The Heart and Blood Vessels
· The individual cells/tissues
that make up a large organism exchange substances with their immediate
environment by diffusion, osmosis and active transport across their cell
surface membrane.
· The problem is that their immediate environment is made up of other
cells/tissues of the organism and the fluid between cells.
· The cells/tissues could
quickly take up all the oxygen, glucose and other essential substances in this
environment and excrete toxic concentrations of carbon dioxide and other waste
substances (such as urea) into it.
· This means that new supplies
of essential substances have to be brought continuously to the immediate
environment and toxic wastes have to be removed.
· In large organisms, simple
diffusion will not do this quickly enough.
· Instead, large organisms
have transport systems to carry substances to and from the immediate
environment of the cells/tissues.
· The transport systems use moving fluids to carry substances to and from the
cells/tissues.
· The fluids (such as blood and sap) are mainly water.
· Substances are carried along
with the water in solution, in suspension, or bound to some component of the
fluid.
· So, there is a mass flow of fluid/water, carrying
substances to and from the immediate environment of the cells/tissues.
· The transport systems are linked to specialised exchange surfaces, such
as the lungs, small intestine, kidney, roots, or leaves.
· The transport fluids carry substances to and from these exchange
surfaces and the immediate environment of the cells/tissues of the rest of the
organism.
· At the exchange surfaces,
the transport system will help to
maintain concentration gradients.
· Uptake of substances, such as oxygen, food molecules and water.
· The transport system continuously carries useful substances away from the
exchange surface:
· this keeps the concentration of the substance lower on the organism side of
the exchange surface than on the external environment side,
· which maintains a concentration gradient across the exchange surface and
uptake continues.
· Excretion of substances, such as carbon dioxide and
urea:
· the transport system
continuously carries excretory/waste
substances to the exchange surface,
· this keeps the concentration of the waste substance higher
on the organism side of the exchange surface
· which maintains a concentration gradient across the exchange surface and
excretion continues.
You
should be able to relate the structure of the mammalian heart to its functions.
· The heart.
· The mammalian heart has two
atria and two ventricles, with muscular walls.
· The atria collect blood
returning to the heart along veins.
· The ventricles pump blood
away from the heart along arteries.
· The right atrium receives
blood from all of the body, except the
lungs.
· The right ventricle pumps blood only to the lungs.
· The left atrium receives
blood from the lungs.
· The left ventricle pumps blood to all
of the body, except the lungs.
· The left ventricle has more
work to do than the right ventricle and so has a thicker muscular wall.
· The left and right sides of
the heart pump synchronously (both atria and then both ventricles together).
· For blood to travel around
the whole body it has to travel twice through the heart.
· This is called double circulation.
· Blood leaves the heart under
high pressure, due to the pumping actions of the ventricles.
· Blood returns to the heart
with a very low pressure.
· This is because the blood
pressure is reduced as it flows through all the arteries, arterioles,
capillaries, venules and veins of the circulatory system.
· The coronary arteries carry blood to the heart muscle.
· This muscle needs a very
good blood supply, to bring in the oxygen and glucose that is needed for respiration and to get rid of waste
carbon dioxide.
· Respiration supplies the ATP
that the muscle needs to contract.
What happens during a heartbeat - the cardiac
cycle.
· When the ventricles are
contracting, the atria are filling with blood.
· Blood can only flow from the
atria into the ventricles when the pressure in the ventricles is lower than in
the atria - along a pressure gradient.
· The atria have thin muscle
walls and can only generate a small amount of pressure.
· After the ventricles have contracted,
they relax, their walls recoil and the volume
of the ventricles increases.
· This lowers the pressure in the ventricles until it is below that in the
atria.
· The higher pressure in the atria pushes the atrio-ventricular valves open
and blood flows into the ventricles.
· The ventricles then contract
and the atria relax.
· To stop blood flowing back
into the atria when the ventricles
contract, the atrio-ventricular valves close.
· The valves are closed by the rising pressure of the blood in the ventricles.
· These one-way valves make sure that blood can only flow from the atria to
the ventricles.
· When the ventricles are filling, the pressure inside is much lower than the blood pressure in the
arteries.
· The semilunar valves are closed
by the higher pressure in the artery and this prevents backflow of blood
from the arteries into the ventricles.
· As the ventricles contract,
the pressure inside rises until it is greater than the pressure in the arteries.
· The pressure difference then
forces the semilunar valves open and blood flows into the arteries along the
pressure gradient.
· These one-way valves make sure that the blood can only flow from the
ventricles into the arteries.
· The whole cycle then repeats
itself (about 70 times per minute).
· The cardiac cycle is often
presented in questions as graphs showing pressure changes in parts of the heart
and aorta with time.
· The graph shows changes in
pressure in the left atrium, left ventricle and the aorta with time.
·
At point A,
the ventricle is contracting and the pressure inside it becomes greater than
the pressure in the aorta.
· The semilunar valve is then
forced open, allowing blood to flow into the aorta.
