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The Circulatory System

Understanding the body's vital transport network

Introduction

Energy Delivery to Every Cell

The cells of each organ receive energy from food substances taken into the body. Food contains stored energy that converts into active energy through catabolism—the chemical process where food and oxygen combine in cells.

Each cell depends on a constant supply of food and oxygen to receive sufficient energy for proper functioning. The cardiovascular system ensures this delivery through blood, vessels, and the heart.

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System Overview

The Cardiovascular Transport System

The cardiovascular system consists of blood (fluid), vessels (transport pathways), and the heart (muscular pump). This system transports food and oxygen to all organs and cells whilst removing waste materials.

Oxygen Absorption

Blood vessels in the lungs absorb oxygen inhaled from air

Nutrient Uptake

Blood vessels in small intestine absorb food substances from digestive tract

Waste Removal

Blood vessels carry carbon dioxide and urea to lungs and kidneys for elimination

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Blood Vessels

Three Major Types of Blood Vessels

01

Arteries

Large vessels leading blood away from heart with strong, elastic walls

02

Capillaries

Microscopic vessels with walls one cell thick for nutrient exchange

03

Veins

Vessels conducting oxygen-poor blood back toward the heart

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Arteries: The High-Pressure Pathways

Arteries are large blood vessels leading blood away from the heart. Their walls comprise connective tissue, elastic fibres, and an innermost layer of epithelial cells called endothelium.

Endothelial cells line all blood vessels and secrete substances affecting vessel size (dilation), blood clotting, and blood vessel growth. Because arteries carry blood away from the heart, they must withstand high pressure from the heart's pumping action.

Their elastic walls expand as the heartbeat forces blood into the arterial system throughout the body. Smaller branches called arterioles carry blood to the tiniest vessels—the capillaries.

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Capillaries: The Exchange Network

Capillaries have walls only one endothelial cell thick. These delicate, microscopic vessels carry nutrient-rich, oxygenated blood from arteries and arterioles to body cells.

Oxygen & Nutrients Out

Thin walls allow passage into tissue fluid surrounding cells

Energy Release

Nutrients burn with oxygen (catabolism) to release energy within cells

Waste Products In

Carbon dioxide and water pass from cells into capillaries

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Veins: The Return Journey

Veins are thinner-walled than arteries and conduct blood (having given up most oxygen) toward the heart from tissues. They contain little elastic tissue and less connective tissue than arteries.

Blood pressure in veins is extremely low compared with arterial pressure. To keep blood moving back toward the heart, veins have valves preventing backflow and maintaining one-directional flow. Muscular action also helps blood movement in veins.

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Heart Anatomy

The Human Heart: A Remarkable Pump

<1 lb

Weight

Less than a pound

4

Chambers

Two atria and two ventricles

2

Pump Stations

Double pump synchronized carefully

The human heart weighs less than a pound, is roughly the size of an adult fist, and lies in the thoracic cavity just behind the breastbone in the mediastinum (between the lungs). It consists of four chambers: two upper chambers called atria and two lower chambers called ventricles.

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The Double Pump System

Pump station number one (right side) sends oxygen-deficient blood to the lungs, where blood picks up oxygen and releases carbon dioxide. The newly oxygenated blood returns to the left side (pump station number two) and doesn't mix with oxygen-poor blood.

Pump station number two forces oxygenated blood out to all body parts. At body tissues, blood loses oxygen and returns to pump station number one, where oxygen-poor blood is sent to lungs to begin the cycle anew.

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Blood Flow

Right Side: Oxygen-Poor Blood Pathway

Venae Cavae Entry

Superior and inferior vena cava bring oxygen-poor blood to right atrium

Right Atrium Contraction

Forces blood through tricuspid valve into right ventricle

Right Ventricle Pumps

Contracts to pump blood through pulmonary valve into pulmonary artery

To the Lungs

Pulmonary artery branches carry oxygen-deficient blood to each lung

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Pulmonary Circulation: Gas Exchange

Blood entering lung capillaries from the pulmonary artery soon loses its large quantity of carbon dioxide into lung tissue, and the carbon dioxide is expelled.

Simultaneously, oxygen enters the capillaries of the lungs and is brought back to the heart within the pulmonary vein. Several pulmonary veins transport oxygen-rich blood back to the heart.

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Left Heart

Left Side: Oxygen-Rich Blood Pathway

Oxygen-rich blood returns to the heart from pulmonary veins. The left atrium walls contract to force blood through the mitral valve into the left ventricle.

