7.5: Summary

The main parts of the circulatory system are the heart, blood vessels, and blood. There are 3 main types of blood vessels. Thick-walled high-pressure arteries carry blood from the heart and branch extensively into thin-walled, very- low-pressure capillaries that make intimate contact with each of the body’s cells. Capillaries then converge into medium-walled, medium-pressure veins that return the blood back to the heart (veins have valves that help).

Blood contains red blood cells, white blood cells, and platelets suspended in a liquid called plasma. Red blood cells have hemoglobin, which carries oxygen to cells throughout the body and carries carbon dioxide back to the lungs. White blood cells are an essential part of the immune system, and platelets play a key role in blood clotting.

Anemia is a reduced capacity of the blood to carry oxygen due to insufficient red blood cells or hemoglobin. Less oxygen is delivered to the tissues, so an anemic person tires easily and can feel weak, dizzy, drowsy, irritable, and have a headache. Several nutritional deficiencies can cause anemia. The most common ones are deficiencies of iron, folate, and vitamin B₁₂.

Iron deficiency is the most common cause of anemia, and is common among children and premenopausal women. Iron deficiency is unusual among men and postmenopausal women, because they aren’t growing like children and don’t have regular blood losses like menstruating women. Without enough iron, normal amounts of hemoglobin can’t be made, since iron is an essential part of hemoglobin. So red blood cells are smaller because of less hemoglobin.

Folate and vitamin B₁₂ are needed to make red blood cells. Fewer (but bigger) red blood cells are made when there’s a deficiency of either of these B-vitamins. Though the anemia caused by folate deficiency looks the same as that caused by B₁₂ deficiency, the one from a B₁₂ deficiency is called pernicious (deadly) anemia. This is because, in addition to anemia, untreated B₁₂ deficiency can cause permanent nerve damage and ultimately death.

Vitamin B₁₂ is unique in that it’s the only vitamin that plants can’t make. Those who include animal foods like milk or eggs in their diet get enough B₁₂ in their diet. (Of all the vitamins, it’s required in the smallest amount.) B₁₂ deficiency generally occurs only when the stomach doesn’t make enough of the intrinsic factor needed for B₁₂ absorption. The stomach normally makes plenty of intrinsic factor. Longtime vegans (those who eat only plant foods) should see to it that they get enough B₁₂ (e.g., as a supplement or in plant foods fortified with B₁₂). Exclusively breast-fed infants born of long-term vegans are particularly at risk of developing a B₁₂ deficiency.

Plasma is the fluid portion of blood. Plasma proteins (proteins dissolved in the plasma) include:

• Antibodies, which protect against infections.
• Albumin, which carries various substances, holds fluid, and regulates the pH of blood.
• Blood-clotting factors, which are needed to form blood clots.
• Lipoproteins, which transport fat in the blood. Fat (lipid) doesn’t dissolve in water/plasma, but does when combined with protein (lipoprotein).