· At point B, the ventricle starts to relax and
the pressure inside falls below the pressure in the aorta.
· The semilunar valve is then
forced closed, preventing back-flow of blood into the ventricle.
· At point C, the pressure inside the relaxing
ventricle falls below that of the atrium.
· The atrio-ventricular valve
then opens and allows blood to flow into the ventricle from the atrium.
· At point D, the ventricle begins to contract
again and the pressure inside rises above that in the atrium.
· This causes the
atrio-ventricular valve to close, preventing back-flow of blood from the
ventricle into the atrium.
· The points X and Y mark the start and end of one cardiac cycle. Note
that the graph starts to repeat itself after Y.
· The time between X and Y is 0.8 seconds (from the graph).
· The number of beats per
minute can be calculated as;
60 ¸ 0.8
= 75 beats per minute.
Note. It is very common for students to make errors in
calculation and come up with silly answers; like 75,000 or 0.075 beats per
minute!
· Each heartbeat is
started/initiated by the sino-atrial
node (SAN).
· This is a small patch of
specialised heart/cardiac muscle cells.
· At regular intervals (about
0.8 seconds) they start a wave of electrical activity, or impulses, which
travel through the walls of the atria and cause them to contract.
· The wave of electrical
activity eventually reaches a second patch of specialised cells, the atrio-ventricular node (AVN).
· After a brief time delay,
this node sends out impulses along specialised muscle strands ( Bundle of His
and Purkinje Fibres) to the walls of the ventricles and causes them to
contract.
· This sequence means that the
atria contract first, followed by the ventricles.
· The sino-atrial node is a
built-in pacemaker, which means that the heart can beat without any input from
the nervous system (it is a myogenic heart).
· If the pacemaker fails, an
artificial one (an electrode) can be attached to the surface of the heart.
· This delivers regular bursts
of electricity to the area of the sino-atrial node and causes contraction of
the heart.
· There are connections from
the nervous system to the sino-atrial node. This allows the rate of output of
the node to be accelerated or slowed down, depending upon the needs of the
body. This is dealt with in more detail later in this section.
· The sequence of beating of the atria and ventricles and the action of
the heart valves ensures that blood can only flow in one direction through the
heart and the body.
· This is very important to the efficient functioning of the blood transport
system.
Note. If you are asked about the events producing one heart beat, do NOT
answer in terms of the medulla, chemoreceptors in the blood and responses to
changes in blood chemistry.
You
should be able to relate the structure of arteries, veins and capillaries to
their functions.
· Arteries carry blood under high pressure away from the heart.
· Each time the heart beats
the ventricles send a surge of blood along the arteries, pushing the blood
forwards.
· Each surge of blood puts
pressure on the walls of the arteries as it travels along, causing the arterial
wall to bulge slightly. (This is felt as a pulse where the artery is near to
the surface of the body, e.g. at the wrist.)
· The arterial wall has to be
able to resist this pressure, to avoid bursting and maintain a high blood
pressure.
· It also reacts by recoiling,
to squeeze the blood and maintain its high pressure.
· The structure of the wall
allows it to give slightly without splitting and then to squeeze the blood.
· There are three
layers/tunics making up the wall of an artery.
· Elastin fibres have elastic
properties.
· A layer of endothelial cells
covers the inside of all blood vessels.
· In a drawing the endothelial
layer/tunica intima may be shown as highly folded. This folding increases the
surface area, allowing for the expansion of the artery as each pulse passes by.
· Veins carry blood under low pressure towards the heart.
· The high blood pressure
produced by the beating of the heart and carried by the arteries has been
almost completely destroyed by resistance to flow in the smallest arterioles
and capillaries.
· The walls of veins have the
same coats as the arteries but not in the same proportions.
· The blood pressure in the
veins is not enough on its own to lift blood from the lower body back to the
heart.
· As muscles in the legs and
body contract, they press on the veins and squeeze the blood along them.
· To make sure that the blood
travels in one direction, towards the heart, the veins have one-way valves at
regular intervals.
· These are semi-lunar valves.
· In A, blood is being pushed up the vein with enough pressure to open
the semi-lunar valves in B.
· In B, blood flows towards the heart.
· In C, the blood tries to flow back down the vein and this causes the
valves to close.
· Capillaries are where exchange of substances takes place between the
blood and the cells/tissues of the body.
· The cells of the body have
to be within a very short distance of a capillary, to allow for efficient
movement of substances by diffusion.
· This means that there are
very large numbers of capillaries, giving a very large surface area for exchange with the cells/tissues.
· The walls of the capillaries
are only one cell thick (endothelial
cell).
· This gives a very short pathway for the exchange of
substances with the cells/tissues.
You
should understand the role of chemoreceptors, aortic and carotid bodies, the
medulla and the sino-atrial node in the control of the cardiac cycle.
· The amount of blood pumped
by the heart increases and decreases depending upon the requirements of the
body.
· During exercise, the rate of
respiration in the muscle tissues
increases.