Thickest Walls

The left ventricle has walls three times thicker than the right ventricle

Powerful Pumping

Must pump blood with great force to travel through arteries to all body parts

Aortic Exit

Blood pumps out through aortic valve into aorta, which branches throughout body

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Heart Structure: Layers and Septa

Septa

Partitions separating the four heart chambers

Endocardium

Smooth layer of endothelial cells lining interior of heart and valves

Myocardium

Middle, muscular layer—the thickest layer of heart wall

Pericardium

Fibrous and membranous sac surrounding the heart

The pericardium comprises two layers: visceral pericardium (adheres to heart) and parietal pericardium (lines outer fibrous coat). The pericardial cavity normally contains 10-15 ml of fluid, lubricating membranes as the heart beats.

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Cardiac Cycle

Diastole and Systole: The Heartbeat Phases

Diastole (Relaxation)

Ventricle walls relax; blood flows into heart from venae cavae and pulmonary veins

Systole (Contraction)

Ventricle walls contract; blood pumps into pulmonary artery and aorta

This diastole-systole cardiac cycle occurs 70-80 times per minute. The beat of the heart felt through artery walls is called the pulse.

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Conduction System: The Heart's Electrical Network

A small region of specialised muscle tissue in the posterior portion of the right atrium generates electrical impulses. This region is the sinoatrial node (SA Node), also called the pacemaker of the heart.

1

SA Node (Pacemaker)

Generates electrical impulse in right atrium

2

Atrial Contraction

Current causes atria walls to contract, forcing blood into ventricles

3

AV Node

Wave passes to atrioventricular node at interatrial septum

4

Bundle of His

Excitation wave sent to specialised muscle fibres

5

Ventricular Contraction

Bundle branches carry impulse to ventricles, causing systole

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Electrocardiogram

Recording Heart's Electrical Activity

The record used to detect electrical changes in heart muscles as the heart beats is called an electrocardiogram (EKG or ECG).

Like ripples in a pond when a stone is thrown, the wave of electricity passes from the pacemaker across the myocardium. A short rest period follows, then the pacemaker begins the wave of excitation across the heart again.

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Medical Terminology

Essential Cardiovascular Vocabulary

Aorta

Largest artery in the body

Arteriole

Small artery

Capillary

Small blood vessel where materials pass through thin walls

Coronary Arteries

Vessels branching from aorta carrying oxygen-rich blood to heart muscle

Endothelium

Innermost lining of blood vessels

Murmur

Abnormal heart sound from improper valve closure

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More Key Terms

Mitral Valve

Valve between left atrium and left ventricle

Pacemaker

Sensitive tissue in right atrium beginning heartbeat (sinoatrial node)

Pulmonary Circulation

Blood flow from heart to lungs and back to heart

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Pathological Conditions

Arrhythmias: Abnormal Heart Rhythms

Arrhythmias are abnormal heart rhythms that can significantly impact cardiac function. Understanding these conditions is crucial for proper diagnosis and treatment.

1

Heart Block (Atrioventricular Block)

Failure of proper impulse conduction through AV node to bundle of His

2

Flutter

Rapid but regular contractions of atria or ventricles, reaching up to 300 beats per minute in heart disease patients

3

Fibrillation

Rapid, random, ineffectual, irregular contractions (350+ beats per minute). Requires defibrillator or drugs like digitalis

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Cardiac Arrest and Palpitations

Cardiac Arrest

Sudden, often unexpected stoppage of heart movement caused by heart block or ventricular fibrillation resulting from underlying heart disease

Palpitations

Uncomfortable sensations in chest associated with different arrhythmias. Don't necessarily indicate serious heart disease—smoking, caffeine, and antidepressants can produce palpitations

PVCs and PACs

Two cardiac causes of palpitations are premature ventricular contractions (PVCs) and premature atrial contractions (PACs)

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Congenital Defects

Congenital Heart Disease

Congenital heart disease refers to abnormalities in the heart present at birth, resulting from failures in foetal heart development.

1

Coarctation of the Aorta

Narrowing of the aorta

2

Patent Ductus Arteriosus

Small duct between aorta and pulmonary artery remains open instead of closing after birth

3

Septal Defects

Small holes in septa between atria (ASDs) or ventricles (VSDs)

4

Tetralogy of Fallot

Congenital malformation involving four distinct defects: pulmonary artery stenosis, ventricular septal defect, aorta shift to right, right ventricle hypertrophy

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Heart Failure and Coronary Artery Disease

Congestive Heart Failure

The heart is unable to pump its required amount of blood. More blood enters the heart from veins than leaves through arteries.

