Cardiovascular System: Structure, Function, and Blood Composition

Cardiovascular System Overview

General Functions

The cardiovascular system is responsible for the transport of blood, nutrients, hormones, wastes, and heat throughout the body. It also plays a crucial role in protection against disease and fluid loss (clotting).

• Transport: Delivers oxygen, nutrients, and hormones to tissues; removes waste products.

• Protection: Immune cells in blood defend against pathogens; clotting prevents fluid loss.

• Regulation: Maintains homeostasis of temperature, pH, and fluid balance.

Heart

Location and Structure

The heart is located in the mediastinum, the space between the lungs within the thoracic cavity. It is surrounded by a double-walled sac called the pericardium.

Coverings: Pericardium

• Fibrous Pericardium: Dense irregular connective tissue; protects and anchors the heart.

• Serous Pericardium: Composed of two layers:

  • Parietal pericardium (outer layer, attached to fibrous pericardium)

  • Visceral pericardium (inner layer, also called epicardium, attached to heart surface)

• Between layers is pericardial cavity with serous fluid for lubrication.

Heart Wall

• Epicardium: Outer layer, same as visceral pericardium.

• Myocardium: Middle layer, cardiac muscle arranged in spiral/circular patterns.

• Endocardium: Inner layer, simple squamous epithelium lining heart chambers and valves.

Chambers and Associated Blood Vessels

• Right Atrium: Receives deoxygenated blood from body via three major veins.

• Left Atrium: Receives oxygenated blood from lungs via pulmonary veins.

• Right Ventricle: Pumps deoxygenated blood to lungs via pulmonary trunk and arteries.

• Left Ventricle: Pumps oxygenated blood to body via aorta.

Septa

• Interatrial Septum: Separates atria.

• Interventricular Septum: Separates ventricles.

Cardiac (Fibrous) Skeleton

• Dense connective tissue separating atria and ventricles.

• Provides attachment for cardiac muscles and supports heart valves.

• Prevents simultaneous contraction of atria and ventricles.

Heart Valves

• Atrioventricular (AV) Valves:

  • Bicuspid (mitral) valve: Between left atrium and ventricle; two cusps.

  • Tricuspid valve: Between right atrium and ventricle; three cusps.

• Semilunar Valves:

  • Aortic valve: Separates left ventricle and aorta.

  • Pulmonary valve: Separates right ventricle and pulmonary trunk.

Cardiac Muscle Cells

• Contractile Cells: Form majority of myocardium; striated, branched, single nucleus, connected by intercalated discs.

• Conduction System Cells: Modified cardiac muscle cells that produce and conduct electrical impulses; do not contract.

Conduction System Parts

• Sinoatrial (SA) Node: Pacemaker, initiates heartbeat.

• Atrioventricular (AV) Node: Delays impulse, located at base of right atrium.

• AV Bundle (Bundle of His): Conducts impulse from AV node to ventricles.

• Purkinje Fibers: Terminal fibers in ventricles, spread impulse for contraction.

Anatomy of Blood Vessels

General Structure

• Tunica externa: Connective tissue (CT).

• Tunica media: Smooth muscle, elastic fibers.

• Tunica intima/interna: Endothelium (simple squamous epithelium), continuous with endocardium.

• Lumen: Central cavity containing blood.

Types of Blood Vessels

• Arteries: Carry blood away from heart; thick walls, high pressure.

• Arterioles: Small arteries, regulate blood flow and pressure.

• Capillaries: Smallest vessels, one cell layer thick; site of exchange of gases, nutrients, and wastes.

• Venules: Collect blood from capillaries.

• Veins: Carry blood toward heart; thin walls, large lumen, valves prevent backflow.

Blood Vessel Classification Table

Type	Function	Wall Structure
Arteries	Carry blood away from heart	Thick tunica media, elastic fibers
Arterioles	Regulate blood flow/pressure	Thin tunica media
Capillaries	Exchange of gases/nutrients	Endothelium only
Venules	Collect blood from capillaries	Endothelium, thin media
Veins	Carry blood to heart	Thin walls, valves

Circulatory Routes

General Principles

• Closed, double circulation: Blood confined to heart and vessels; two routes: pulmonary and systemic.

Adult Circulation

• Pulmonary Circulation: Right ventricle to lungs (deoxygenated blood), returns to left atrium (oxygenated blood).

• Systemic Circulation: Left ventricle to body (oxygenated blood), returns to right atrium (deoxygenated blood).

• Coronary Circulation: Supplies heart muscle itself; includes coronary arteries, veins, and sinus.

Fetal Circulation

• Umbilical vein: Carries oxygenated blood from placenta to fetus.

• Ductus venosus: Bypasses fetal liver, connects umbilical vein to inferior vena cava.

• Foramen ovale: Shunt between right and left atria, bypasses fetal lungs.

• Ductus arteriosus: Connects pulmonary trunk to aorta, bypasses fetal lungs.

• Umbilical arteries: Return mixed blood to placenta.

Blood Composition

Physical Properties

• Blood is a liquid connective tissue with higher viscosity than water.

• pH: 7.35–7.45

• Volume: 4–6 L in adults

Composition Overview

• Plasma: ~55–60% of blood volume; water, proteins, solutes.

• Formed elements: ~40–45% of blood volume; cells and cell fragments.

Plasma Details

• Water: ~90% of plasma.

• Proteins: ~8% of plasma; includes:

  • Albumin: Most abundant, maintains osmotic pressure.

  • Fibrinogen: Clot formation.

  • Globulins: Immunity (antibodies), transport.

• Other solutes: Nutrients, hormones, electrolytes, wastes, gases.

Formed Elements

• Red Blood Cells (Erythrocytes): Most abundant; biconcave discs, no nucleus when mature, contain hemoglobin for oxygen transport.

  • Life span: 120 days

  • Destroyed in liver and spleen

• White Blood Cells (Leukocytes): Defend against disease; two main types:

  • Granulocytes: Visible granules; includes neutrophils (phagocytosis), eosinophils (parasite defense), basophils (inflammation).

  • Agranulocytes: No visible granules; includes lymphocytes (immunity), monocytes (phagocytosis).

• Platelets: Cell fragments involved in clotting; lifespan 5–10 days.

Hemopoiesis/Hematopoiesis

Formation of Blood Cells

• All blood cells originate from hematocytoblasts (stem cells) in red bone marrow.

• Adult red marrow is found in axial skeleton and pelvic/pectoral girdles.

Clinical Considerations

Congenital Heart Defects

• Most common type: Incomplete closure of foramen ovale.

Atherosclerosis

• Build-up of fat deposits in tunica intima, leading to narrowing/blockage of arteries.

Key Equations

• Blood Flow Equation: $Q = \frac{\Delta P}{R}$ Where $Q$ is blood flow, $\Delta P$ is pressure difference, and $R$ is resistance.

• Cardiac Output: $CO = HR \times SV$ Where $CO$ is cardiac output, $HR$ is heart rate, and $SV$ is stroke volume.

Summary Table: Blood Cell Types

Type	Main Function	Key Features
Red Blood Cells	Oxygen transport	Biconcave, no nucleus, hemoglobin
Neutrophils	Phagocytosis	Granules, most abundant WBC
Eosinophils	Parasite defense	Granules, allergic response
Basophils	Inflammation	Granules, histamine release
Lymphocytes	Immunity	B and T cells, agranular
Monocytes	Phagocytosis	Largest WBC, agranular
Platelets	Clotting	Cell fragments, no nucleus

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.