In small unicellular organisms, substances move around slowly by diffusion.
Diffusion is too slow to move substances round the larger bodies of multicellular organisms.
They have a circulatory system: substances are carried in blood pumped by a heart.
In a closed circulatory system (eg in vertebrates) blood is enclosed in narrow blood vessels. This increases efficiency: blood travels faster as a higher pressure is generated.
Valves ensure blood flows in one direction:
Fish have a single circulation: heart pumps blood to gills for gas exchange, then to tissues and back to the heart.
Birds and mammals have a double circulation: right ventricle pumps blood to lungs. Blood returns to the left atrium and then the left ventricle pumps it to the rest of the body. Blood travels round the body faster, delivering nutrients faster, so the animals have a higher metabolic rate.
Arteries and veins contain collagen: a tough, fibrous protein to make them tough and durable.
The artery wall stretches as blood is pumped in and then recoils as the heart relaxes.
Blood flow is continual and there is a pulse.
Contracting muscles and low pressure in the chest when breathing in assist blood flow in veins. Valves prevent backflow. There is no pulse and pressure is low.
See diagrams and photomicrographs: Figure 1.10 on page 8 of the textbook.
§ narrow lumen
§ thicker walls
§ more collagen, elastic fibres and smooth muscle
§ no valves
§ wide lumen
§ thinner walls
§ less collagen, elastic fibres and smooth muscle
Figure 1.9 on page 8 of the textbook: make sure you know the structure of the heart.
The chambers of the heart (atria and ventricles) fill with blood when they relax (diastole) and pump blood out when they contract (systole).
The cardiac muscle making up the atria and ventricles is supplied with blood by the coronary arteries.
PHASE OF CARDIAC CYCLE
Pressure in the atria increases as they fill with blood returning from the veins.
Increased pressure opens the atrioventricular valves allowing blood to enter the ventricles.
The atria contract to force remaining blood into ventricles.
Ventricles contract from the base up, increasing the pressure and closing the atrioventricular valves.
The semilunar valves open and blood is forced into the arteries.
As the atria and ventricles relax, pressure falls.
In the ventricle, this causes closure of the semilunar valves.
In the atria blood is drawn into the heart from the veins.
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