Coronary Artery Disease

Disease of the arteries surrounding the heart. The three large coronary arteries arise from the aorta and supply oxygenated blood to the heart.

Interestingly, blood flowing through the four hollow chambers doesn't nourish myocardial tissue. Instead, after blood leaves via the aorta, a portion is immediately led back over the heart's surface through coronary arteries.

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Atherosclerosis

Atherosclerosis and Its Dangers

Coronary artery disease usually results from atherosclerosis—the deposition of fatty compounds on the inner lining of coronary arteries (any artery can be similarly affected).

1

Vessel Narrowing

Narrowing due to atherosclerosis causes inflexibility and plugging of the vessel

2

Abnormal Clotting

Roughened artery lining may rupture or cause abnormal blood clotting, leading to thrombotic occlusion

3

Reduced Blood Flow

Blood flow decreases (ischemia) or stops entirely, leading to death (necrosis) of myocardial tissue

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Myocardial Infarction (Heart Attack)

The area of dead myocardial tissue is known as an infarction. The infarcted area is eventually replaced by scar tissue. The severity of a myocardial infarction depends on the size of the blocked artery and the extent of blockage.

Small Artery Block

Death of only small portion of heart; patient may resume normal activity after scar tissue forms

Moderate Block

Larger area affected; requires careful recovery and lifestyle modifications

Major Artery Block

Extensive damage; requires immediate intervention and long-term management

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Angina Pectoris

Angina Pectoris: Chest Pain

Angina pectoris is an episode of chest pain resulting from a temporary difference between oxygen supply and demand to the heart muscle.

Angina can result from low oxygen levels in blood (from smoking or respiratory disease), restricted blood flow to heart (coronary artery disease), or increased heart work beyond normal levels.

Nitroglycerin

Given sublingually for acute attacks; powerful vasodilator and muscle relaxant

Beta-Blockers

Slow heartbeat and force of contraction; block sympathetic nerve stimulation to myocardium

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Other Cardiac Conditions

Endocarditis

Inflammation of inner heart lining caused by bacteria (bacterial endocarditis)

Hypertensive Heart Disease

High blood pressure affecting the heart due to arteriole contraction leading to increased arterial pressure

Mitral Valve Prolapse (MVP)

Improper closure of mitral valve when heart is pumping blood

Pericarditis

Inflammation of membrane (pericardium) surrounding the heart

Rheumatic Heart Disease

Heart disease caused by rheumatic fever, usually occurring in childhood following streptococcal infection

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Vascular Diseases

Blood Vessel Disorders

Aneurysm

Local widening (ballooning) of an artery caused by weakness in arterial wall or breakdown due to atherosclerosis

Hypertension

High blood pressure. Most is essential hypertension (idiopathic cause). Blood pressure ≥140/90 mmHg is considered high in adults

Peripheral Vascular Disease

Blockage of blood vessels (arteries) in lower extremities due to atherosclerosis

Raynaud's Phenomenon

Short episodes of pallor and numbness in fingers and toes due to temporary arteriole constriction in skin

Varicose Veins

Abnormally swollen and twisted veins, usually in legs, due to damaged valves failing to prevent backflow

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Diagnostic Tests

Laboratory Tests and Imaging

Various tests help diagnose cardiovascular conditions and assess heart health.

01

Lipid Tests

Measure cholesterol and triglycerides. Levels below 200 mg/dL associated with low CAD risk

02

Lipoprotein Electrophoresis

Separates lipoproteins. High LDL/VLDL associated with atherosclerosis; high HDL is protective

03

Serum Enzyme Tests

Measure CPK, LDH, myoglobin, and troponin-T released during myocardial infarction

04

Angiography

Dye injected into bloodstream; x-rays taken of heart and large vessels

05

Echocardiography

Ultrasound waves show heart structure and movement

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Clinical Procedures

Cardiac Procedures and Interventions

1

Cardiac Catheterisation

Thin, flexible tube guided into heart to detect pressures and blood flow patterns

2

Cardioversion (Defibrillation)

Brief electrical discharges applied across chest to stop arrhythmia and restore normal rhythm

3

Coronary Bypass Surgery (CABG)

Vessel grafts anastomosed to existing coronary arteries to detour around blockages

4

Endarterectomy

Surgical removal of innermost artery lining when thickened by fatty deposits

5

Heart Transplantation

Donor heart transferred to recipient; diseased heart removed and replaced

6

PTCA (Balloon Angioplasty)

Catheter with balloon inflated to compress plaque and open artery

7

Thrombolytic Therapy

Drugs like tPA and streptokinase dissolve clots; must be given within 12 hours of heart attack

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