Red blood cell (erythrocytes):

• Biconcave disc-shaped.
• Size: 8 µm (diameter) and 2 µm (thickness).
• No nucleus.
• Contains hemoglobin.
• Elastic; allows it to squeeze into thin capillaries.
• 5 million cells/mL.
• Life span 120 days.
• Function: transport oxygenated and deoxygenated blood in the form of oxyhemoglobin and carbaminohemoglobin respectively.
• Produced in the bone marrow.
• Destroyed in the liver and spleen.

• Transport oxygen.
• Transport water.
• Transport heat.
• Transport wastes.
• Transport nutrients.
• Transport hormones.
• Transport carbon dioxide.

Arteries:

• Size of the lumen; small.
• Muscular wall: thick.
• Presence of valve: absent.
• Blood pressure: high.
• Function: transport oxygenated blood except for the pulmonary artery. 
• Blood flow: transport blood from the heart to all body parts.

Capillaries: 

• Size of the lumen: absent.
• Muscular wall: one cell thick.
• Presence of valve: absent.
• Blood pressure: higher than veins and lower than arteries.
• Function: the site of exchange for gases and nutrients.
• Blood flow: transport blood from arteries to the veins.

Veins:

• Size of the lumen: large.
• Muscular wall: thin.
• Presence of valve: present (prevent the backward flow of blood).
• Blood pressure: lower than arteries and capillaries.
• Function: transport deoxygenated blood except for pulmonary vein. 
• Blood flow: transport deoxygenated blood from body tissues to the heart.

1. The weight of a human's heart is around 250g until 350g and has the size of a fist.
2. The heart is situated at the thorax cavity, above the diaphragm, and protected by the ribcage.
3. The heart's muscles are myogenic, which means they contract and relax automatically and are not controlled by the nervous system.
4. Covered by pericardium to protect the muscle tissues of the heart. 
5. The function of the valve is to ensure that the blood flows only in one direction and to prevent the backward flow of blood.
6. It has a four-chambered heart; atria (right and left); thinner than ventricles and ventricles (right and left); thicker.