Arteries and red blood cells

Open cirulatory system - blood bathes tissues directly

Closed circulatory system- Stays inside vessels

Arteries-- Lumen is small for high pressure, thick wall withstands high pressure elastic tissue to stretch and recoil smooth muscle narrows the lumen limits flow.

Veins-- Veins carry blood back to the heart, exception = pulmanary vein--The lumen is relatiively large to ease the pressure of the thinner layers of collegen that do not recoil contains valves that prevent back flow.

Capillaries-- Thin walls that allow material exchange walls consist of flattened endothelial cells narrow lumen same as that of a red blood cell helps them retain oxygen presses them close to the capillary walls reducing the diffusion path to tissue.

Produced by stem cells in marrow Oxygen is absorbed into the blood by haemoglobin oxygen pressure must be high before the oxygen can be absorbed, total oxygen = 4 molecules. Difficult to absorb 1st and 4th oxygen atom, fetal haemoglobin has a greater affinity for oxygen.

Red blood cells gives up more oxygen to working muscles than tissue at rest.