Dr. Adel Hussien

Assistant Prof. of Physiology

Physiology dept.-Faculty of

Medicine – Minia University

Cardiovascular System

Learning objectives:

- What is the CVS system?

- Describe structure of the heart?

- Mention types of circulation inside the body

- Discuss functions of blood circulation

- Define properties of cardiac muscle (rhythmicity, excitability,

conductivity and contractility)

Introduction to the Cardiovascular System

- Cardiovascular system includes heart and blood vessels.

- Heart pumps blood into the blood vessels. Blood vessels circulate the

blood throughout the body.

- Blood transports nutrients and oxygen to the tissues

and removes carbon dioxide and waste products

from the tissues.

- Heart is made up of four chambers,

two atria ( right atrium - left atrium) and

two ventricles (right ventricle - left ventricle).

- The musculature of ventricles is thicker

than that of atria.

Right atrium receives venous (deoxygenated) blood via two large

veins:

1. Superior vena cava that returns venous blood from the head, neck

and upper limbs

2. Inferior vena cava that returns venous blood from lower parts of the

body

- Right atrium communicates with right ventricle

through tricuspid valve.

- Venous blood from the right atrium enters

the right ventricle through this valve.

- From the right ventricle, pulmonary artery

arises. It carries the venous blood from right

ventricle to lungs.

In the lungs, the deoxygenated blood is oxygenated.

- Left atrium receives oxygenated blood from the

lungs through pulmonary veins.

- This is the only exception in the body, where an artery carries venous

blood and vein carries the arterial blood.

Blood from left atrium enters the left ventricle through mitral valve

(bicuspid).

- Wall of the left ventricle is very thick.

- Left ventricle pumps the arterial blood to

different parts of the body through the aorta.

VALVES OF THE HEART

- There are four valves in human heart. Two valves in between atria and

the ventricles, artio-ventricular valves (tricusped and mitral).

- The other two are the semilunar valves, placed at the opening of blood

vessels arising from ventricles, (pulmonary and aortic valves).

- Valves of the heart permit the flow of blood through heart in only one

direction.

DIVISIONS OF CIRCULATION

Blood flows through two divisions of circulatory system:

1. Systemic circulation

2. Pulmonary circulation.

SYSTEMIC CIRCULATION

- Systemic circulation is also known as greater circulation.

- Blood pumped from left ventricle passes through a series of blood

vessels, arterial system and reaches the tissues.

- Exchange of various substances between blood and the tissues occurs

at the capillaries.

- After exchange of materials, blood enters the venous system and

returns to right atrium of the heart. From right atrium, blood enters the

right ventricle.

- Thus, through systemic circulation, oxygenated blood is supplied

from heart to the tissues and venous blood returns to the heart from

tissues.

PULMONARY CIRCULATION

Pulmonary circulation is also called lesser circulation. Blood is pumped

from right ventricle to lungs through pulmonary artery. Exchange of

gases occurs between blood and alveoli of the lungs at pulmonary

capillaries. Oxygenated blood returns to left atrium through the

pulmonary veins.

Thus, left side of the heart contains oxygenated or arterial blood and the

right side of the heart contains deoxygenated or venous blood.

General functions of the circulatory system

1. Respiratory function: by transporting O2 from the lungs to the

tissues and CO2 from the tissues to the lungs

2. Nutritional function: by transporting nutritive products of digestion

from the intestine to various tissues in the body

3. Excretory function: by removing waste products from the tissues to

the excretory organs such as, the kidneys or the lungs

4. Endocrine function: by carrying the hormones from the endocrine

glands to their target organs

5. Body temperature regulation: by transferring heat from deep organs

to be dissipated at the body surface

6. Homeostasis: by removing any substance in excess and providing any

deficient substance in any part of the internal environment. This

maintains the equilibrium of all parts of the internal environment

.

