Cardiovascular System Overview ch 21

• The heart keeps the blood in motion
• When blood stops moving, nutrients and oxygen supplies are exhausted, and wastes accumulate
• The heart beats about 100,000 times per day, equivalent to about 70 beats per minute
• The heart pumps about 1.5 million gallons of blood per year, approximately 2.9 gallons per minute
• The heart pumps between 5 and 30 liters of blood per minute; the volume can vary widely depending on circumstances like excercise or rest.

Overview of the Cardiovascular System

• The heart is about the size of a clenched fist
• The heart consists of four chambers: two atria and two ventricles
• The heart pumps blood into two circuits: the pulmonary circuit and the systemic circuit

Basic Circulatory Components

• Each circuit involves arteries, veins, and capillaries
• Arteries transport blood away from the heart
• Veins transport blood toward the heart
• Capillaries are vessels that interconnect arteries and veins

The Pericardium

• The heart is surrounded by a pericardium, consisting of two parts
• The outer fibrous pericardium
• The inner serous pericardium consists of two layers
• The inner visceral layer, also called epicardium, is attached to the surface of the heart
• The outer parietal layer, which is adjacent to the fibrous pericardium
• The space between the two serous layers contains pericardial fluid and is called the pericardial cavity
• Pericardial fluid lubricates the space to reduce friction

Structure of the Heart Wall

• The walls of the heart consist of three layers: the epicardium, the myocardium, and the endocardium
• The epicardium is the external surface and consists of visceral pericardium
• The myocardium consists of cardiac tissue, including cardiac muscle cells, connective tissue, blood vessels, and nerves
• The endocardium is the internal, endothelial surface

Cardiac Muscle Tissue

• Cardiac muscle tissues have a striated appearance and is involuntary because it contracts without information from the CNS
• It depends on aerobic respiration and has lots of mitochondria and myoglobin
• The circulatory supply of cardiac muscle tissue is extensive
• Cardiac muscle cells are interconnected by intercalated discs

Intercalated Discs

• Cardiac cells have specialized cell-to-cell junctions, called the intercalated discs
• The plasma membranes of two adjacent cardiac cells are bound together by desmosomes
• Intercalated discs bind the myofibrils of adjacent cells together
• Cardiac muscle cells are connected by gap junctions, allowing ions to move directly from one cell to another and creating a direct, electrical connection
• Intercalated discs allow all the muscle cells to form a functional syncytium and to contract as one unit

The Cardiac Skeleton

• Each cardiac cell is wrapped in an elastic sheath
• Each muscle layer is wrapped in a fibrous sheet
• Fibrous sheets separate the superficial layer from the deep layer muscles
• Fibrous sheets also encircle the base of the pulmonary trunk, ascending aorta, and valves

Functions of the Cardiac Skeleton

• Stabilizes the position of cardiac cells and heart valves
• Provides support for the blood vessels and nerves in the myocardium
• Helps to distribute the forces of contraction
• Helps to prevent overexpansion of the heart
• Provides elasticity so the heart recoils after contraction
• Isolates atrial cells from ventricular cells

Orientation and Superficial Anatomy of the Heart

• The heart lies slightly to the left of the midsagittal plane and is located in the mediastinum
• The base is the superior border of the heart
• The apex is the inferior portion of the heart
• The right border is formed by only the right atrium, and the inferior border is formed by the right ventricle
• The heart is rotated slightly toward the left
• The anterior surface consists of the right atrium, right ventricle, and the left ventricle
• The posterior surface consists of the left atrium and a small portion of right atrium
• The diaphragmatic surface is composed of the right and left ventricles

External Surface Features

• The heart chambers are identified by sulci (grooves) on the external surface
• The interatrial groove separates the left and right atria
• The coronary sulcus separates the atria and the ventricles
• The anterior interventricular sulcus separates the left and right ventricles
• The posterior interventricular sulcus also separates the left and right ventricles
• The left and right atria are positioned superior to the coronary sulcus and have thin walls
• Both contain an expandable anterior portion called an auricle
• The left and right ventricles are positioned inferior to the coronary sulcus and have thicker walls than the atria
• The left ventricular wall is thicker than the right ventricular wall

Internal Anatomy and Organization of the Heart

• A frontal section of the heart reveals:
• The left and right atria are separated by the interatrial septum
• The left and right ventricles are separated by the interventricular septum
• The atrioventricular valves are formed from folds of endocardium
• The atrioventricular valves are situated between the atria and the ventricles

The Right Atrium

• Receives oxygen-poor venous blood via the superior vena cava, inferior vena cava, and coronary sinus
• Coronary sinus enters the posterior side of the right atrium
• Contains pectinate muscles in the anterior wall and auricle
• The interatrial septum contains the fossa ovalis, a fetal remnant of the foramen ovale that allowed fetal blood to bypass the lungs

The Right Ventricle

• Receives oxygen-poor blood from the right atrium, passing through the right atrioventricular valve, also called right AV valve or tricuspid valve
• Blood leaves the right ventricle by passing through the pulmonary valve, also called pulmonary semilunar valve
• The pulmonary valve leads to the pulmonary trunk, then to the right and left pulmonary arteries
• The right AV valve is connected to papillary muscles via chordae tendineae
• Three fibrous flaps (cusps) and three papillary muscles exist within the ventricle
• Each of the three cusps is connected by the chordae tendineae to separate papillary muscles
• Papillary muscles and chordae tendineae prevent valve inversion when the ventricles contract
• The internal surface of the right ventricle consists of trabeculae carneae (muscular ridges) and a moderator band
• A moderator band is found only in the right ventricle
• It is a muscular band that extends from the interventricular septum to the ventricular wall
• It prevents overexpansion of the thin-walled right ventricle

