Tuesday, 24 April 2012

Cardiovascular System- Revision Notes

Cardiovascular System- Revision Notes

Right side of the heart pumps blood to the lungs

Gas exchange occurs

Carbon dioxide (CO2) leaves the blood and enters the lungs

Oxygen (O2) leaves the lungs and enters the blood



Left side of the heart pumps blood around the rest of the body

Tissue wastes are passed into the blood for excretion

Body cells extract nutrients and oxygen from the blood



The Heart ensures a continuous flow of blood to all cells

The heart’s function is to provide continual physiological adjustments in order to maintain an adequate blood supply.

If the heart loses this function (blood supply) it can result in tissue damage and cell death

The heart pumps blood into vessels that vary in structure, size and cell death

Arteries

Arterioles

Capillaries

Veins

Venules




Arteries and Arterioles

Blood vessels that transport blood away from the heart

Vary in size

Walls consist of three layers of tissue:

Tunica adventitia --> outer layer of fibrous tissue

Tunica media --> middle layer of smooth and elastic tissue

Tunica intima --> inner lining of squamous epithelium (endothelium)

Amount of muscular and elastic tissues varies in the arteries depending on their size and function

In the large arteries the tunica media consists of more elastic tissue than smooth muscle, to allow the vessel wall to stretch; it also absorbs the pressure wave generated by the heart

As arteries branch off these proportions change until the tunica media is almost completely smooth muscle, this enables their diameter to be precisely controlled, regulating the pressure within them.

Systemic blood pressure is mainly determined by the resistance these tiny blood vessels offer to blood flow ( resistance vessels)

Arteries have thicker walls than veins and this enables them to withstand the high pressure of arterial blood.



Veins and Venules

Veins --> blood vessels that return blood at low pressure to the heart

The walls of veins are thinner than arteries, they have the same three layers of tissue

They are thinner because there is less muscle and elastic tissue in the tunica media, this is due to the fact that veins carry blood at a lower pressure

When cut veins collapse

Some veins contain valves to prevent back flow of blood, this ensures the blood goes back towards the heart

The valves are formed by a fold of tunica intima and are strengthened by connective tissue

They are abundant in the lower limbs, where blood has to travel a longer distance against gravity

Small veins are called venules

Veins are called capacitance vessels because they are very distensible and can hold a large proportion of the body’s blood

About two-thirds of the body’s blood is in the venous system at any one time.

This allows the vascular system to absorb sudden changes in blood volume, such as haemorrhage

The veins can recoil, helping to prevent a sudden fall in blood pressure



Capillaries and Sinusoids

Capillary walls consist of a single layer of endothelial cells sitting on a very thin basement membrane

Water and other small molecule substances can pass

Blood cells and larger molecule substances do not usually pass through the capillary walls

Capillaries form a vast network of tiny vessels that link the smallest arterioles to the smallest venules.

Their diameter is approximately that of an erythrocyte (white blood cell) which is 7µm

The capillary bed is the site of exchange of substances between the blood and the tissue fluid, which bathes the body’s cells

Rings of smooth muscle (pre-capillary sphincters) which guard entry to the capillaries and also directs blood flow

Hypoxia (low levels of oxygen in tissues) or high levels of tissue waste, indicates high levels of activity, therefore the sphincters dilate and allow the blood flow through the affected bed to be increased

Sinusoids --> wider than capillaries and have extremely thin walls separating blood from the neighbouring cells.

In some there are distinct spaces between the endothelial cells

Among the endothelial cells there can be many phagocytic macrophages

Sinusoids are found in the bone marrow, endocrine glands, spleen and liver

They have a large lumen

The blood pressure is lower

Blood flow is slower



Blood Supply

The outer layers of tissue of thick-walled blood vessels receive their blood supply via a network a network of blood vessels (vasa vasorum)

Vessels with thin walls and the endothelium receive oxygen and nutrients by diffusion from the blood passing through them



Control of Blood Vessel Diameter

All blood vessels, except capillaries, have smooth muscle fibres in the tunica media which are supplied by nerves of the autonomic nervous system.

