood Vessels

Blood vessels can be classified into three types:

  • Veins: Carry blood to the heart, generally thicker connective tissue, larger lumen, and contain valves to prevent backflow.
  • Arteries: Carry blood away from the heart. They possess thicker smooth muscle layers allowing for vasoconstriction and vasodilation, leading to smaller lumens.
  • Capillaries: The smallest vessels, facilitating nutrient exchange between arteries and veins, composed of only two thin layers: endothelium and basement membrane, without smooth muscle.
    • Arterioles, capillaries, and venules are known as microcirculation, visible only under a microscope.

Anatomy of Blood

Blood is a connective tissue with the following components:

  • Plasma: Approximately 55% of blood volume, consisting of water, proteins, and electrolytes.
  • Buffy Coat: Contains platelets and leukocytes, less than 1% of total blood volume.
  • Erythrocytes: Red blood cells that contain hemoglobin and carry oxygen. Their biconcave shape maximizes surface area for gas exchange and enables flexible movement through narrow capillaries.

Circulatory System Circuits

There are two main circuits in the human circulatory system:

  1. Pulmonary Circuit: Transports oxygen-poor blood to the lungs for oxygenation and returns oxygen-rich blood to the heart.
  2. Systemic Circuit: Delivers oxygen-rich blood to capillary beds throughout the body, returning oxygen-poor blood to the heart.

Blood converges into the right atrium via superior and inferior venae cavae, resulting in "mixed venous blood" due to varying oxygen levels. Each circuit pumps equal volumes of blood, with the right ventricle pumping to the lungs and the left ventricle pumping to the rest of the body.

Blood Flow Dynamics

Blood can circulate in two styles:

  • Series Blood Flow: Blood path follows a sequential route, essential for oxygenation in the pulmonary circuit before delivering oxygen to the systemic circuit. The heart acts as a muscular double pump.
  • Parallel Blood Flow: Occurs in the systemic circuit, allowing simultaneous blood flow to multiple organs; this system reduces the time for oxygen delivery and adapts flow according to organ needs.

Electrical Activity of the Heart

The heart's ability to pump is reliant on synchronized contractions, which are coordinated by an intricate conduction system in the myocardium.

Myogenic Contraction

  • Achieved by specialized cardiac myocytes, known as autorhythmic cells. There are two main types:
    • Pacemaker Cells: Generate spontaneous action potentials found in the SA and AV nodes.
    • Conduction Fibers: Large-diameter myocytes transmitting action potentials throughout the heart.

Sinoatrial (SA) Node

  • Located in the right atrium, the SA node contains pacemaker cells crucial for establishing heart rhythm. The electrical activity path is:
    1. From SA node to AV node.
    2. AV bundle (Bundle of His)
    3. Down bundle branches to the heart's apex.
    4. Signal spreading through ventricles via Purkinje fibers.

This ensures atrial contraction precedes ventricular contraction, giving time for valves closure.

Electrocardiograms (ECGs)

ECGs measure the heart’s electrical activity to infer mechanical functions.

  • Each lead consists of two electrodes (one positive, one negative). Common leads include I, II, and III, where the direction and amplitude of electrical activity are displayed on a graph.
  • Basic ECG output parts:
    • P Wave: Represents atrial depolarization prior to contraction.
    • QRS Complex: Represents ventricular depolarization, with atrial repolarization occurring simultaneously but overshadowed.
    • T Wave: Represents ventricular repolarization.

The Cardiac Cycle

The cardiac cycle encompasses all events during a complete heartbeat, divided into:

  1. Ventricular Filling: AV valves are open, allowing blood influx from atria to ventricles.
  2. Isovolumetric Contraction: Ventricles begin contracting but volume remains unchanged as valves are still closed.
  3. Ventricular Ejection: Blood is expelled from ventricles as semilunar valves open due to pressure differences.
  4. Isovolumetric Relaxation: Following ejection, ventricles relax while no blood volume changes occur.

This cycle repeats continuously as the heart beats.

Pressures Throughout the Cardiac Cycle

Various pressures (atrial, ventricular, aortic) vary during each heartbeat:

  • Atrial Pressure: Shows modest fluctuations compared to ventricular pressure.
  • Ventricular Pressure: Significant changes, notably rising during filling and contraction phases.
  • Aortic Pressure: Measured during diastole (relaxation) and systole (contraction), showing a characteristic dicrotic notch upon closure of the aortic valves.

Stroke Volume and Cardiac Output

  • Stroke Volume (SV): The volume of blood pumped from the ventricles in one beat, calculated as:
    extStrokeVolume(SV)=extEnd−DiastolicVolume(EDV)−extEnd−SystolicVolume(ESV)ext{Stroke Volume (SV)} = ext{End-Diastolic Volume (EDV)} - ext{End-Systolic Volume (ESV)}
  • Ejection Fraction (EF): Indicates the efficiency of the heart, defined as:
    ext{Ejection Fraction (EF)} = rac{ ext{Stroke Volume}}{ ext{End-Diastolic Volume}}
  • Cardiac Output (CO): Volume of blood the heart pumps per minute:
    extCardiacOutput(CO)=extHeartRate(HR)imesextStrokeVolume(SV)ext{Cardiac Output (CO)} = ext{Heart Rate (HR)} imes ext{Stroke Volume (SV)}

Factors Affecting Stroke Volume

Three key determinants of stroke volume include:

  1. End-Diastolic Volume: Influences preload and directly relates to stroke volume (Starling's Law).
  2. Ventricular Contractility: Refers to the strength of contraction independent of volume; enhanced by sympathetic nervous activity and related hormones.
  3. Afterload: The resistance ventricles must overcome during ejection. Increased arterial pressure elevates afterload, requiring greater ventricular force to maintain stroke volume.

Summary of Cardiac Regulation

Cardiac output is dynamically controlled by factors affecting heart rate and stroke volume. Intrinsic mechanisms of myocardial contractility and external controls from autonomic nervous system responses play distinct roles in maintaining cardiovascular health and efficiency. Changes in sympathetic or parasympathetic outputs adjust the heart's function to meet physiological demands throughout varying states of activity and rest.