Several capillaries join to form successively wider branches , ultimately forming a vein .
Veins carry oxygen-depleted blood back to the heart (the pulmonary artery and veins which are mentioned below are exceptions to this description).
Blood is the carrier of oxygen and other substances.
The normal total circulating volume is about 8% of the body weight (5.6 liters in a 70 kg man) .
Some tissue fluids enter another system of closed vessels – this forms the lymph, in the lymphatics.
Lymph and its secretion are discussed elsewhere.
The heart is a muscular pear shaped organ, the size of a clenched fist.
It is protected by the bony rib cage, and its rarely damaged by a blow.
It is located in the chest a little to the left of the center line.
The heart’s function, like its position, is central to life.
Without a continuous supply of blood (oxygen, sugar) the body cannot function. To maintain this supply, the heart has to work non-stop.
INTERNAL VIEW OF THE HEART
THE HEART AS A PUMP
The heart receives deoxygenated blood on the right side and pumps it to the lungs to be oxygenated.
Oxygenated blood returns from the lungs to the left side of the heart, which pumps it to the rest of the body.
THE HEART AS A PUMP VENTRICLE VALVE ATRIUM To the lungs From the body (CO2) From the lungs (O2) To the body
Flows into the right upper chamber ( right atrium ) which is separated from the lower chamber ( ventricle ) by a one – way valve.
The deoxygenated blood is transported by two veins – the superior and the inferior vena cava, bringing blood from the upper and lower halves of the body, respectively.
When pressure in the atrium is greater than that in the ventricle, the valve opens, and blood flows from the atrium into the ventricle.
The ventricle is filled with blood.
The valve closes once more and this marks the beginning of the contraction of the ventricle.
The ventricle has very thick, powerful muscular walls.
The right ventricle contracts, forcing blood out through the pulmonary artery , which also has a valve at its opening.
The pulmonary artery takes the deoxygenated blood to the lungs, where it gets oxygenated once more.
As most of the blood in the ventricle is pumped out the pressure in the ventricle drops, and the valve at the opening of the pulmonary artery closes.
The ventricle relaxes.
The cycle goes on as once more blood from the atrium enters the ventricle.
Oxygenated blood flows from the lungs through the pulmonary veins into the upper chamber on the left side of the heart.
The sequence of events is similar to that described above for incoming blood .
As the pressure in the left atrium increases, blood passes through the one – way valve into the left ventricle below it.
As the ventricle is filled with blood, the one – way valve closes.
The ventricle contracts, the valve at the entrance to the aorta opens, and blood is pumped through the aorta (= the main artery arising from the heart), to be circulated throughout the body.
The blood ultimately returns the right side of the heart in the deoxygenated condition.
The contraction phase is called the systole .
The atrial systole is followed by the ventricular systole.
The phase in between contractions, of relaxation and ‘filling’ is called the diastole .
During the diastole all chambers of the heart are relaxed.
THE HEART’S PACEMAKER
The parts of the heart normally beat in an orderly sequence, as described above.
The heart continues to beat after all the nerves to it are sectioned.
Indeed, if the heart is cut into pieces, the pieces continue to beat.
The heartbeat originates in a specialized cardiac conduction system and spreads via this system discharge rhythmically because of special properties of their cell membrane and the way in which charged particles (ions) pass through it .
The specialized system includes the sinoatrial node (SA node), internodal atrial pathways, the atrioventricular node (AV node), the Bundle of His , and the Purkinje system .
The SA node is located at the point where the superior vena cava (a vein which brings deoxygenated blood from the upper half of the body) joins the right atrium.
The AV node is located in the inter atrial septum.
The AV node is continuos with the Bundle of His, which gives off a left and a right bundle branch at the top of the interventricular septum.
These fibers join the Purkinje system , which spreads to all parts of the ventricular muscle.
The SA node fires faster than the other parts, hence the SA node sets the pace at which the heart pumps .
It is called the cardiac pacemaker .
Hence the heartbeat starts in the SA node and its impulses spread in one-tenth of a second across the two atria, causing them to contract.
There is a delay of one-tenth of a second before the impulses are picked up by the AV node and passed on to the Bundle of His.
