chapter 21
Introduction to the Heart and Cardiovascular Circuits
The heart is a vital muscular pump that circulates blood throughout the body, maintaining the transport of substances and overall homeostasis.
Heart Basics
Size: Approximately the size of a clenched fist.
Average Heart Rate: 70 beats/minute (approximately 100,000 beats/day, 36.5 million beats/year).
Pumping Volume: 5 to 30 liters of blood/minute (varies based on the need of peripheral tissues).
Chambers: Four chambers - two atria and two ventricles.
Two Main Circuits of Blood Flow
Pulmonary Circuit:
Connects the heart with the lungs.
Path: Carbon dioxide-rich (deoxygenated) blood leaves heart → lungs for gas exchange → oxygen-rich (oxygenated) blood returns to heart.
Systemic Circuit:
Connects oxygen-rich blood from the heart to the body's tissues for gas exchange → carbon dioxide-rich blood returns to heart.
Types of Blood Vessels in Circuits
Arteries: Carry blood away from the heart.
Veins: Carry blood to the heart.
Capillaries: Connect arteries and veins, facilitating exchange with tissues.
Location and Orientation of the Heart
The heart is centrally located in the thoracic cavity, with a specific orientation.
Location: More or less in the center of the thoracic cavity, nestled between the lungs.
Borders: Broad base (superior) and pointed apex (inferior).
Orientation:
Slightly left of the midline.
Rotated a little bit to the left such that the right atrium and right ventricle are slightly anterior.
Pericardium and Heart Wall
The heart is enclosed by protective membranes and has distinct layers in its wall.
Pericardium (Heart Coverings)
Outer Fibrous Pericardium:
Tough, dense outer layer that anchors the heart and prevents overfilling.
Inner Serous Pericardium:
Divided into two parts:
Outer Parietal Layer: Lines the inner surface of the fibrous pericardium; forms outer boundary of pericardial cavity.
Inner Visceral Layer (Epicardium): Fused to the outer surface of the heart itself.
Pericardial Cavity:
Space between parietal and visceral layers.
Contains pericardial fluid, which reduces friction as the heart beats.
Heart Wall Layers (from superficial to deep)
Epicardium (Visceral Layer of Serous Pericardium):
Outermost layer.
Composition: Mesothelium (simple squamous epithelium) and underlying areolar tissue connecting it to the myocardium.
Myocardium:
Thick middle layer, composed of cardiac muscle tissue.
Function: Responsible for the heart's pumping action/contraction.
Composition: Cardiac muscle cells, connective tissue, and blood vessels and nerves.
Endocardium:
Innermost layer.
Composition: Areolar tissue connecting myocardium to endothelium.
Endothelium: Simple squamous epithelium that lines heart chambers and blood vessels.
Cardiac Muscle Tissue Histology
Characteristics:
Has a striated and involuntary contraction appearance.
Cells connected by intercalated discs (complex junctions).
Intercalated Discs (Detailed)
Complex junctions linking adjacent cardiac muscle cells:
Desmosomes: Provide strong mechanical connections.
Gap Junctions: Allow direct electrical connections; facilitate rapid spread of electrical impulse.
Bind together myofibrils of adjacent cells.
Anatomy of the Heart
Chambers and Septa
The heart contains four chambers that direct blood flow through the circuits.
Four Main Chambers:
Two Atria (Right and Left):
Superior/receiving chambers.
Have lumpy, wrinkled flaps called auricles.
Two Ventricles (Right and Left):
Inferior/pumping/ejecting chambers.
Have thicker walls compared to atria.
The left ventricle has significantly thicker walls than the right ventricle.
Basic Blood Flow Path:
Deoxygenated blood from systemic circuit → Right Atrium → Right Ventricle → Lungs.
Oxygenated blood from lungs → Left Atrium → Left Ventricle → Rest of body.
Septa (Internal Divisions):
Interatrial Septum: Lies between the atria.
Interventricular Septum: Separates the ventricles.
Heart Valves
Valves ensure unidirectional blood flow and prevent backflow within the heart.
General Function:
Prevent backflow (ensuring unidirectional blood flow).
