Transport in Animals - Blood and lymphatic vessels

Arteries have thick walls with a high proportion of $elastic$ $fibers$ and $smooth$ $muscle$ to withstand and maintain high blood pressure from the heart.

Capillaries are the site of exchange; their walls are composed of a single layer of $endothelial$ cells, providing a short diffusion path for $O_2$, $CO_2$, and $C_6H_{12}O_6$.

Veins contain $valves$ to ensure the one-way flow of blood back to the heart against gravity under low pressure.

The mammalian circulatory system is a $double$ $circulatory$ $system$: the pulmonary circulation (lungs) and the systemic circulation (body).

Tissue fluid is formed when high $hydrostatic$ $pressure$ at the arterial end of capillaries forces water and small solutes out of the $plasma$.

The lymphatic system collects excess tissue fluid, now called $lymph$, and returns it to the circulatory system via the $subclavian$ $veins$.

Red blood cells contain $haemoglobin$ ($Hb$), which binds reversibly with oxygen to form $oxyhaemoglobin$ ($HbO_8$ or $Hb(O_2)_4$).