In another tenth of a second, impulses from the AV node spread through the Bundle of His, to the Purkinje system, making the ventricles contract.
It is essential to remember that through the cardiac pacemaker has its own intrinsic rhythmically, this is normally regulated by the nerve supply to the SA node.
Sympathetic nerves increase the heart rate, while parasympathetic nerves reduce it.
These nerve supply, the heart rate increases during exercise or anxiety, and reduces during sleep.
The heart rate of a normal adult may vary between 60-90 beats/min, though the average is 72 beats/min.
The heart rate increases with emotion, exercise, and fever. The heart rate decreases during rest and sleep.
THE CARDIAC OUTPUT
The output of the heart per unit time is called the cardiac output .
In a normal man, while lying down the average cardiac output is 5.5l/min.
The cardiac output is significantly influenced by a factor which is not related to the nerve supply.
This factor is based on the observation that “the force with which the heart contacts depends on the extent to which the muscle is stretched to begin with.”
The more the ventricle is filled, greater is the initial stretch.
Filling is dependent on various factors.
The most important is the venous return , or the amount of blood returning to the heart, via the veins.
This depends on factors like the position of the body (standing impedes venous blood from the legs returning to the heart) and the ‘pumping action’ of the calf muscles.
While walking, the contraction of the calf muscles facilitates the venous return of blood from the legs.
Since the cardiac output is dependent on the heart rate and the force with which the heart pumps, it is also influenced by the innervation.
As described above, sympathetic nerves increase heart rate, and also the force of contraction, thus increasing the cardiac output.
In contrast, parasympathetic nerves reduce cardiac output by reducing both the heart rate, and the force with which it pumps.
Blood is transported in three main types of closed vessels – arteries, veins, and capillaries .
Vein Artery Capillary more muscle than veins thin walls allow materials to pass through V V = valves in veins maintain one way flow toward the heart
Arteries – Blood is ejected from the left ventricle through the aorta. This large artery (2.5 cm in diameter in the adult), has numerous branches, so that the oxygenated blood reaches the different parts of the body.
Whenever blood flows through an artery, its flexible wall expands at each heartbeat and this can be felt as a the pulse , by passing a superficial artery gently against the bone beneath.
The pulse is usually taken at the wrist, where an artery (the radial artery) passes over the bone.
The blood also exerts pressure on the vessel walls as it is pumped through.
Pressure is necessary for circulation to take place.
The pressure has a rhythmic fluctuation – the pressure is maximum during systole or when the ventricles are contracting.
It is least when the ventricles are relaxed – in the diastolic phase .
The blood pressure hence has two readings – systolic and diastolic.
The walls of arteries are lined with smooth cells (endothelium).
Around this is elastic tissue and muscle.
The outer layer is made up of connective tissue.
The elastic and muscle layers make the arteries efficient in pumping blood through the body.
Sometimes in later life, arteries become less efficient.
Fatty mineral deposits accumulate in the walls and on the inner surface of arteries, and the walls become thickened and less elastic.
There are different causes.
An important cause is consumption of a diet rich in saturated fat.
These deposits may eventually block the artery.
In case the arteries which supply the heart muscle are affected, it can result in ischemic heart disease , and even lead to a heart attack.
Many parts of the body have alternate routes of blood supply.
In case the main route is blocked, these alternate routes may still supply blood to maintain normal functioning.
The diameter of the artery can be varied, hence altering the amount of blood supplies by it.
One of the important factors which changes vessel diameters is the control by the nerves .
Sympathetic nerves for example, simultaneously cause narrowing of blood vessels in the skin, while widening those in the muscles.
Arteries carry blood which is ‘rich’ in oxygen and hence the blood appears bright red.
If an artery is cut, the blood gushes out in spurts.
Capillaries – Most arteries divide as successively smaller branches, until they form very fine capillaries, in all the body tissues.
These are minute vessels with very thin walls.
The thin walls allow for the movement of oxygen and other essential substances from the blood of the body cells.
Waste material from the cell such as carbon dioxide diffuses from the cell to the capillary.
