6.2 THE TRANSPORT
SYSTEM
Our circulatory system provides a delivery and collection service for the whole body.
The heart, blood and blood vessels make up an efficient transport system that
reaches all cells, bringing the substances they need and taking away wastes.
Humans and other mammals have a closed circulatory system with blood contained
inside a network of arteries, veins and capillaries
The Heart
Blood is kept moving by the pumping
action of the heart muscle.
The heart will beat 2.5x109 times in a
lifetime on average, sending more
than 1.5 million liters of blood from
each ventricle.
The heart is about the size of your
fist.
It is a double pump with two separate
sides.
The right hand receives
deoxygenated blood from all over the
body and pumps it to the lungs to
pick up more oxygen.
The left hand side receives
oxygenated blood from the lungs and
pumps it to cells all over the body.
The heart has four chambers- two
smaller atria (atrium singular) at the top.
Two larger ventricles below.
The right and left hand sides are
completely separated from one another.
Atria have thin walls as the blood
received from the veins is under
relatively low pressure.
Ventricles are stronger and more
muscular as their job is to pump blood
out of the heart.
Both ventricles hold the same volume of
blood but the left ventricle is thicker than http://www.youtube.com/watch?v=DAXa4eR
the right since it must generate enough 1s0M
pressure to pump blood all around the
http://www.youtube.com/watch?v=oE8tGkP5
body. The right pumps only to the lungs.
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Atria are separated from ventricles by
atrioventricular valves, which prevent the
blood flowing backwards into the atria.
A second set of valves in the aorta and
pulmonary arteries- the semilunar valvesprevent backflow into the ventricles as
they relax after contraction.
Heart muscle works continuously, beating
75 times per minute when an average
person is resting.
Coronary arteries extend over the surface
of the heart and penetrate deep into the
muscle fibers to supply oxygen and
nutrients.
The Cardiac Cycle
The cardiac cycle is the sequence of
events that takes places during one
heart beat.
As the heart’s chambers contract, blood
inside them is forced on its way.
Valves in the heart and arteries stop the
blood flowing backwards.
1.
4.
3.
2
1. Atrial Systole- The muscles of the atrium wall contract, pushing blood through
the atrioventricular valves in the ventricles. Both atria contract at the same time.
2. Ventricular Systole- Blood forced into the ventricles causes blood pressure
inside to rise, so the antroventricular valves snap closed. When the ventricles are
full, ventricle muscles contract, generating the pressure that drives blood through
the semilunar valves in the aorta and pulmonary artery. A pulse is produced that
can be felt in arteries in other parts of the body.
1.
4.
3.
http://www.youtube.com/watch
?v=rguztY8aqpk
2
3. Diastole- Ventricles and atria now relax, and the pressure inside them is low. The
semilunar valves are closed by the back pressure of blood in the arteries.
4. Blood flows into the atria from the veins, opens the atrioventricular valves, and
begins to fill the ventricles. Blood from the body enters the right atrium via the vena
cava. Blood from the lungs enters the left atrium from the pulmonary vein.
5. The whole cycle repeats when the atria contract again.
Control of the Heart Beat
Heart tissue is made of a special type of muscle that is different from other muscles
in our bodies.
Cardiac muscle is unique because it contracts and relaxes without stimulation from
the nervous system. It is said to be myogenic.
Natural myogenic contractions are initiated at an inbuilt pacemaker, which keeps
cardiac muscle working in a coordinated, controlled sequence.
The pacemaker, or sinoatrial node (SAN), is a special region of muscle cells in the
right atrium that sets the basic pace of the heart. The rate set by the SAN is also
influenced by stimulation from the nervous system and by hormones.
At the start of every heart beat, the SAN produces an impulse that
stimulates both atria to contract.
A second structure, the atrioventricular node (AVN) at the base of the right
atrium, is also stimulated. It delays the impulse briefly until the atrila
contraction finishes and then transmits it on a bundle of modified muscle
fibers to the base of the ventricles.
