TOPIC 1.CLASSIFICATION KINGDOM FUNGI
The endothelium is the
innermost layer of the artery. It has only one layer of cells. The endothelium
surrounds the lumen (the central tube of the vessel). The lumen of an
artery is narrow and smooth so that it can transport blood at high pressure.
The muscular layer is made of smooth
muscle and elastic fibres. Smooth muscle is arranged in circles round the
endothelium. This layer makes it possible for the artery to contract and relax
for the efficient movement of blood.
The outermost layer is the fibrous
layer made of connective tissues such as collagen. The fibres are arranged
parallel to the length of the vessel. They enable the artery to withstand the
pressure caused by the blood coming from the heart.
When the ventricles contract, the
arteries relax allowing blood from the heart to flow into them. When the
ventricles relax, the arteries contract, forcing the blood forward. This
contraction and relaxation of arteries is felt as a pulse.
Pulse rate is the number of pulses
per minute. The pulse rate reflects the heartbeat. An adult human’s heart beats
at an average of 72 times a minute. However, this can increase or
decrease due to physical activity, emotional state or health factors
Arteries branch to form arterioles.
Arterioles in turn branch to form capillaries. Capillaries are joined at
the other end by venules which join to form veins.
Veins
Veins are vessels that transport
blood to the heart from all parts of the body. All veins transport deoxygenated
blood except the pulmonary vein. The pulmonary vein transports oxygenated blood
from the lungs to the heart
