Cardiovascular System: Heart Anatomy

• The cardiovascular system is a closed system of the heart and blood vessels.
• The main functions of the cardiovascular system are to deliver oxygen and nutrients to cells and tissues, and remove carbon dioxide and other waste products.

Heart Location and Orientation

• The heart is located in the thorax, between the lungs in the inferior mediastinum.
• The pointed apex of the heart is directed towards the left hip, while the base points toward the right shoulder.
• The heart is about the size of a fist.

Coverings and Walls of the Heart

• The heart is enclosed in the pericardium, a double-walled sac.
• The fibrous pericardium is the loose and superficial outer layer.
• The deep serous membrane has two layers: the parietal pericardium lining the fibrous pericardium, and the visceral pericardium (epicardium) next to the heart.
• Serous fluid fills the space between the layers of the pericardium.
• The heart wall has three layers: the epicardium (visceral pericardium), the myocardium (mostly cardiac muscle), and the endocardium (endothelium).

Chambers and Great Vessels

• The right and left sides of the heart act as separate pumps.
• The heart has four chambers: the right and left atria (receiving chambers) and the right and left ventricles (discharging chambers).
• The right atrium receives blood from the vena cava, and the left atrium receives blood from the pulmonary veins.
• The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the aorta.
• The interventricular septum separates the two ventricles, and the interatrial septum separates the two atria.

Pulmonary Circulation

• Pulmonary circulation carries blood to the lungs for gas exchange and then back to the heart.
• Blood is pumped from the right ventricle through the pulmonary trunk to the pulmonary arteries, carrying oxygen-poor blood to the lungs.
• Oxygen-rich blood returns to the left atrium via the pulmonary veins.

Systemic Circulation

• Systemic circulation involves blood flow from the left side of the heart through body tissues and back to the right side of the heart.
• Blood is pumped from the left ventricle into the aorta.
• Oxygen-poor blood returns to the right atrium via systemic veins, which empty into the superior and inferior vena cava.

Heart Valves

• Heart valves ensure unidirectional blood flow.
• Atrioventricular (AV) valves located between the atria and ventricles include the bicuspid (mitral) valve on the left and the tricuspid valve on the right.
• Semilunar valves are located between the ventricles and arteries: the pulmonary semilunar valve and the aortic semilunar valve.
• AV valves are anchored by chordae tendineae and open during heart relaxation, closing during ventricular contraction.
• Semilunar valves close during heart relaxation and open during ventricular contraction.
• Valve operation is based on pressure changes in the heart.

Cardiac Circulation

• The blood in heart chambers does not nourish the myocardium.
• The heart possesses its own circulatory system: coronary arteries supply oxygenated blood, while cardiac veins drain the myocardium.
• The coronary sinus, a large vein on the posterior of the heart, receives blood from cardiac veins and empties into the right atrium.

Blood Flow Through the Heart

• Blood from the superior and inferior vena cava flows into the right atrium, passes through the tricuspid valve into the right ventricle.
• From the right ventricle, blood goes through the pulmonary semilunar valve into the pulmonary trunk, which splits into the pulmonary arteries to the lungs.
• In the lungs, blood picks up oxygen, drops off carbon dioxide, and returns to the heart through the pulmonary veins, entering the left atrium.
• Blood travels through the bicuspid valve into the left ventricle, then exits the heart via the aortic semilunar valve and aorta.

Intrinsic Conduction System

• Cardiac muscle can initiate its own contraction influenced by intrinsic and extrinsic factors.
• The intrinsic conduction system increases heart contraction rate and ensures the heart beats as a unit.
• The sinoatrial (SA) node acts as the heart's pacemaker; impulses spread through the atria to the atrioventricular (AV) node, then to the AV bundle, bundle branches, and Purkinje fibers.
• This system results in a "wringing" contraction of the ventricles.

Heart Contractions

• The intrinsic conduction system enforces a rate of about 75 beats per minute.
• Contraction is initiated at the SA node, spreading to the AV node, causing atrial contraction.
• The impulse passes through the AV bundle, bundle branches, and Purkinje fibers, leading to ventricular contraction and blood ejection into the aorta and pulmonary trunk.

Homeostatic Imbalances

• Heart block occurs when a damaged AV node releases the ventricles from SA node control, resulting in a slower heart rate.
• Ischemia is a lack of oxygen supply to the heart muscle.
• Fibrillation is a rapid, uncoordinated shuddering of the heart muscle.

Cardiac Cycle

• The cardiac cycle is one complete heartbeat.
• Systole is contraction, and diastole is relaxation.
• The heart beats approximately 75 times per minute, with a cardiac cycle length of 0.8 seconds.

Blood Pressure

• Blood pressure is expressed as systolic pressure (pressure when the heart beats) over diastolic pressure (pressure when the heart rests between beats).
• Mid-to-late diastole involves low heart pressure, passive blood flow into the atria and ventricles, closed semilunar valves, and open atrioventricular valves.

Ventricular Systole

• During ventricular systole, blood pressure rises, AV valves close ("lub" sound), and semilunar valves open, pushing blood into the pulmonary trunk and aorta while the atria relax and fill with blood.

Early Diastole

• Early diastole features all four valves briefly closed, the "dup" sound as semilunar valves close, and atrial refilling as heart pressure drops and AV valves open.

Homeostatic Imbalance: Faulty Valves

• Faulty valves reduce heart efficiency, causing abnormal heart sounds (murmurs).

Regulation of Heart Rate

• Heart rate is regulated by neural (ANS) controls, hormones and ions, and physical factors.

Vascular System

• The vascular system transports blood to the tissues and back to the heart.
• Arteries and arterioles carry blood away from the heart, capillary beds facilitate exchanges between tissue and blood, and venules and veins return blood to the heart.

Blood Vessel Anatomy

• Blood vessels (except capillaries) have three layers: tunica intima (friction-reducing lining), tunica media (smooth muscle & elastic tissue controlled by the sympathetic nervous system), and tunica externa (protective outermost covering).
• Arteries have a thicker tunica media than veins.
• Veins have a thinner tunica and valves to prevent backflow, and a larger lumen than arteries.
• Skeletal muscle contractions aid venous blood flow.
• Capillaries are one cell layer thick (tunica intima) to permit exchange.
• Capillary beds consist of a vascular shunt and true capillaries.
• Precapillary sphincters regulate blood flow into true capillaries; when closed, blood bypasses the local area via the vascular shunt.

Major Arteries of the Systemic Circulation

• The aorta is the largest artery, leaving the left ventricle.
• Its regions include the ascending aorta, aortic arch, thoracic aorta, and abdominal aorta.

Arterial Branches of the Abdominal Aorta

• The celiac trunk (first abdominal aorta branch) divides into the left gastric artery (stomach), splenic artery (spleen), and common hepatic artery (liver).
• The superior mesenteric artery supplies most of the small intestine and the first half of the large intestine.
• The left and right renal arteries supply the kidneys.
• The inferior mesenteric artery serves the second half of the large intestine.
• The left and right common iliac arteries are the final branches of the aorta.
• Internal iliac arteries serve the pelvic organs.
• External iliac arteries become the femoral artery in the thigh, then the popliteal artery, and finally the anterior and posterior tibial arteries.