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Circulation
Multi-cellular animals have a much smaller surface area: volume ratio than
single-celled organisms (e
...
amoeba)
...

All circulatory systems include;
- A fluid in which substances are transported (e
...
blood)
- Vessels through which the fluid can flow (e
...
arteries)
- A mechanism for moving the fluid through the vessels (e
...
the heart)
There are two circulations in the human circulatory system;
1
...

2
...

The path of blood around the
human circulatory system is as
follows; all the blood arriving at the
right side of the heart is
deoxygenated because it has given
up its oxygen to respiring cells in
muscles and organs
...


Definitions;
- Pulmonary: related to the lungs
- Hepatic: related to the liver
- Renal: related to the kidneys
- Carotid: related to the brain
- Systemic: related to the whole body
- Mesenteric: related to the stomach and intestines

- In addition to the network of blood vessels throughout the body, there is also
a network of lymphatic vessels
...

Blood Vessels
All blood vessels have an endothelial lining which is only 1 cell thick
...

ARTERIES
- Arteries carry oxygenated blood away from the heart
- Arteries have thick walls to withstand high blood pressure
- Arteries have lumens with small diameters
- Elastic tissue allow the artery to expand and recoil under high pressure
- Arteries have a thick fibrous layer made of collagen which resists pressure to
prevent arteries from bursting
VEINS
- Veins carry deoxygenated blood towards the heart
- Veins have valves to prevent backflow of blood in conditions with low blood
pressure
- Veins have thinner walls and bigger lumens
CAPILLARIES
- Capillaries have walls which are one cell thick (squamous endothelial cells) to
decrease diffusion distance for more efficient exchange of substances
- Capillaries are very small, meaning that they can carry blood close to any cell
- Capillaries are selectively permeable, allowing for diffusion and osmosis
between the blood and body cells
...
This causes the arteries to become narrower and restricts
blood flow
...
This causes a
buildup of lactic acid, lowering the pH and stopping muscle cells from
contracting
...


Low density lipoproteins (LDLs) can stick to the arterial walls and build up
between the endothelial layer and muscle tissue
...
If an atheroma
breaks through the lining of the blood vessel, a thrombus (blood clot) may form
which can then block the blood vessels
...
Having a diet high in cholesterol- excess levels to cholesterol can cause
coronary blood vessels to swell inwards and makes them more likely to
become blocked by a blood clot
...
Smoking- smoking can increase blood pressure, which can cause damage to
the inside of the coronary arteries
...

3
...
Diabetes is also
associated with obesity; this can also result in damage to blood vessels
...

Structure of the heart
Semi-lunar valves: the pulmonary and
aortic valves
...


The cardiac cycle
1
...

2
...
The semi-lunar valves are closed; the bicuspid and tricuspid valves
are open
...
Ventricular systole: the left and right ventricles contract, forcing blood out of
the heart through the aorta and the pulmonary artery
...

4
...

Control of heart rate
- nervous control of the heart beat is mainly by reflex actions which involve
reflex arcs originating from sensory receptors
...

- Chemoreceptors are sensitive to pH changes in the blood caused by the
concentration of O2 and CO2
...


- The receptors pass on the impulse from via the sensory nerve to the medulla
...




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