CHAPTER 7 – THE CIRCULATORY SYSTEM


Blood vessels Blood flows from and returns to the heart through the blood vessels. There are three types of blood vessels: • arteries


• veins Arteries


Arteries carry blood away from the heart. They have thick muscle walls to withstand the high pressure of blood pumped out of the heart.


Arteries usually carry oxygenated blood (blood that contains high levels of oxygen and low levels of carbon dioxide).


Veins Veins carry blood to the heart. They have thinner walls than arteries.


The blood pressure in veins is lower than in arteries, so veins have valves to prevent the backflow of blood.


Veins usually carry deoxygenated blood (blood that contains low levels of oxygen and high levels of carbon dioxide).


CONCENTRATE! In diagrams, oxygenated blood is shown as red and deoxygenated blood is shown as blue. In reality, blood with a lower concentration of oxygen is still red, but not quite as bright as blood with a higher concentration of oxygen.


Capillaries Capillaries are very small blood vessels that link arteries and veins.


Capillary walls are only one cell thick and have lots of pores. This allows molecules, such as oxygen and carbon dioxide, to pass in and out of the surrounding cells.


To heart Away from heart • capillaries


Vein Tissue cells Waste


(carbon dioxide and urea)


Oxygen and


nutrients


Artery


Capillaries ? TEST TUBE


4. Identify two differences between arteries, veins and capillaries. 5. What structures in veins prevent blood from flowing backwards?


Elements of Science 59


: Fig. 7.5 The three types of blood vessels


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