Cardiovascular System:Heart & Circulation

I. Anatomy of the Heart

A. Coverings

1. Pericardium

• fibrous pericardium

• serous pericardium• parietal pericardium• visceral pericardium

B. Heart wall layers 1. Epicardium – (visceral pericardium)

• protects heart 2. Myocardium – cardiac muscle

3. Endocardium – epithelial/ connective/ fibers

C. Chambers, Vessels, and Valves

1. Four chambers

• upper chambers – rt and left atria

Collects blood

• lower chambers – rt and left ventricle

pumps blood from heart

2. Arteries

• carry blood away from heart

3. Veins

• blood toward heart

4. Valves

• AV valves

Tricuspid – rt atrium

Bicuspid or mitral - lt atrium

• Semilunar valves

Pulmonary semilunar (pulmonary trunk)

Aortic semilunar (aortic arch)

II. Pulmonary and Systemic Circulation

A. Pulmonary pathway

oxygenated blood –rt ventricle from rt atrium

myocardium (rt ventricle) contracts

low oxygen blood through pulmonary semilunar

pulmonary trunk -> arteries -> lungs

oxygenated blood -> rt & lt pulmonary veins ->

left atrium

B. Systemic pathway

Oxygenated blood -> Lt atrium to Lt ventricle

Left ventricle contracts ->aortic semilunar

Aortic arch -> arteries to tissues

oxygen depleted blood from tissues ->

veins to heart -> rt atrium (vena cava)

Passage of Blood Through the Heart

Blood follows this sequence through the heart: superior and inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk and arteries to the lungs → pulmonary veins leaving the lungs → left atrium → bicuspid valve → left ventricle → aortic semilunar valve → aorta → to the body.

Direction of blood flow through the heart

The relationship between the systemic and pulmonary circulations

Blood supply to the heart orcoronary circulation

The coronary arteries

Blood supply to the heartArterial supply

The heart is supplied with arterial blood by the right and left coronary arteries, which branch from the aorta immediately distal to the aortic valve

The coronary arteries receive about 5% of the blood pumped from the heart, although the heart comprises a small proportion of body weight

This large blood supply, especially to the left ventricle, highlights the importance of the heart to body function

The coronary arteries traverse the heart, eventually forming a vast network of capillaries

Blood supply to the heart –

Venous drainage

Most of venous blood is collected into several small veins that join to form coronary sinus, which opens into right atrium

The remainder passes directly into the heart chambers through little venous channels

THE CARDIAC CYCLE

What is the cardiac cycle

The cardiac cycle is the sequence of events that occur when the heart beats There are two phases of this cycle:

Diastole - Ventricles are relaxedSystole - Ventricles contract

The cardiac cycle

Exercise increases blood flow through the heart so that the cardiac cycle accelerates to accommodate the increased demand for oxygen

The normal cycle is around 0.8 seconds. This accelerates with faster and more powerful atrial and ventricular contraction, which is stimulated by the cardiac centre in the brain

Heart rate:- is defined as the number of heart contractions in each minute

# There are two distinct periods in the cardiac cycle- one of the heart muscle relaxation (cardiac diastole), the other of contraction (cardiac systole)

Cardiac diastole

During cardiac diastole

• The bicuspid and tricuspid valves are closed and the atrium is full

• Once full with blood, the atria forces the bicuspid and tricuspid valves to open and fill the ventricles

• This lasts for around 0.4 seconds at rest

Cardiac systoleCardiac systole

• The atria contract and send blood via the bicuspid and tricuspid valves into the ventricles

• When full, these contract causing blood to be expelled from the heart via the semi-lunar valves

• (the bicuspid and tricuspid valves are closed at this time)

• This lasts around 0.4 seconds at rest

The Cardiac CycleHeart at rest– Blood flows from large veins into atria– Passive flow from atria into ventricles

