The Cardiovascular System: Structure, Function, and Circulation

The Cardiovascular System

Overview

The cardiovascular system is a closed system composed of the heart and blood vessels. Its primary function is to deliver oxygen and nutrients to tissues and remove carbon dioxide and waste products.

• Heart: Pumps blood throughout the body.

• Blood Vessels: Allow blood to circulate to all parts of the body.

The Heart

Location and Orientation

• Located in the thorax between the lungs, within the inferior mediastinum.

• The pointed apex is directed toward the left hip; the base points toward the right shoulder.

• Approximately the size of a fist.

Heart Coverings

• Pericardium: A double-walled sac surrounding the heart.

• Fibrous pericardium: Loose and superficial outer layer.

• Serous pericardium: Deep to the fibrous pericardium, composed of two layers:

  • Visceral pericardium (epicardium): Next to the heart.

  • Parietal pericardium: Lines the inner surface of the fibrous pericardium.

  • Serous fluid: Fills the space between the layers, reducing friction.

Heart Wall Structure

• Epicardium: Outer layer (visceral pericardium).

• Myocardium: Middle layer, composed mostly of cardiac muscle, responsible for contractions.

• Endocardium: Inner layer, made of endothelium.

Heart Chambers

• The heart is divided into right and left sides, acting as separate pumps.

• Four chambers:

  • Atria: Receiving chambers (right and left atrium).

  • Ventricles: Discharging chambers (right and left ventricle).

Heart Septa

• Interventricular septum: Separates the two ventricles.

• Interatrial septum: Separates the two atria.

Heart Valves

Valves ensure unidirectional blood flow and prevent backflow.

• Atrioventricular (AV) valves: Between atria and ventricles.

  • Bicuspid (mitral) valve: Left side of heart.

  • Tricuspid valve: Right side of heart.

  • Anchored by chordae tendineae ("heart strings").

  • Open during heart relaxation, closed during ventricular contraction.

• Semilunar valves: Between ventricle and artery.

  • Pulmonary semilunar valve

  • Aortic semilunar valve

  • Closed during heart relaxation, open during ventricular contraction.

Circulatory Pathways

Systemic and Pulmonary Circulations

• Systemic circulation: Blood flows from the left side of the heart through the body tissues and back to the right side.

• Pulmonary circulation: Blood flows from the right side of the heart to the lungs and back to the left side.

Associated Great Vessels

• Arteries:

  • Aorta: Leaves the left ventricle.

  • Pulmonary arteries: Leave the right ventricle.

• Veins:

  • Superior and inferior vena cava: Enter the right atrium.

  • Pulmonary veins (four): Enter the left atrium.

Blood Flow Through the Heart

1. Superior and inferior venae cavae dump blood into the right atrium.

2. Blood passes through the tricuspid valve to the right ventricle.

3. From the right ventricle, blood leaves via the pulmonary semilunar valve into the pulmonary trunk.

4. Pulmonary trunk splits into right and left pulmonary arteries, carrying blood to the lungs.

5. Oxygen is picked up and carbon dioxide is dropped off in the lungs.

6. Oxygen-rich blood returns to the heart through the four pulmonary veins.

7. Blood enters the left atrium, passes through the bicuspid valve into the left ventricle.

8. From the left ventricle, blood is pumped through the aortic semilunar valve into the aorta and systemic circulation.

Coronary Circulation

• Blood in the heart chambers does not nourish the myocardium.

• The heart has its own nourishing circulatory system:

  • Coronary arteries: Branch from the aorta to supply the heart muscle with oxygenated blood.

  • Cardiac veins: Drain the myocardium of blood.

  • Coronary sinus: Large vein on the posterior of the heart, receives blood from cardiac veins and empties into the right atrium.

Heart Conduction System

Intrinsic Conduction System (Nodal System)

• Heart muscle cells contract without nerve impulses, in a regular, continuous way.

• Special tissue sets the pace:

  • Sinoatrial node (SA node): Pacemaker, located in the right atrium.

  • Atrioventricular node (AV node): Junction of atria and ventricles.

  • Atrioventricular bundle (AV bundle or bundle of His): In the interventricular septum.

  • Bundle branches: In the interventricular septum.

  • Purkinje fibers: Spread within the ventricle wall muscles.