These nerves arise from the vasomotor centre in the medulla oblongata and they change the diameter of the lumen of blood vessels, controlling the volume they contain

Medium and small sized arteries have more muscle than elastic in their walls

These respond to nerve stimulation

In large arteries the middle layer is almost all elastic tissue

Their diameter depends on the amount of blood they contain



Vasodilation and Vasoconstriction

Sympathetic nerves supply the smooth muscle of the tunica media of blood vessels.

There is no parasympathetic nerve supply to most blood vessels and therefore the diameter of the vessel lumens and the tone of the smooth muscle is determined by the degree of sympathetic nerve stimulation

There is a baseline (resting level) of nervous activity supplying the smooth muscle in the vessel walls, which can then be increased or decreased as required

Decreased nerve stimulation relaxes the smooth muscle, thinning the vessel wall and enlarging the lumen

This is known as vasodilation

It results in increased blood flow under less resistance

Increased nerve activity causes the smooth muscle of the tunica media to contract and thicken

This is known as vasoconstriction

Resistance to flow of fluids along a tube is determined by three factors:

The diameter of the tube

The length of the tube

The viscosity of the fluid involved

The important factor determining how easily the blood flows through blood vessels is the first of these variables, this can also be called the peripheral resistance

The length and viscosity of blood vessels could also contribute, in health however; these are constant and are therefore not significant determinants of changes in blood flow.



Auto-regulation

The accumulation of metabolites in local tissues also influences the degree of dilation of arterioles and capillaries

This mechanism ensures that local blood flow is increased or decreased in response to tissue need:

Exercise --> lactic acid accumulates in muscle or a rise in tissue temperature causes vasodilation

Excess CO2 or hypoxia --> signify increased tissue metabolism, causing local vasodilation to improve blood supply

Release of vasodilators such as nitric oxide (NO) --> increase blood flow through capillary beds, controls regional blood flow, reduced clotting, and aids body defences

Tissue damage for example, inflammation --> mediators such as histamine, prostaglandins and bradykinin lead to vasodilation

Situations where the circulation to vital organs, such as the brain and heart is threatened



Capillary Exchange

Internal respiration --> the exchange of gases between capillary blood and local body cells

Oxygen is carried from the lungs to the tissues in combination with haemoglobin as oxyhaemoglobin.

Exchange in the tissues takes place between blood at the arterial end of the capillaries and the tissue fluid, and then between the tissue fluid and the cells.

Oxygen diffuses down its concentration gradient, from the oxygen-rich arterial blood into the tissues, where oxygen levels are lower because of constant tissue consumption.

Oxyhaemoglobin is an unstable compound and dissociates (breaks up) easily to liberate oxygen

Carbon dioxide is one of the waste products of cell metabolism and towards the venous end of the capillaries it diffuses into the blood down the concentration gradient

Blood transports carbon dioxide to the lungs for excretion by three different mechanisms:

Dissolved in the water of the blood plasma --> 7%

In chemical combination with sodium in the form of sodium bicarbonate --> 70%

Combination with haemoglobin --> 23%



Exchange of other Substances

The nutrients required by the cells of the body are transported around the body in the blood plasma

In passing from the blood to the cells, the nutrients pass through the semipermeable capillary walls into the tissue fluid bathing cells

It then passes through the cell membrane into the cell

The mechanisms of the, transfer of water, and other substances from the blood capillaries depends on diffusion and osmosis.




Diffusion

Diffusible substances include:

Dissolved oxygen

Dissolved carbon dioxide

Dissolved glucose

Amino acids

Fatty acids

Glycerol

Vitamins

Mineral salts

Water



Osmosis

Osmotic pressure across a semipermeable membrane draws water from a dilute to a more concentrated solution in an attempt to establish a state of equilibrium.