Atria (R & L) contract simultaneously– Blood forced into ventricles

Ventricles (R & L) contract simultaneously– Atrioventricular valves close “lubb” sound– Blood forced into large arteries

Ventricles relax– Semilunar valves close “dub” sound

Heart at rest

Heart valves• Valves are flap-like structures that allow blood to flow in

one direction

• The heart has two kinds of valves, atrioventricular and semilunar valves

Heart sounds

• The audible sounds that can be heard from the heart are made by the closing of the heart valves

• These sounds are referred to as the “lub-dupp” sounds

• The “lub” sound is made by the contraction of the ventricles and the closing of the atria-ventricular valves

• The “dupp” sound is made by the semi-lunar valves closing

Conducting system of Heart

Stimulation of the heart originates in the cardiac centre, in the “medulla oblongata.”

The “sympathetic and parasympathetic nervous systems” work antagonistically and provide the stimulation for acceleration and deceleration of the heart rate

Cardiac systole (contraction) is initiated by the electrical cardiac impulse from the “sinu-atrial node” (the pace-maker found in the right atria wall)

This distributes electrical stimulus through the “myocardial” (heart muscle) wall between the heart chambers

where the “atrio-ventricular node” (between the right atrium and right ventricle) continues distribution of the electrical signal across the ventricles

• In the upper part of the right atrium of the heart is a specialized bundle of neurons known as the sino-atrial node (SA node)

• Acting as the heart's natural pacemaker, the SA node "fires" at regular intervals to cause the heart of beat with a rhythm of about 60 to 70 beats per minute for a healthy, resting heart

• The electrical impulse from the SA node triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart

The SA node

The AV node

• The AV node (AV stands for atrioventricular) is an electrical relay station between the atria (the upper) and the ventricles (the lower chambers of the heart)

• Electrical signals from the atria must pass through the AV node to reach the ventricles

AV node (bundle of his)

The bundle of His is located in the proximal interventicular septum

It emerges from the AV node to begin the conduction of the impulse from the AV node to the ventricles

Purkinje fibersPurkinje fibers

Purkinje fibers are heart muscle tissues that are specialized to conduct electrical impulses to ventricular cells, which induce the lower chambers of the heart to contractImpulses from the upper chambers of the heart are relayed by this node to large bundles of Purkinje fibers referred to as the Bundle of HisThese bundles branch into smaller elements and eventually form terminal ends that burrow into left and right ventricular chamber musclesAs the impulse is passed to the ventricles, the muscles contract and pump bloodThe contraction caused by the specialized fibers begins from the bottom of the ventricles and move upwards so that the blood leaves the lower chambers through the pulmonary arteries and the aorta

Signal Conduction Pathway

• SA action potentials -> contraction in atrium

• AV action potentials (slower) -> bundle of HIS->

through septum -> Purkinje fibers -> contraction

C. Electrocardiography

• electrical events corresponding to mechanical

• P wave: atrial fibers depolarize

• QRS complex: ventricles depolarize

• T wave: ventricles repolarize

Electrocardiography

Cardiac Output (CO) CO is the amount of blood pumped by each

ventricle in one minute CO is the product of heart rate (HR) and

stroke volume (SV) HR is the number of heart beats per minute SV is the amount of blood pumped out by a

ventricle with each beat

Cardiac Output: Example CO (ml/min) = HR (75 beats/min) x SV

(70 ml/beat) CO = 5250 ml/min (5.25 L/min)

Regulation of Stroke Volume SV = end diastolic volume (EDV) minus

end systolic volume (ESV) EDV = amount of blood collected in a

ventricle during diastole ESV = amount of blood remaining in a

ventricle after contraction

Factors Affecting Stroke Volume Preload – amount ventricles are stretched

by contained blood Contractility – cardiac cell contractile force

due to factors other than EDV Afterload – back pressure exerted by blood

in the large arteries leaving the heart

Preload and Afterload

Regulation of Heart Rate Positive chronotropic factors increase heart

rate Negative chronotropic factors decrease

heart rate Autonomic nervous system Hormones

Blood Pressure & its regulation Blood pressure is the force or pressure that the

blood exerts on the walls of the blood vessels BP = CO x TPR CO= SV x HR BP = Blood Pressure CO = Cardiac Output TPR = Total Peripheral Resistance SV = Stroke Volume HR = Heart rate