· This increases the demand
for oxygen and glucose and the need to get rid of waste carbon dioxide.
· There will be a lowering of
the concentrations of oxygen and glucose in the blood and an increase in the
concentration of carbon dioxide.
· Homeostatic mechanisms will try to return these
concentrations to their normal values.
· There will be an increase in
ventilation of the lungs (section 3.3), more glucose will be released from
reserves (section 3.5) and the rate of circulation of the blood will increase;
to increase transport to and from the tissues.
· To increase the rate of circulation, the heart rate increases,
· the volume of blood pumped with each beat increases (the stroke volume)
· and the strength of contraction of the cardiac muscles increases; leading
to higher blood pressure.
· After exercise, and when
blood concentrations are back to normal, the heart has to return to its
normal/resting state.
· There are nervous and chemical mechanisms
involved in the regulation of cardiac activity.
· Nervous control of heart action is controlled by cardiovascular centres in the medulla of the brain.
· This is part of the
autonomic nervous system, involved in the non-conscious control of body
functions.
· In the medulla there is a cardio-inhibitory centre (part of the parasympathetic nervous system) which
can send nerve impulses to the sino-atrial node of the heart (via the vagus
nerve).
· When the nerve impulses
reach the node, they cause the release of the neurotransmitter acetylcholine.
· This slows down the
generation of electrical impulses by the sino-atrial node and slows down the rate of the heartbeat.
· It also reduces the stroke volume and strength of contraction of the
cardiac muscle.
· In the medulla there is also
a cardio-acceleratory centre (part of the sympathetic nervous system) which can
also send nerve impulses to the sino-atrial node.
· In this case the
neurotransmitter released is noradrenaline
and this speeds up the activity of the node, thus increasing the rate of
heartbeat.
· It also increases the stroke
volume and strength of contraction of the heart muscle.
· The hormone, adrenaline, is very similar in structure to noradrenaline
and also causes the heart to speed-up.
· Adrenaline is released as
part of the ‘fight or flight’ reaction to danger.
· The activities of the
cardio-inhibitory and acceleratory centres are governed by nerve impulses from receptors
in the aortic and carotid bodies.
· These are specialised areas
in the walls of the aorta and carotid arteries, containing stretch receptors and chemoreceptors.
· If heart rate and blood pressure increase, the aorta and
carotid arteries expand slightly.
· The stretch receptors detect
this stimulus and send nerve impulses
to the medulla, which acts as the co-ordinating centre.
· Some nerve impulses go to
the cardio-inhibitory centre, causing it to send nerve impulses that slow the
heart (the effector) and thus lower
blood pressure (the response).
· Some nerve impulses go to
the acceleratory centre and inhibit its action.
· As blood pressure falls, the
stretch receptors produce fewer nerve impulses and the inhibitory effect on the
heart reduces.
· This is an example of negative feedback.
· If blood pressure falls below normal, the stretch receptors send nerve
impulses that inhibit the cardio-inhibitory centre and stimulate the
acceleratory centre.
· As a result, the heart
speeds-up and blood pressure rises until negative feedback reduces the
stimulatory effect.
· The actions of the
cardio-inhibitory and acceleratory centres will tend to return blood pressure
and heart activity to some normal levels.
· This is an example of an homeostatic mechanism.
· During exercise, the muscles
in the limbs squeeze the veins harder and increase the flow of blood back to
the heart.
· This increases the filling
of the heart chambers; a greater volume of blood enters for each beat/the stroke
volume increases.
· The heart responds by
beating more strongly, which increases blood pressure. (You may have been
taught about this as Starling’s law.)
· Exercise also leads to more
oxygen leaving the blood and more carbon dioxide entering.
· The chemoreceptors in the aortic and carotid bodies detect these
stimuli and send nerve impulses to centres in the medulla which increase the rate of ventilation. (See
section 3.3)
· This increase in ventilation
causes an increase in the heart rate.
Common
Mistakes
Not being able to say what mass flow is.
Not understanding how the sequence of events in one
heart beat relate to sequences of pressure changes and valve openings and
closings. This leads to great difficulties when trying to interpret
graphs/charts which show pressure changes in the heart, for example. Study the
examples in the text and sample questions carefully!
Not appreciating the importance of one-way flow in the
vascular system and how this is achieved. This would include the sequence in
which the heart chambers contract, controlled by the SAN, the one-way valves in
the heart and large veins and the constant pushing forward of blood by the
heart.
Mixing up the SAN and AVN. (The AVN is not even on the
syllabus!)
Not understanding the role of the medulla in
controlling heart rate. In particular, not being able to explain the
homeostatic function of this control and the role of negative feedback. Which
is why these aspects are discussed in some detail in the notes.
Candidates
often fail to answer questions as set. If asked about the control
of heart rate, they talk about the SAN and the events in the heart, when they
should be talking about the medulla, its receptors and nervous connections to
the SAN. On the other hand, when asked about the events of one heartbeat, they
talk about the medulla!