Heart Contractions

1. Contraction is initiated by the SA node.

2. Impulse spreads to the AV node, then the atria contract.

3. At the AV node, the impulse passes through the AV bundle, bundle branches, and Purkinje fibers, causing the ventricles to contract.

• Tachycardia: Rapid heart rate over 100 beats per minute.

• Bradycardia: Slow heart rate less than 60 beats per minute.

The Cardiac Cycle and Output

Cardiac Cycle

• Atria contract simultaneously, then relax as ventricles contract.

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Events of one complete heartbeat:

  1. Mid-to-late diastole: Blood flows from atria into ventricles.

  2. Ventricular systole: Blood pressure builds before ventricular contraction, pushing out blood.

  3. Early diastole: Atria finish refilling, ventricular pressure is low.

Cardiac Output (CO)

• Amount of blood pumped by each side (ventricle) of the heart in one minute.

• Formula: $CO = HR \times SV$

• Heart rate (HR): Typically 75 beats per minute.

• Stroke volume (SV): Volume of blood pumped by each ventricle in one contraction (about 70 mL/beat).

• Example calculation: $CO = 75\, \text{beats/min} \times 70\, \text{mL/beat} = 5250\, \text{mL/min}$

• Changing heart rate is the most common way to change cardiac output.

• Starling's Law of the Heart: The more the cardiac muscle is stretched, the stronger the contraction.

Regulation of Heart Rate

• Increased heart rate:

  • Sympathetic nervous system (crisis, low blood pressure)

  • Hormones (epinephrine, thyroxine)

  • Exercise

  • Increased blood volume

• Decreased heart rate:

  • Parasympathetic nervous system

  • High blood pressure or blood volume

  • Decreased venous return

Blood Vessels: The Vascular System

Functions and Types

• Transport blood to the tissues and back.

• Arteries: Carry blood away from the heart.

• Arterioles: Small branches of arteries.

• Capillary beds: Sites of exchange between tissues and blood.

• Venules: Small veins returning blood toward the heart.

• Veins: Carry blood toward the heart.

Microscopic Anatomy of Blood Vessels

• Three layers (tunics):

  • Tunica intima: Endothelium (inner layer).

  • Tunica media: Smooth muscle, controlled by the sympathetic nervous system.

  • Tunica externa: Mostly fibrous connective tissue (outer layer).

Differences Between Blood Vessels

• Walls of arteries are the thickest.

• Lumens of veins are larger.

• Larger veins have valves to prevent backflow.

• Skeletal muscle "milks" blood in veins toward the heart.

• Walls of capillaries are only one cell layer thick to allow for exchanges.

Capillary Beds

• Consist of two types of vessels:

  • Vascular shunt: Directly connects an arteriole to a venule.

  • True capillaries: Exchange vessels where oxygen and nutrients cross to cells, and carbon dioxide and metabolic waste products cross into blood.

Major Arteries and Veins of Systemic Circulation

Aorta

• Largest artery in the body.

• Leaves the left ventricle of the heart.

• Regions:

  • Ascending aorta: Leaves the left ventricle.

  • Aortic arch: Arches to the left.

  • Thoracic aorta: Travels downward through the thorax.

  • Abdominal aorta: Passes through the diaphragm into the abdominopelvic cavity.

Arterial Branches of the Aorta

• Brachiocephalic trunk: Splits into the right common carotid artery and right subclavian artery.

• Left common carotid artery: Splits into left internal and external carotid arteries.

• Left subclavian artery: Branches into the vertebral artery; in the axilla, becomes the axillary artery, then brachial, radial, and ulnar arteries.

Arterial Branches of the Thoracic and Abdominal Aorta

• Thoracic aorta supplies the lungs (bronchial arteries), esophagus (esophageal arteries), and diaphragm (phrenic arteries).

• Abdominal aorta branches:

  • Celiac trunk: First branch, divides into left gastric artery (stomach), splenic artery (spleen), and common hepatic artery (liver).

  • Superior mesenteric artery: Supplies most of the small intestine and first half of the large intestine.

  • Renal arteries: Serve the kidneys.

  • Gonadal arteries: Ovarian arteries in females, testicular arteries in males.

  • Lumbar arteries: Serve muscles of the abdomen and trunk.

  • Inferior mesenteric artery: Serves the second half of the large intestine.

  • Common iliac arteries: Final branches, serve the pelvic organs and lower limbs.

Major Veins of Systemic Circulation

• Superior and inferior vena cava: Enter the right atrium.