Several capillaries join together to form a venule (a small vein), ultimately leading to a vein. If capillaries are cut blood oozes out, slowly.
Veins – transport blood which is depleted of oxygen to the heart, which pumps it to the lungs to be oxygenated.
Venous blood hence contains carbon dioxide and waste products picked up from the tissues.
Veins were earlier thought to be mere passageways for the flow of blood into the heart.
It is now known that they are capable of constricting and enlarging, of storing large quantities of blood, and making it available when required, and regulating the cardiac output by varying the venous return.
Veins have significantly thinner walls compared to arteries.
They are provided with valves, which allow the blood to flow in one direction, i.e. towards the heart .
Veins contain deoxygenated blood, hence if a vein is severed dark red blood oozes out steadily.
As described above, circulating blood presses against the walls of the blood vessels with a force/pressure.
The pressure is greatest during systole , when the heart contracts and forces blood into the arteries of the body.
This maximum pressure is known as the systolic pressure .
During the diastole , when the heart muscle relaxes, the pressure drops to a minimum. This is known as the diastolic pressure .
Blood pressure varies from person to person, as well as from time to time in the same person.
It is influenced by the general health, heredity, age, activity, position of the body and emotional state.
The blood pressure is determined by the cardiac output and the peripheral vascular resistance (i.e. the diameter of the blood vessels).
Hence, if the heart pumps with more force and also at a greater rate, the cardiac output increases, increasing the blood pressure.
Also, if the arteries are more narrow, the pressure required to pump blood through them has to be more.
During the stress response sympathetic nerves increase the rate and force of the heart beat.
Sympathetic nerves also cause the arteries supplying the skin to narrow, hence increasing the peripheral resistance.
Hence the stress response during about a sympathetic mediated increase in blood pressure.
RECORDING THE BP
Is done with a sphygmomanometer .
It is conventionally recorded as the systolic pressure over the diastolic.
A normal young adult may have a BP of 120/80 mmHg, this means that the systolic pressure is 120, while the diastolic is 80.
Considerable variation within the normal range is possible.
A systolic pressure above 140 mm, and a diastolic pressure above 90 mm, should be further investigated.
High blood pressure ( hypertension ) can give rise to serious complications and hence should be treated and evaluated periodically.
NEURAL REGULATION OF THE HEART AND BLOOD VESSELS
The normal activity of the heart (force and speed of pumping) is controlled by centers in the lower brain stem.
These are called vital centers , as injury to them is usually fatal.
There is one center which has a stimulating effect, increasing the heart rate and force of contraction.
Another center has an inhibitory effect – reducing heart rate and force of contraction.
The former acts through the sympathetic nerves, whereas the latter acts via the parasympathetic nervous system.
There are many factors which can modify the output (instructions to the heart) from these centers.
There are special receptors which send important information to them.
These receptors lie close to the important blood vessels (the aorta or the carotid artery which supplies the brain) and detect the level of distension/stretch of the blood vessels.
In case the receptors sense that the blood vessels are stretched.distended they activate the inhibitory center, so that the cardiac output decreases and the blood vessel is no longer stretched.
Other stimuli also influence these centers.
Anxiety and anger are two emotions which are well known to ‘speed up’ the heart.
CIRCULATION THROUGH SPECIAL REGIONS
As described elsewhere, the brain is extremely sensitive to oxygen deprivation.
Occlusion of its blood supply produces unconsciousness in as short a period as 10 seconds.
If deprivation is continued for 120 second, the small stores of energy are used up and irreversible damage and brain death occurs.
For this reason the regulation of the blood supply to the brain has unique features.
Though these blood vessels do have a nerve supply, this does not normally modify the vessel diameter – and hence the body supply.
The vessels show the property of autoregulation.
When the CO2 rises the vessels dilate.
They increase blood supply helps to wash away the CO2.
Absorption of digested food materials takes place through the villi of the small intestine, into the capillaries.
These capillaries join to form a portal vein, which transports the blood rich in absorbed substances, to the liver.
Substances are stored in the liver.
When required they are broken down, and enter the circulation via the hepatic vein.
It joins the inferior vena cava, and hence transports blood to the right atrium.