Circulatory System Structures and Cardiac Cycle

Structures of the Circulatory System and Great Vessels

• The circulatory system includes the heart, blood vessels (arteries, veins, capillaries), and the lymphatic system
• The great vessels include the aorta, superior vena cava, and pulmonary artery
• The heart is composed of the endocardium, myocardium, visceral pericardium (epicardium), parietal pericardium, pericardial cavity and fibrous pericardium

Cardiac Cycle

• One cardiac cycle has one contraction and one relaxation. This happens for each heartbeat
• Diastole is when the ventricles relax and fill with blood
• Systole is when the ventricles contract and blood leaves the ventricles

Coronary Vessels

• The right coronary artery (RCA) supplies blood to the right atrium, right ventricle, the bottom portion of the left ventricle,and the back of the septum
• The left coronary artery divides into two branches: the circumflex artery and the left anterior descending artery
• The circumflex artery supplies blood to the left atrium and the side and back of the left ventricle
• The left anterior descending artery (LAD) supplies blood to the front and bottom of the left ventricle and the front of the septum
• Coronary veins take oxygen-depleted blood from the heart muscle and return it to the right atrium

Myocardial Cells and Contraction

• Myocardial cells include striations, intercalated disks, muscle fibers and the nucleus
• Myocardial cells also allow for longitudinal portion with large gap junctions and sarcomeres

ANS Innervation

• The autonomic nervous system (ANS) is involved in regulating bodily functions, including heart rate and blood pressure
• Sympathetic nerve fiber stimulation (+) increases heart rate and contractility
• Parasympathetic nerve fiber stimulation (-) decreases heart rate and contractility
• Inotropic deals with the strength of contraction
• Chronotropic deals with the heart rate

Factors Affecting Cardiac Output

• Cardiac output (CO) is the product of heart rate and stroke volume
• Preload is the volume of blood in the ventricles at the end of diastole (end-diastolic pressure)
• Afterload is the resistance the left ventricle must overcome to circulate blood
• Normal cardiac output at rest is 5 liters per minute (5/L)

Frank Starling Law of the Heart

• States that the energy of contraction is proportional to the initial length of the cardiac muscle fiber
• The law defines the relationship between stroke volume and end-diastolic volume
• Myocardial stretch determines the force of myocardial contraction

Blood Pressure

• Blood pressure is the product of cardiac output and peripheral resistance
• Cardiac Output is affected by alterations to the heart rate, stroke volume, or both
• Peripheral resistance is impacted by neural control of resistance and hormones
• Factors that regulate blood pressure include baroreceptors, chemoreceptors (O2, CO2, pH), epinephrine/norepinephrine, antidiuretic hormone, the RAAS system (aldosterone, angiotensin), and natriuretic peptides (ANP, BNP)
• Vasodilators like nitric oxide lower blood pressure

Structure of Blood Vessels

• Arteries are thicker and carry oxygenated blood in systemic circulation
• The two types of arteries: Elastic Arteries and Muscular arteries
• Capillaries allow for substances to move through junctions between endothelial cells, fenestrations, active transport, and diffusion. Blood flow is controlled by contraction/relaxation of smooth muscle
• Veins have thin walls, are fibrous, and do not recoil quickly after distention
• Some veins contain valves, and muscle pump aids in venous return

Factors Affecting Blood Flow

• Blood flow is affected by the diameter of the vessel
• Viscosity affects blood flow, higher viscosity decreases speed
• Laminar flow is normal vs turbulent flow is faster and disorganized
• Stiff arteries versus Elastic arteries determine vascular compliance

Lymphatic System

• The lymphatic system aids in returning fluid from blood capillary bed to the heart

Assessments and Tests of the Cardiovascular Function

• Assessments include thorough history and physical examination, risk factors, symptoms, vital signs, LOC, and mucous membrane color
• Tests to assess cardiovascular function include chest x-ray, ECG (single strip vs 12 lead), echocardiography, stress test, electrophysiology, catheterization with angiography, vascular doppler ultrasonography, vascular CT and MRI, venography, and arteriography

Aging and the Cardiovascular System

• Cardiovascular disease is the most common cause of morbidity and mortality in older adults
• The physiologic changes include arterial stiffening, changes in neurogenic control over vascular tone, increased occurrence of atrial fibrillation, loss of exercise capacity, and left ventricular hypertrophy and fibrosis
• Arterial stiffening involved increased collagen, changes in elastin, and decreased baroreceptor activity