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The blood system

Topic 6
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2

Biology SL

➢ When blood leaves an arteriole it enters a capillary bed rather than a single
capillary
○

Much of the blood pressure is lost

➢ They are extremely thin (one cell thick) and therefore chemical exchanges occur
here, as it’s more efficient
➢ Blood cells make their way through capillaries one cell at a time
Veins
➢ Veins are blood vessels that collect blood from capillaries and returns it to the heart
➢ Pulmonary veins are blood vessels that lead oxygenated blood from the lungs back
to the heart
➢ The blood flow through veins is slower than through arteries, because the blood has
lost a great deal of pressure
○

That’s why they have thin walls and larger diameter

➢ They also have internal “one-way” valves that help keep the blood flowing toward
the heart
➢ Vena cava is the biggest vein in our bodies
➢ Vanule is the smallest type of vein

Artery

Capillary

Vein

Thick walled

1 cell thick

Thin walled

No exchanges

Exchanges

No exchanges

No internal valves

No internal valves

Internal valves

High internal pressure

Low internal pressure

Low internal pressure

The heart
➢ Each side of the heart has a collection of chambers for the blood that moves in from
the veins
○

They are thin-walled, muscular chambers that are called atria (sing
...
2

Biology SL

The muscle around the left ventricle is the thickest, because it sends blood
to the body

○

The force by which the blood is sent out is referred to as blood pressure

➢ The blood that is pumped out of the heart makes a circuit through the following
sequence of blood vessels
○

Aorta

○

A large artery

○

Smaller arteries

○

An arteriole

○

A capillary bed

○

A venule

○

Larger veins

○

A large vein

○

Vena cava

➢ The right side of the heart sends out blood along our pulmonary circulation
○

Oxygen poor blood goes from the heart to the lungs and oxygen rich blood
from the lungs to the heart

➢ The left side of the heart sends blood along the systemic circulation
○

Blood goes from the heart to our organs leaving oxygen and taking carbon
dioxide

Control of the heart rate
➢ The heart consists of a special kind of muscle called cardiac muscle
○

It spontaneously contracts and relaxes without any control by the nervous
system
■

This is known as myogenic muscle contraction

➢ The myogenic activity needs to be controlled in order to make the timing of the
contractions unified and useful
➢ “Du - dank” sound
○

“Du” ⇒ the atrioventricular valves close

○

“Dunk” ⇒ the semilunar valves close
■

It’s louder because they’re smaller and thus the pressure is higher

➢ Within the right atrium, there’s a mass of specialized tissue that has properties of
both muscle and nervous system cells within its walls
○

This tissue is called the sinoatrial node (SA node)

3

The blood system

Topic 6
...
8 seconds
giving you a heart rate of 72-75 beats/minute

○

It is controlled by nerves from the brain

➢ Also in the right atrium s another mass of specialized muscle tissue known as the

atrioventricular node (AV node)
○ The AV node receives the signal from the SA node, delays for approximately
0
...
2

Biology SL

➢ Heart valves open and close based on blood pressure differences on either side of
any one valve
➢ Both sides have similar pressures and volumes of blood at the same time
○

All pressures are measured in mm/Hg (Hg = mercury)

➢ Diastole is the term used for a chamber of the heart that isn’t contracting
○

When both chambers are in rest, then they’re both undergoing diastole

➢ Systole is the term used for a chamber of the heart that is contracting
➢ Both chambers in diastole
○

The atrial pressure is slightly higher than the ventricular pressure keepin
the atrioventricular valve open

○

Blood returning to the atrium via the veins moves passively down to the
ventricle through the open valve

○

The pressure in the aorta is much higher than in the left ventricle
■

This difference in pressure keeps the left semilunar valve closed and
prevents backflow into the ventricle

➢ Atria in systole, ventricles in diastole
○

When the SA node “fires”, the pressure in the atria increases and blood goes
into the ventricle

○

The pressure produced by this systole is low, because:
■

The walls of both atria are very thin

■

There’s no need for high pressure, as much of the volume of blood
has already moved passively within the ventricle through the open
atrioventricular valve

➢ Atria in diastole, ventricles in systole
○

When the AV node “fires”, the pressure in the ventricle increases a lot, so
the atrioventricular valve closes to prevent backflow of the blood to the atria
■

○

Called early ventricular systole

The pressure in the aorta is still far higher than in the ventricle, so the
semilunar valve is closed

○

The large volume of blood in the ventricle and its thick muscular walls
permit the pressure to build up as systole continues

○

The pressure in the ventricle becomes greater than that in the aorta and the
semilunar valve opens allowing the ventricle to pump blood in the aorta
■

○

Called late ventricular systole

As the ventricle finishes its contraction, the pressure inside it once again
drops below the pressure in the aorta and the semilunar valve closes
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The blood system

Topic 6

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