The force of the osmotic pressure depends on the number of non-diffusible particles in the solution separated by the membrane

The main substances responsible for the osmotic pressure between blood and tissue are:

Plasma proteins, especially albumin



Capillary Fluid Dynamics

The two main forces determining overall fluid movement across the capillary wall are:

The hydrostatic pressure (blood pressure), which tends to push fluid out of the blood stream

The osmotic pressure of the blood, which tends to pull it back in, and is due mainly to the pressure of plasma proteins

At the arterial end, the hydrostatic pressure is about 35 mmHg, and the opposing osmotic pressure of the blood is only 25mmHg

The overall force therefore drives fluid out of the capillary and into the tissue

This net fluid loss of fluid from the blood stream must be reclaimed in some way

At the venous end of the capillary, the situation is reversed, blood flow is slower than at the arterial end because of the hydrostatic pressure drops along the capillary to only 15 mmHg, the osmotic pressure remains unchanged at 25 mmHg

This now exceeds the hydrostatic pressure, fluid moves back into the capillary

This transfer of substances, including water, to the tissue spaces is dynamic process

As blood flows slowly through the large network of capillaries from the arterial to the venous end, there is constant change

Not all the water and cell waste products return to the blood capillaries.

Of the 24 litres or so fluid that moves out of the blood across capillary walls every day, only about 21 litres returns to the bloodstream at the venous end

The excess is drained away from the tissue spaces in the minute lymph capillaries which originate as blind-end tubes with wall similar to, but more permeable than, those of the blood capillaries

Extra tissue fluid and some cell waste materials enter the lymph capillaries and are eventually returned to the blood stream

The heart is a roughly cone-shaped, hollow, muscular organ

10cm long and is about the size of the individual’s fist

Weighs approximately 225g in women and about 310g in men



Position

Situated in the thoracic cavity, in the middle of the sternum

It lies obliquely, a little more to the left than the right and presents a base above and an apex below

The apex is about 9cm to the left of the midline at the level of the 5th intercostal space

The base extends to the level of the 2nd rib



Organs Associated with the Heart

Inferiority

The apex rests on the central tendon of the diaphragm



Superiority

The great blood vessels (the aorta, superior vena cava, pulmonary artery and pulmonary veins)



Posteriorly

The oesophagus, trachea, left and right bronchus, descending aorta, inferior vena cava and thoracic vertebrae



Laterally

The lungs – the left lung overlaps the left side of the heart



Anteriorly

The sternum, ribs and intercostal muscles



Structure

The heart is composed of three layers of tissue:

Pericardium

Myocardium

Endocardium



Pericardium

Made up of two sacs

The outer sac consists of fibrous tissue and the inner consists of a continuous double layer of serous membrane

The outer sac is continuous with the tunica adventitia of great blood vessels above, and is adherent to the diaphragm below

Its inelastic, fibrous nature prevents over-distension of the heart

The outer layer of the serous membrane, the parietal pericardium, lines the fibrous sac

The inner layer, the visceral pericardium, is adherent to the heart muscle

A similar arrangement of a double membrane forming a closed space is also seen with the pleura (the membrane enclosing the lungs)

The serous membrane consists of flattened epithelial cells

It secrets serous fluid into the space between the visceral and parietal layers, which allows smooth movement between them when the heart beats

The space between these layers is only a potential space, in health the two layers are in close contact, with only the thin film of serous fluid between them



Myocardium

Composed of specialised cardiac muscle found only in the heart

It is not under voluntary control, but, like skeletal muscle, cross-stripes are see on microscopic examination

Each fibre (cell) has a nucleus and one or more branches

The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells.

Microscopically these intercalated discs can be seen as thicker, darker lines than the ordinary cross-stripes

This arrangement gives cardiac muscle the appearance of being a sheet of muscle rather than a very large number of individual cells

Due to the continuity of the end-to-end fibres each one does not need to have a separate nerve supply

When an impulse is transmitted it spreads from cell to cell via the branches and intercalated discs over the whole sheet of muscle which in turn causes the contraction

The sheet arrangement of the myocardium enables the atria and ventricles to contract in a coordinated and effective manner

The myocardium is thickest at the apex and thins out towards the base

This reflects the amount of work each chamber contributes to the pumping of blood

Its thickest in the left ventricle, which has the greatest workload

The atria and ventricles are separated by a ring of fibrous tissue, which does not conduct electrical impulses.