Control of Arterial Blood Pressure

Control of Blood Pressure Short term control :

Baroreceptors Chemoreceptors Higher centres in the brain

Long term control:

RAAS system

Summary of the main mechanisms in blood pressure control

Baroreceptors

Located in walls of aortic arch and left and right carotid sinus

Mechanical stretch receptors Neuronal circuits in brainstem compare

actual value of BP provided by the baroreceptors with the set point or optimal value

The baroreceptor reflex

Chemoreceptor Nerve ending situated in the carotid and aortic bodies

control of respiration

Sensitive to changes in the levels of Co2 , O2 and acidity of the blood

The relationship between stimulation of chemoreceptors and arterial blood pressure

Higher centres in the brain

Fear

Anxiety

Pain

Anger

Long term Blood pressure regulation

Renin-angiotensin-aldosterone system (RAAS)

Anti-diuretic harmone (ADH) Atrial natriuretic peptide (ANP)

harmone released by heart

sodium and water loss from the kidney

reduces blood pressure

RAAS System

Disorder of CVS• Hypertension

• Hypotension

• Congestive heart failure

• Cardiac Arrhythmia

• Angina Pectoris

• Arteriosclerosis

• Myocardial Infarction

Hypertension

Essential hypertension

Benign (chronic ) hypertension

Malignant ( accelerated) hypertension

Secondary hypertension

Kidney disease

Endocrinal disorders

Heart failure

Acute heart failure

Chronic heart failure

Right-sided (congestive) heart failure

Left-sided (left ventricular) heart failure

Cardiac arrhythmia

Sinus bradycardia

Sinus tachycardia

Asystole

Fibrillation

Atrial fibrillation

Ventricular fibrillation

Heart block

Blood VesselsFunctions: Distribution of blood Exchange of materials with tissues Return of blood to the heart

Structure: Most have the same basic structure:

– 3 layers surrounding a hollow lumen

General Structure of Blood Vessels

Arteries and veins are composed of three tunics:

tunica interna tunica media tunica externa

Capillaries are composed of endothelium

General Structure

The Vessels1. Tunica Intima innermost smooth layer simple squamous epithelium continuous with the endocardium present in all vessels

The Vessels2. Tunica Media layer of smooth muscle - circular arrangement –

contains elastin

supplied by sympathetic division of the ANS

depending on body’s needs – lumen is narrowed (vasoconstriction) or widened (vasodilation)

The Vessels3. Tunica Externa (Adventitia) thin layer of CT elastic & collagen fibres

The VesselsTypes of Vessels:

Arteries – carry blood away from the heart Veins – carry blood towards the heart Capillaries – the most important part of

the vascular system; site of exchange of materials

Types of Blood vessels: Arteries

Elastic Arteries: Thick-walled arteries near the heart; the aorta and its

major branches

Large lumen allows low-resistance conduction of blood

Contain lots of elastin in all three tunics

walls stretch and recoil to propel blood

Withstand and regulate large blood pressure fluctuations

Types of Blood vessels: Arteries

Muscular (distributing) arteries medium sized vessels tunica media more smooth muscle;

less elastin major area of vaso-constriction &

dilation to regulate blood flow

The VesselsArterioles (diameter of 0.3 mm or less)

- smallest arteries; lead to capillary beds

- close to capillaries - single layer of muscle spiralling around the endothelial

lining

- regulates blood flow to capillary

The VesselsCapillaries Smallest vessels – diameter just large

enough for a red blood cell walls consist of tunica intima only

(i.e. layer of endothelium) thinness facilitates exchange of

materials