The Left Atrium

• Receives oxygenated blood from the lungs via the right and left pulmonary veins
• Pectinate muscles are restricted to the auricle
• Blood passes through the left atrioventricular valve (bicuspid valve or left AV valve), also called the mitral valve

The Left Ventricle

• Has the thickest wall, needed for strong contractions to pump blood throughout the entire systemic circuit
• Compared to the right ventricle, this has a thin wall since it only pumps blood through the pulmonary circuit
• This does not have a moderator band but has prominent trabeculae carneae
• The left AV valve has chordae tendineae connecting to two cusps and to two papillary muscles
• Blood leaves the left ventricle by passing through the aortic valve (aortic semilunar valve)
• It enters the ascending aorta, then travels to the aortic arch, then down the descending aorta and to all body parts (systemic)

Structural Differences between the Right and Left Ventricles

• The right ventricle has a thinner wall and weaker contraction and contains a moderator band
• The left ventricle has a thicker wall and powerful contraction, which is six to seven times more powerful than the right ventricle

The Structure and Function of Heart Valves

• A normal heart has four valves: two AV valves (tricuspid and bicuspid valves) and two semilunar valves (aortic and pulmonary valves)

• Each AV valve consists of a ring of connective tissue that connects to heart tissue and is part of the fibrous skeleton ; it also consists of cusps, chordae tendineae, and papillary muscles

• AV valve function during the cardiac cycle includes:

• Papillary muscles relaxing

• Pressure in the atria causes AV valves to open with blood flowing from atria to ventricle

• When ventricles contract, pressure causes the AV valves to close and the semilunar valves to open

• Closure of AV valves prevents regurgitation (backflow) into the atria, forcing blood through the open semilunar valves

Coronary Blood Vessels

• Originate at the base of the ascending aorta
• They supply the cardiac muscle tissue via the coronary circulation
• Major coronary arteries include:
• Right coronary artery (RCA) with atrial branches, right marginal branch, posterior interventricular branch and conducting system branches
• Left coronary artery (LCA) with a circumflex branch and left marginal branch along with an anterior interventricular branch
• The right coronary artery passes between the right auricle and pulmonary trunk
• Major branches from the right coronary artery include atrial branches, right marginal branch, posterior interventricular branch, and conducting system branches
• Major branches from the left coronary artery include the anterior interventricular branch which branches to the posterior interventricular branch called anastomoses
• A circumflex branch branches to form the left marginal branch and branches to form the posterior left ventricular branch
• The coronary veins drain cardiac venous blood ultimately into the right atrium
• Main coronary veins include the great cardiac vein, which delivers blood to the coronary sinus, the middle cardiac vein which delivers blood to the coronary sinus, and the coronary sinus, which drains directly into the posterior aspect of the right atrium

Additional Main Coronary Veins

• Posterior vein of the left ventricle: parallels the posterior left ventricular branch
• Small cardiac vein: parallels the right coronary artery
• Anterior cardiac veins: branches from the right ventricle cardiac cells

The Coordination of Cardiac Contractions

• The cardiac cycle consists of alternate periods of contraction (systole) and relaxation (diastole)
• During atrial systole, blood flows into the ventricles
• During ventricular systole, blood is ejected into the pulmonary trunk and the ascending aorta
• During diastole, chambers are filling with blood as the heart relaxes

Cardiac Cycle Management

• Cardiac contractions of the cardiac cycle are coordinated by conducting cells
• There are two kinds of conducting cells: nodal cells and conducting cells
• Nodal cells include sinoatrial nodes and atrioventricular nodes
• The establish the rate of contractions and cell membranes automatically depolarize (autorhythmic)
• Conducting cells distribute the contractile stimulus to the myocardium

The Sinoatrial (SA) Node

• Located in the posterior wall of the right atrium near the entrance of the superior vena cava
• Called the cardiac pacemaker
• Pacemaker cells in the SA node automatically generate 80-100 action potentials per minute
• Bradycardia is a slower-than-normal heart rate
• Tachycardia is a faster-than-normal heart rate

The Atrioventricular (AV) Node

• Sits within the floor of the right atrium

Summary of Cardiac Conduction System

• Impulse travels from the SA node to the AV node and is conducted by internodal pathways; atrial contraction occurs
• The AV node slows impulse and transmits it to the AV bundle, which conducts the impulse along the interventricular septum. It then divides and branches to form the right and left bundle branches (servin right and left ventricles)
• The bundle branches conduct impulses to the Purkinje fibers, which connect to cardiac muscle cells, and subsequent ventricular contraction occurs

Autonomic Control of Heart Rate

• The SA node sets the heart rate but can be altered
• Impulses from the autonomic nervous system modify the pacemaker activity
• Nerves associated with the ANS innervate the SA node, the AV node, cardiac cells, and smooth muscles in the cardiac blood vessels
• Effects of norepinephrine (NE) and acetylcholine (ACh) on nodal tissue:
• Norepinephrine from the sympathetic division of the ANS causes an increase in the heart rate and the force of contractions
• Acetylcholine from the parasympathetic division of the ANS causes a decrease in the heart rate and a decrease in the force of contractions
• Cardiac centers in the medulla oblongata modify heart rate
• Stimulation of cardioacceleratory center: activates sympathetic neurons, heart rate increases
• Stimulation of cardioinhibitory center: activates parasympathetic neurons (Vagus [N X] is involved), heart rate decreases