Blood supply of the heart

1. Arterial supply:

- Right and left coronary arteries supply the heart with arterial blood

They arise from the aorta just above the aortic valve

2. Venous drainage:

- Coronary sinus which opens into right atrium

- Thebesian veins opening directly into all cardiac chambers

Nerve supply of the heart

The heart receives both parasympathetic and sympathetic supply

Parasympathetic supply comes through the vagus nerve which

supplies only the atria while the ventricles did not receive

parasympathetic supply (vagal escape)

Parasympathetic stimulation inhibit all cardiac functions and cause

coronary vasoconstriction (decrease coronary blood flow)

Sympathetic supply comes from cervical sympathetic ganglia and

reaches the heart through superficial and deep cardiac plexuses

Sympathetic nerves supply all structures of the heart including the

ventricles (unlike the vagus)

Sympathetic stimulation increase all cardiac functions and cause

coronary vasodilatation (increase coronary blood flow)

Properties of Cardiac Muscle

There are 4 properties for the cardiac muscle:

1-RHYTHMICITY

2- EXCITABILITY

3- CONDUCTIVITY

4- CONTRACTILITY

1- RHYTHMICITY Rhythmicity is the ability of the heart to beat (contract and relax) regularly at a constant

rate.

It is an inherent property of the cardiac muscle itself and is not dependent on any

nervous tissue i.e. myogenic in origin and not neurogenic, the nerves control the rate

but do not initiate the beat.

2- EXCITABILITY

- Excitability is the ability of a living tissue to give response to a

stimulus.

- The resting membrane potential of the contractile muscle fibers is

stable at -90 mV.

- When an effective stimulus is applied a propagated action potential is

produced.

3- CONDUCTIVITY Ability to conduct the excitation wave through the heart. Human heart has a specialized

conductive system, through which impulses from SA node are transmitted to all other

parts of the heart

4- CONTRACTILITY

Ability of cardiac muscle to contract and push blood into the circulation

Heart rate and its regulation

Learning objectives:

- What is the heart rate?

- Mention variations of heart rate

- Discuss regulation of heart rate

Heart rate

Definition: Heart rate is the number of heart beats/min

Physiological standards of heart rate

Average heart rate in adults 70 beat/min

Range: 50-95 beat/min

Heart rate above 100 beat/min called Tachycardia

Heart rate below 50 beat/min called Bradycardia

The heart rate is dependent upon the strength of the vagal tone. The

higher the vagal tone the slower the heart rate

The vagal tone is greater in males than females, in adults than in

children and in athletes than in non trained persons

What is the vagal tone?

Vagal tone is the continuous stream of inhibitory impulses

reaching the heart via vagus nerve (parasympathetic). Heart rate is kept

under control because of vagal tone. No sympathetic tone at rest

Physiological variations of heart rate

1- Variations with age:

The heart in the newly born is about 120 beat/min

It decreases gradually till it reaches the normal adult rate at the age of 20

years (70 beat/min). Then it remains steady till old age

2- Variations with sex:

The vagal tone ig greater in males than in females, so the resting heart

rate in females is faster than that of males

3- Physical training

The vagal tone in athletes is greater than in non trained persons

(sedentary) so the heart rate in athletes is slower (about 50 beat/min)

4- Sleep: Heart rate decrease during sleep because of increase

parasympathetic activity

Regulation of heart rate

Under normal resting conditions the heart rate is maintained at about 70

beat/min by 3 mechanisms

1.Nervous regulation

2.Chemical regulation

3.Physical regulation

1. Nervous Regulation of heart rate

Afferent impulses from different parts of the body reach the Cardiac

Acceleratory Center (CAC) and the Cardiac Inhibitory Center (CIC) in

the medulla oblongata which changes the heart rate.

2. Chemical regulation of heart rate

1- Oxygen lack

Hypoxia (decrease oxygen supply to tissues) causes increase heart rate due to

- Direct stimulation of the CAC

- Reflex secretion of adrenaline

2- CO2 excess

Increase in the CO2 tension leads to stimulation of the chemoreceptors in the aortic and carotid bodies causing reflex increase in heart rate

3- Blood hormones

Adrenaline and thyroxine increase the heart rate due to direct stimulation of SA node increasing its rhythmicity

3- Physical regulation of heart rate

1- Increased blood temperature: causes increase in the heart rate, a

rise of 1ºC leads to increase in heart rate by 10 beats/min

Mechansim:

- Directing stimulating effect on the SA node and cardiac conductive

system

- Stimulation of the CAC and inhibition of the CIC

2- Decreased blood temperature: causes decrease in the heart rate by

the opposite mechanisms described above