oxygen.
Figure 18.4: Capillaries connect arteries and veins. ( 1 )Blood is carried back to the heart in blood vessels calledveins. Veins have thinner walls
than arteries do, as you can see inFigure18.5. The blood in veins is not under pressure.
Veins have valves that stop blood from moving backward. Blood is moved forward in veins
when the surrounding skeletal muscles squeeze the veins. Blood that is carried by veins is
usually low in oxygen. The exception is the pulmonary veins that return oxygen-rich blood
to the heart from the lungs.
Figure 18.5: The walls of veins are not as thick as artery walls; veins have valves that stop
blood from flowing backward. ( 17 )
Bloodis a body fluid that is a type of connective tissue. Blood is made of blood cells, and
a fluid calledplasma.The main types of cells found in blood are red blood cells and white
blood cells. Red blood cells are the cells that carry oxygen. Oxygen-rich blood is bright red
and oxygen-poor blood is dark red. You will learn more about blood in a later lesson in this
chapter.
The cardiovascular system of humans isclosed.That means the blood never leaves the large
loop of blood vessels in which it travels. Other animals such as invertebrates have open
circulatory systems, in which their blood can leave the blood vessels.