The Cardiovascular System (Heart)
I. Structure of the Heart
A. Size and location
- The human heart is about 14cm long and 9cm wide
- Located within the mediastinum resting on the diaphragm.
B. Coverings of the Heart Diagram
- Fibrous Pericardium
- Serous Pericardium
- Parietal layer
- Pericardial cavity
- Visceral layer (epicardium)
C. Walls of the Heart composed of three distinct layers: Diagram
- Epicardium - protective layer, serous membrane with connective tissue covered by epithelium
- Myocardium - cardiac muscle, fibers arranged in planes separated by connective tissue
- Endocardium - epithelium and connective tissue containing elastic and collagenous fibers as well as Purkinje fibers which are specialized muscle fibers that conduct cardiac impulses
D. Heart Chambers, Valves, and Other Structures Diagram1 Diagram2 Diagram3
- atria (upper chambers)
- protruding appendages called auricles
- lined with pectinate muscles
- shallow depression, fossa ovalis (foramen ovale in fetal heart)
- ventricles (lower chambers)
- atrioventricular septum
- interventricular septum (longitudinal partition)
- trabeculae carneae (irregular muscle ridges)
- papillary muscles
- chordae tendinea
- semilunar valves
- pulmonary arteries and veins
- atrioventricular valves
E. Pathway through the Heart Diagram
Heart passages open vertically (atrium and ventricle) and the heart is divided into side-by-side pumps serving two blood circuits:
- Pulmonary circuit - receives blood returning to the heart from the body and sends it through the lungs for oxygenation
- Systemic circuit - supplies the entire body with oxygenated blood
BLOOD FLOW (low oxygen, high carbon dioxide) → venae cavae → rt. atrium → rt. atrium contraction → blood passes through tricuspid valve → blood enters rt. ventricle → rt. ventricle contracts and tricuspid closes → blood moves through pulmonary semilunar valve → blood moves into pulmonary trunk → pulmonary arteries transport blood to the lungs → blood is oxygenated → oxygenated blood transported back to heart (lft atrium) via pulmonary veins → left atrium contracts, blood moves through the bicuspid valve into lft ventricle → lft ventricle wall contracts, bicuspid valve closes → blood passes through the aortic semilunar valve into the aorta → blood sent to tissue/cells → blood becomes deoxygenated (low oxygen, high carbon dioxide) → blood flows through venous system back to venae cavae
F. Blood Supply to the Heart (Coronary Circulation) Diagram
i. Blood is supplied to the heart by the first two branches of the aorta, right and left coronary arteries. These arteries arise from the base of the aorta and encircle the heart in the atrioventricular groove.
Left coronary artery:
- anterior interventricular artery - supplies blood to the interventricular septum and anterior walls of both ventricles
- circumflex artery - supplies blood to the left atrium and the posterior walls of the left ventricle
Right coronary artery:
- Posterior interventricular artery - runs to the apex and supplies blood to the posterior ventricular walls
- Marginal artery - supplies blood to the myocardium of the right side of the heart
Coronary veins: Small, middle, and great cardiac leading to coronary sinus
ii. The myocardium needs a constant supply of oxygen in order for the heart to continually pump. Myocardium capillaries are branches of cardiac veins which join to form the coronary sinus, an enlarged vein which empties into the right atrium.
II. Heart Physiology
A. Cardiac Muscle Fibers are interconnected in branching networks (via intercalated disks) that spread in all directions through the heart. When any portion of the net is stimulated, an impulse travels to all of its parts, and the whole structure contracts as a unit. Unit contraction = functional syncytium (e.g. atrial and ventricular syncytium).
B. Cardiac Conduction System is a system of specialized muscle fibers that conduct cardiac impulses from the sinoatrial node into the myocardium.
- The sinoatrial (S-A) node, also called the pacemaker, is a group of specialized cells in the wall of the right atrium that initiates cardiac cycles and regulates regular rhythmic contractions (sinus rhythm) of the heart. Cardiac impulses travel from the S-A node into atria causing atrial syncytium (contraction).
- The cardiac impulse then travels along fibers to the atrioventricular (A-V) node which is specialized muscle tissue localized in the floor of the right atrium.
- Junctional fibers conducting impulses from the S-A node into the A-V node have small diameters slightly hindering the impulse so as to allow the atria to empty into the ventricles.
- From the A-V node, cardiac impulses travel into a group of large fibers(A-V bundle, bundle of his) then runs down the interventricular septum.
- Towards the apex of the heart, the A-V bundle divides into right and left branches that give rise to Purkinje fibers which are widely distributed throughout the cardiac muscle and when stimulated causes ventricular contraction.
NOTE: Heart is controlled by the cardiac centers in the medulla oblongata and innervated by the autonomic nervous system
- Sympathetic - neuron from cardioacceleratory center synapses at thoracic spinal cord. Postganglionic fibers innervate the SA node, AV node and left ventricle
- Parasympathetic - neuron from cardioinhibtory center synapses in the dorsal motor nucleus of the medulla. Vagus nerve sends impulse to the SA and AV node.
C. Electrocardiogram (ECG) is a recording of the electrical changes in the myocardium during a cardiac cycle and consists of a series of three distinguishable waves called deflection waves: Diagram
- P wave = results from the movement of the depolarization wave from the S-A node through the atria, leading to atrial contraction
- QRS wave = ventricular depolarization
- T wave = ventricular repolarization
D. Cardiac Cycle (Pressure within the chambers of the heart rises and falls) Diagram1 Diagram2
1. Ventricular Filling
- heart in total relaxation, mid-to-late diastole
- pressure in heart is low, blood returning to heart
- rapid ventricular filling (70% of ventricle filling), blood flows passively into atrium through open AV valves into ventricles
- AV valve flaps drift upward to closed position
- atriole systole, atrial pressure rises, propels blood into resting ventricles
- ventricles now contain the maximum volume of blood called the end-diastolic volume (EDV)
2. Ventricular Systole
- blood is pushed up against AV valves, forcing them shut
- All four valves closed (isovolumetric contraction)
- Pressure rises
- Semilunar valves open
- Ventricular ejection occurs
- Ventricles start to relax, semilunar valves close
- Blood remaining - end systolic volume (ESV)
2. Relaxation (Quiescent) Period (from T wave to P wave)
- Ventricles start to relax, all four chambers in diastole
- Pressure within chambers drops, blood starts to flow from pulmonary trunk and aorta toward ventricles
- Blood becomes trapped in semilunar cusps, valves close
- Rebound of blood closed cusps (dicrotic wave on aortic pressure curve)
- Isovolumetric relaxation occurs --> interval when ventricular blood volume does not change because both semilunar and atrioventricular valves are closed
- As ventricles relax, space inside expands, and pressure falls
- When ventricular pressure drops below atrial pressure, atrioventricular valves open and ventricular filling begins
NOTE: CARDIAC OUTPUT = HR X SV. Stroke volume is equal to the difference between EDV and ESV. At rest CO = HR (75 beats/min) X SV (70 ml/beat) = 5250 ml/min
III. Clinical Aspects