Textbook Note Summary of the Heart (Pages 242-245)

Maintenance of the Human Body

Cardiovascular System Overview

Major Components

  • Epithelial Tissue Functions

    • Protection: Acts as a barrier against physical and chemical harm.

    • Secretion: Produces substances such as mucus and enzymes.

    • Absorption: Facilitates the uptake of nutrients and other substances.

    • Excretion: Removes waste products from the body.

    • Senses: Contains sensory nerves that respond to stimuli.

Heart Structure and Function

  • Location: The heart is a cone-shaped muscular organ, roughly the size of a fist, located between the lungs behind the sternum with its apex tilted left.

  • Components:

    • Myocardium: The thick middle layer consisting mainly of cardiac muscle.

    • Pericardium: A thick, membranous sac enclosing the heart secreting lubricating fluid.

    • Endocardium: The inner lining consisting of connective and endothelial tissues.

  • Chambers: The heart has four chambers—two atria (upper, thin-walled) and two ventricles (lower, thick-walled)—which work together to pump blood.

Heart Valves

  • Valves: Four main valves regulate blood flow:

    • Atrioventricular Valves:

    • Tricuspid Valve (Right Side): Contains three flaps/cusps.

    • Bicuspid Valve (Mitral Valve, Left Side): Contains two flaps.

    • Semilunar Valves:

    • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

    • Aortic Semilunar Valve: Between left ventricle and aorta.

Blood Flow Pathway Through the Heart

  • O2-Poor Blood to Lungs:

    1. O2-poor blood enters right atrium via superior and inferior vena cavae.

    2. Blood flows through tricuspid valve into right ventricle.

    3. Blood is pumped through pulmonary semilunar valve into pulmonary trunk and into lungs via pulmonary arteries.

  • O2-Rich Blood to Body:

    1. O2-rich blood returns to left atrium via pulmonary veins.

    2. Blood flows through bicuspid (mitral) valve into left ventricle.

    3. Blood is pumped through aortic semilunar valve into the aorta, delivering O2-rich blood to the body.

  • Separation of Blood: O2-poor and O2-rich blood do not mix.

Heart Function and Cardiac Cycle

  • Heartbeat Definition: Each heartbeat is a cardiac cycle consisting of:

    • Systole: Contraction phase of the heart muscle.

    • Diastole: Relaxation phase of the heart muscle.

  • Rate: Normal heartbeat is around 70 beats per minute, lasting approximately 0.85 seconds.

  • Heartbeat Sounds:

    • “Lub” Sound: Caused by closure of atrioventricular valves.

    • “Dup” Sound: Caused by closure of semilunar valves.

    • Heart Murmur: Can indicate ineffective valves where blood leaks backward.

Intrinsic Control of Heartbeat

  • Nodal Tissue: Unique cardiac muscle with both muscular and nervous characteristics. Consists of:

    • SA Node (Sinoatrial Node): Pacemaker of the heart located in the right atrium that initiates heartbeat every 0.85 seconds.

    • AV Node (Atrioventricular Node): Located near the septum, it receives impulses from the SA node. A brief delay allows for atrial contraction before ventricular contraction.

    • AV Bundle & Purkinje Fibers: Transmit impulses to contract ventricles.

  • Pacemaker Function: If the SA node fails, the AV node can maintain a slower heartbeat (40-60 BPM). An artificial pacemaker can be implanted if necessary.

Extrinsic Control of Heartbeat

  • Autonomic Nervous System: Adjusts heart rate via the cardiac control center in the brain (medulla oblongata).

    • Parasympathetic System: Decreases heart rate.

    • Sympathetic System: Increases heart rate during stress or exercise via hormones (epinephrine and norepinephrine).

Electrocardiogram (ECG)

  • Definition: A recording of electrical changes in myocardium during a cardiac cycle.

  • Components:

    • P Wave: Indicates atrial contraction.

    • QRS Complex: Indicates ventricular contraction.

    • T Wave: Indicates recovery of ventricles post-contraction.

  • Abnormalities: Conditions like ventricular fibrillation lead to irregular contractions, requiring defibrillation to restore normal rhythm.

Visual Aid Figures

  • Figure 13.3: Depicts external heart anatomy (arteries and veins).

  • Figure 13.4: Internal view of the heart showing chambers and valves.

  • Figure 13.5: Stages of the cardiac cycle showing atrial and ventricular contractions.

  • Figure 13.6: Conduction system of the heart and electrocardiogram readings.