When a wave of electrical activity passes over the atrial muscle it can only spread to the ventricles through the conducting system that bridges the fibrous ring from the atria to the ventricles



Endocardium

Lines the chambers and valves of the heart

It is a thin, smooth, glistening membrane that permits smooth flow of blood inside the heart

Consists of flattened epithelial cells

Continuous with the endothelium lining the blood vessels



Interior of the Heart

The heart is divided into a right side and a left side by the septum

Septum --> partition consisting of myocardium covered by endocardium

After birth blood cannot cross the septum

Each side is divided by an atrioventricular valve into an upper chamber (atrium) and a lower chamber (ventricle)

The atrioventricular valves are formed by double folds of endocardium, strengthened by some fibrous tissue

The right atrioventricular valve (tricuspid valve) has three cusps (flaps)

The left atrioventricular valve (mitral valve) has two cusps (flaps)

Flow of blood in the heart is one way --> blood enters the heart via the atria and passes into the ventricles below

The valves between the atria and ventricles open and close passively (requires no energy) according to changes in pressure in the chambers

They open when the pressure in the atria is greater than that in the ventricles

During ventricular systole (contraction) the pressure in the ventricles rises above that in the atria and the valves close --> preventing backflow of blood

The valves are prevented from opening upwards into the atria by tendinous cords (chordae tendineae) --> these extend from the inferior surfaces of the cusps to little projections of myocardium into the ventricles, covered with endothelium (papillary muscles)




Flow of Blood through the Heart

The superior and inferior vena cava empty their contents into the right atrium

This blood passes via the right atrioventricular valve into the right ventricle à it is then pumped into the pulmonary artery

The opening of the pulmonary artery is guarded by the pulmonary valve, formed by three semilunar cusps

This valve prevents the backflow of blood into the right ventricle when the ventricular muscle relaxes

After leaving the heart the pulmonary artery divides into left and right pulmonary arteries

These carry the venous blood to the lungs where exchange of gases take place

Two pulmonary veins from each lung carry oxygenated blood back to the left atrium

Blood then passes through the left atrioventricular valve into the left ventricle à it is then pumped into the aorta

The opening of the aorta is guarded by the aortic valve, formed with three semilunar cusps

From this sequence of events it can be seen that the blood passes from the right to the left side of the heart via the lungs (pulmonary circulation)

Both the atria contract at the same time and this is followed by the simultaneous contraction of both ventricles

The muscle layer of walls of the atria is thinner than that of the ventricles

This is consistent with the amount of work they do

Assisted by gravity

Propel the blood only through the atrioventricular valves into the ventricles

The ventricles actively pump blood to the lungs and around the entire body

The pulmonary artery leaves the heart from the upper part of the right ventricle

The aorta leaves from the upper part of the left ventricle



Blood Supply to the heart

Arterial supply --> 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

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

The coronary arteries transverse the heart, eventually forming the network of capillaries

Venous drainage --> most venous blood is collected into several small veins that join to form the coronary sinus, which opens into the right atrium

The remainder passes directly into the heart chambers through venous channels



Conducting System of the Heart

The heart is an intrinsic system whereby the cardiac muscle is automatically stimulated to contract without the need for external stimulation

This is known as autorythmicity

The intrinsic system can be stimulated or depressed by nerve impulses initiated in the brain and by circulating chemicals, including hormones

Small groups of specialised neuromuscular cells in the myocardium initiate and conduct impulses, causing coordinated and synchronised contraction of the heart muscle



Sinoatrial node (SA node)

Small mass of specialised cells

Lies in the wall of the right atrium near the opening of the superior vena cava

The SA node is the pacemaker of the heart

Normally initiates impulses more rapidly than other groups of neuromuscular cells

Firing of the SA node causes atrial contraction



Atrioventricular node (AV node)

Small mass of neuromuscular tissue

Situated in the wall of the atrial septum near the atrioventricular valves

Normally the AV node conducts impulses that arrive via the atria and that originated from the SA node.

There is a delay here --> the electrical signal takes 0.1 of a second to pass through into the ventricles.