Impulses radiate up through the ventricles, about 0.1 seconds after the
atria.
The natural rhythm of the pacemaker is modulated by the nervous system
so that the heart rate is adjusted to our activity levels.
It speeds up when exercising and more oxygen and nutrients are needed,
and slows down when we sleep.
Changes to our heart rate are not under our conscious control but result
from impulses sent from a control center in the part of the brain stem known
as the medulla.
http://www.youtube.com/watch?v=yGlFBzaTuoI
Impulses to speed up the heart pass along
the sympathetic nerve, which stimulates the
pacemaker to increase rate.
Impulses sent along he parasympathetic
nerve cause the heart rate to slow down.
The medulla monitors blood pressure and
CO2 levels.
Emotions such as stress, as well as
increases in activity level, can cause increase
in heart rate.
During periods of excitement, hear or stress
the adrenal glands release the hormone
adrenalin, which travels in the blood to the
pacemaker and stimulates it to increase the
heart rate.
Blood and Blood Vessels
Endotheliumvery smooth
single layer of
cells
Lumen
Collagen fibers
Arteries are blood vessels that carry blood away from the heart.
Elastic fibers and
smooth muscle
They branch and divide many times before forming arterioles and eventually tiny capillaries that reach all the
tissue in the body.
Arteries have thick outer walls of collagen and elastic fibers which withstand high blood pressure and prevent
vessels becoming overstretched or bursting.
Just beneath the outer covering is a ring of circular smooth muscle that contracts with each heart beat to
maintain blood pressure and keep blood moving.
Inside the artery, the lumen is narrow to keep blood pressure high.
The lumen’s lining of smooth epithelial cells reduces friction and keeps blood moving.
Capillaries
Capillaries are the smallest vessels
The lumen of the capillary is only about 10um in diameter and some are so small that red blood cells
must bold up in order to pass along.
Networks of these tiny capillaries reach almost every body cell.
Blood flow here is very slow, at less than 1mm per second, but capillary walls are only one cell thick so
the distance for diffusion of materials in and out of them is as small as possible.
Some capillaries have spaces between their cells enabling plasma and phagocytes to leak out into the
tissues.
Veins
Veins carry blood back towards the heart from body tissues.
Small veins called venules join up to form large veins, which have much thinner
walls than arteries. They also contain few elastic and muscle fibers.
Blood inside a vein does not pulse along and the lumen is large to hold slowmoving blood.
The thin walls can be compressed by adjacent muscles and this helps squeeze
blood along and keep it moving.
Many veins contain valves to prevent blood from flowing backwards.
Differences and Similarities of the Vessels
Artery
Vein
Capillary
Thick walls
Thin walls
Walls one cell thick
No valves
Valves present
No valves
Blood pressure high
Blood pressure low
Blood pressure low
Carry blood away
from the heart
Carry blood to the
heart
Link small arteries
to small veins
Composition of Blood
Blood plasma is a pale yellow liquid that makes up 50-60% of blood volume.
Suspended in plasma are three important groups of cells:
Eryhrocytes (red blood cells), whose job is to carry oxygen
Leucocytes (white blood cells), which fight disease.
Platelets (cell fragments), which are needed for blood clotting
Functions of Blood
Blood has two important roles:
Carrying dissolved materials to all cells,
Helps fight infectious diseases.
Substances Transported by Blood
Substance Transported
Source and Destination
Nutrients
Glucose, amino acids, vitamins and
minerals carried in plasma from the small
intestine to the cells
Oxygen
Carried by red blood cells from the lungs
to all tissues
Carbon Dioxide
Returned to the lungs in plasma and red
blood cells from respiring tissues
Urea
Carried in plasma from cells to the
kidneys for disposal
Hormones
Transported in plasma from glands to
target cells
Antibodies
Protein molecules produced by certain
lymphocytes to fight infection and
distributed in plasma
Heat
Distributed from warm areas to cooler
ones to maintain core temp.