• Superior vena cava: Drains the head and arms.

• Inferior vena cava: Drains the lower body.

• Veins draining into the superior vena cava include:

  • Radial and ulnar veins → brachial vein → axillary vein

  • Cephalic vein (lateral aspect of arm), basilic vein (medial aspect), joined by the median cubital vein

  • Subclavian vein, external and internal jugular veins, vertebral vein

  • Brachiocephalic veins (right and left) form the superior vena cava

  • Azygos vein drains the thorax

• Veins draining into the inferior vena cava include:

  • Anterior and posterior tibial veins, fibial veins, popliteal vein, femoral vein, external iliac vein

  • Great saphenous veins (longest veins in the body)

  • Common iliac veins (formed by the union of internal and external iliac veins)

  • Right gonadal vein drains the right ovary/testis; left gonadal vein drains into the left renal vein

  • Renal veins drain the kidneys

  • Hepatic veins drain the liver

Special Circulations

Arterial Supply of the Brain

• Internal carotid arteries: Supply most of the cerebrum.

• Vertebral arteries: Join to form the basilar artery, serving the brain stem and cerebellum.

• Circle of Willis: A complete circle of connecting blood vessels at the base of the brain, uniting anterior and posterior blood supplies.

Fetal Circulation

• Fetus receives exchanges of gases, nutrients, and wastes through the placenta.

• Umbilical vein: Carries oxygen and nutrient-rich blood to the fetus.

• Umbilical arteries (2): Carry carbon dioxide and waste-laden blood from fetus to placenta.

• Blood bypasses the liver via the ductus venosus and enters the inferior vena cava.

• Blood bypasses the lungs via the foramen ovale (shunt between atria) and ductus arteriosus (connects aorta and pulmonary trunk).

Hepatic Portal Circulation

• Veins of hepatic portal circulation drain digestive organs, spleen, and pancreas.

• Major vessels: Inferior and superior mesenteric veins, splenic vein, left gastric vein.

Pulse and Blood Pressure

Pulse

• Pressure wave of blood, felt at "pressure points" in arteries.

• Normal pulse: 70–76 beats per minute at rest.

Blood Pressure (BP)

• Measured in large arteries.

• Systolic pressure: At the peak of ventricular contraction.

• Diastolic pressure: When ventricles relax.

• Normal BP: 120/80 mm Hg (systolic/diastolic).

• BP decreases as distance from the heart increases.

Factors Affecting Blood Pressure

• Age, weight, time of day, exercise, body position, emotional state.

• Cardiac output (CO): Amount of blood pumped out of the left ventricle per minute.

• Peripheral resistance (PR): Amount of friction blood encounters as it flows through vessels.

• BP formula: $BP = CO \times PR$

• Narrowing of blood vessels and increased blood volume increase PR and BP.

• Neural factors: Autonomic nervous system adjustments (sympathetic division).

• Temperature: Heat causes vasodilation; cold causes vasoconstriction.

• Chemicals: Various substances can increase or decrease BP.

Blood Pressure Variations

• Normal range: 110–140 mm Hg systolic, 75–80 mm Hg diastolic.

• Hypotension: Low systolic BP (below 110 mm Hg), often associated with illness.

• Hypertension: High systolic BP (above 140 mm Hg), can be dangerous if chronic.

Capillary Exchange and Fluid Movements

Mechanisms of Capillary Exchange

• Substances exchanged due to concentration gradients.

• Oxygen and nutrients leave the blood; carbon dioxide and wastes leave the cells.

• Mechanisms:

  • Direct diffusion across plasma membranes

  • Endocytosis or exocytosis

  • Some capillaries have gaps (intercellular clefts) or fenestrations (pores)

Fluid Movements in Capillary Beds

• Blood pressure forces fluid and solutes out of capillaries at the arterial end.

• Osmotic pressure draws fluid into capillaries at the venous end.

• Balance between these pressures determines net fluid movement.

Developmental Aspects and Aging

• The heart becomes a four-chambered organ by the end of seven weeks of embryonic development.

• Few structural changes occur after the seventh week.

• Aging problems associated with the cardiovascular system include:

  • Venous valves weaken (leading to varicose veins)

  • Progressive atherosclerosis

  • Loss of elasticity of vessels leads to hypertension

  • Coronary artery disease from fatty, calcified deposits