Two Main Sets of Valves:
Atrioventricular Valves:
Separate atria from ventricles.
Right Atrioventricular Valve (Tricuspid Valve): Made of three fibrous flaps (cusps).
Left Atrioventricular Valve (Mitral Valve / Bicuspid Valve): Made of two fibrous flaps (cusps).
Semilunar Valves:
Regulate blood flow out of ventricles into great arteries.
Each has three cusps.
Pulmonary Valve: Separates right ventricle from the pulmonary trunk/artery.
Aortic Valve: Separates left ventricle from the aorta.
Atrioventricular Valve Mechanism (Chordae Tendineae and Papillary Muscles):
The cusps of Atrioventricular valves are attached to papillary muscles via chordae tendineae.
Ventricles Relaxed: Chordae tendineae are loose/relaxed. Atrioventricular valve is open.
Ventricles Contract: Papillary muscles contract, tensing chordae tendineae; pulls cusps together, closing the valve.
Fetal Circulation Remnants
Structures present during fetal development that typically close after birth:
Foramen Ovale:
Fetal Function: Opening within the interatrial septum that allows fetal blood to bypass the lungs.
Post-birth: Normally closes after birth.
Adult Remnant: Becomes the fossa ovalis.
Electrical Activity of the Heart: The Conducting System
The heart has an intrinsic electrical conducting system that initiates and coordinates contractions.
Electrocardiogram (ECG or EKG):
Measures the electrical activity of the heart.
Conducting System Components:
Sinoatrial (SA) Node:
Location: In the right atrium.
Function: "Pacemaker of the heart."
Comprised of specialized cardiac muscle/pacemaker cells with autorhythmicity.
Autorhythmicity:
Ability to generate their own electrical impulses.
Internodal Pathways:
Conduct impulse from SA node to Atrioventricular node stimulating atria.
Atrioventricular Node:
Slows the electrical impulse slightly.
Atrioventricular Bundle (Bundle of His):
Conducts impulse from AV node to bundle branches.
Left and Right Bundle Branches:
Extend towards the apex of the heart.
Purkinje Fibers:
Rapidly convey impulses to the contractile cells of the ventricular myocardium.
Sequence of Conduction:
SA Node → Internodal Pathways → Atrioventricular Node → AV Bundle → Bundle Branches → Purkinje Fibers.
Contraction Coordination:
Ensures atria contract first, then ventricles contract.
The Cardiac Cycle
The cardiac cycle describes the sequence of events in one complete heartbeat, involving alternating contraction and relaxation.
Two Main Phases:
Systole: Technical term for contraction (chamber ejects blood).
Diastole: Technical term for relaxation (chamber fills with blood).
Coordination:
Both atria contract together, then both ventricles contract together. Atria contract before ventricles.
Sequence of Events (simplified):
Atrial Systole/Ventricular Diastole (Start):
Atria contract, forcing blood into relaxed ventricles, completing ventricular filling.
Atrial Diastole Begins:
Atria relax.
Ventricular Systole (First Phase):
Ventricles begin to contract (but pressure not high enough yet to open semilunar valves); AV valves close. Semilunar valves are closed.
Ventricular Systole (Second Phase):
Ventricular pressure rises/exceeds arterial pressure; semilunar valves open; blood is ejected from heart.
Ventricular Diastole (Early):
Ventricles relax; semilunar valves close. Blood begins to flow into relaxed atria and ventricles.
Ventricular Diastole (Late):
All chambers relaxed/filling. Atrioventricular valves open; ventricles passively fill with blood.
Coronary Blood Vessels
These blood vessels specifically supply the heart muscle (cardiac muscle tissue) itself.
Function:
Provide oxygen and nutrients for cardiac muscle tissue to support its high energy demands.
Coronary Arteries:
Origin: At the base of the ascending aorta.
Major Arteries: Right Coronary Artery (RCA) and Left Coronary Artery (LCA).
Function: Supply cardiac muscle tissue via coronary circulation.
Coronary Veins:
Function: Drain deoxygenated cardiac venous blood.
Drainage Pathway: Various cardiac veins collect blood → feed into the large coronary sinus → empties directly into the right atrium.