This allows the atria to finish contracting before the ventricles start

The AV node also have a secondary pacemaker function and takes over this role if there is a problem with the SA node itself, or with the transmission of impulses from the atria

Its intrinsic firing rate is slower than that set by the SA node



Atrioventricular bundle (AV bundle of His)

Mass of specialised fibres that originate from the AV node

The AV bindle crosses the fibrous ring that separates the atria and ventricles

At the upper end of the ventricular system it divides into the right and left bundle branches

Within the ventricular myocardium the branches break up into fine fibres -->Purkinje fibres

The AV bundle, bundle branches and Purkinje fibres convey electrical impulses from the AV node to the apex of the myocardium where the wave of ventricular contraction begins

It then sweeps upwards and outwards, pumping blood into the pulmonary artery and the aorta



Nerve Supply to the Heart

The heart is also influenced by autonomic nerves originating in the cardiovascular centre in the medulla oblongata which reach it through the autonomic nervous system

These consist of parasympathetic and sympathetic nerves and their actions are antagonistic

The vagus nerves (parasympathetic) supply mainly the SA and AV nodes and atrial muscle

Parasympathetic stimulation reduces the rate at which impulses are produced -->decreasing the rate and force of the heartbeat

The sympathetic nerves supply the SA and AV nodes and the myocardium of the atria and ventricles

Sympathetic stimulation increases the rate and force of the heartbeat



Factors affecting Heart Rate

Gender

Autonomic (sympathetic and parasympathetic) nerve activity

Age

Circulating hormones

Adrenaline

Thyroxin

Activity and exercise

Temperature

The baroreceptor reflex

Emotional stress



The Cardiac Cycle

Function of the heart --> to maintain a constant circulation on blood throughout the body

The heart acts as a pump and its action consists of a series of events known as the cardiac cycle

During each heartbeat (cardiac cycle) the heart contracts and then relaxes

The period of contraction is called systole and that of relaxation, diastole



Stages of the Cardiac Cycle

Normal number of cardiac cycles per minute ranges from 60 to 80

Taking 74 as an example each cycle lasts about 0.8 of a second and consists of:

Atrial systole --> contraction of the atria

Ventricular systole --> contraction of the ventricles

Complete cardiac diastole --> relaxation of the atria and ventricles

The superior vena cava and the inferior vena cava transport deoxygenated blood into the right atrium at the same time as the four pulmonary veins bring oxygenated blood into the left atrium.

The atrioventricular valves are open and blood flows passively through to the ventricles

The SA node triggers a wave of contraction that spreads over the myocardium of both atria, emptying the atria and completing ventricular filling (atrial systole 0.1 s)

When the electrical impulse reaches the Av node it is slowed down, delaying atrioventricular transmission

This delay means that the mechanical result of atrial stimulation, atrial contraction, lags behind the electrical activity by a fraction of a second

This allows the atria to finish emptying into the ventricles before the ventricles begin to contract

After this brief delay the AV node triggers its own electrical impulse, which quickly spreads to the ventricular muscle via the AV bundle, the bundle branches and Purkinje fibres

This results in a wave of contraction which sweeps upwards from the apex of the heart and across the walls of both ventricles pumping the blood into the pulmonary artery and the aorta (ventricular systole 0.3 s)

The high pressure generated during ventricular contraction is greater than that in the aorta and forces the atrioventricular valves to close, preventing backflow of blood into the atria

After contraction of the ventricles there is complete cardiac diastole, a period of 0.4 seconds, when atria and ventricles relax

During this time the myocardium recovers in preparation for the next heartbeat, and the atria refill in preparation for the next cycle

The valves of the heart and of the great vessels open and close according to the pressure within the chambers of the heart

The AV valves are open while the ventricular muscle is relaxed during arterial filling systole

When the ventricles contract there is a gradual increase in the pressure the atrioventricular valves close

When the ventricular pressure rises above in the pulmonary artery and in the aorta, the pulmonary and aortic valves open and blood flows into these vessels

When the ventricles relax and the pressure within them falls, the reverse process occurs

First the pulmonary and aortic valves close, then the atrioventricular valves open and the cycle begins again

This sequence of opening and closing valves ensures that the blood flows in only one direction