Circulatory System: Blood Components, Vessels, Function

Circulatory System: Blood and Lymph

The circulatory system is responsible for distributing food (nutrients) and oxygen throughout the body. Blood, lymph — HCH-HCH.

Circulatory Blood

Circulatory blood (a tissue of cells suspended in a liquid medium) includes several components:

Plasma

Plasma: acellular liquid (about 90% water) containing dissolved substances, including:

• Nutrients: glucose, amino acids, lipids, water and minerals — transported from the digestive tract to the rest of the body;
• Waste products: transported from cells to the excretory organs (e.g., urea), excess salts and carbon dioxide;
• Hormones: carried from producing glands to the organs where they are used;
• Other substances dissolved in plasma.

Red Blood Cells / Erythrocytes

Red blood cells (erythrocytes): anucleate (in mammals), contain hemoglobin, which gives the cells their red color due to iron and enables them to bind and transport oxygen. Function: carry oxygen required for cellular respiration. Approximately 1 mm³ contains ~5,000,000 red blood cells. A deficit of red blood cells is anemia, which can be caused by a lack of iron or vitamin B12.

White Blood Cells / Leukocytes

White blood cells (leukocytes): have a nucleus; primary function is defense and cleaning. Approximately 1 mm³ contains ~8,000 white blood cells. There are different types:

• Phagocytes: cells that engulf (phagocytose), capture, destroy or digest microbes and old cells;
• Lymphocytes: produce antibodies that destroy bacteria and bind to unknown molecules to neutralize harmful effects.

Pus consists of dead white blood cells and microbes produced during an infection in a wound.

Platelets

Platelets: cell fragments without a nucleus. Approximately 1 mm³ contains 150,000–300,000 platelets. Function: promote blood coagulation. Platelets adhere to a wound, stick to the edges of broken vessels and release substances that lead to the formation of thrombin. Thrombin transforms fibrinogen (in plasma) into fibrin, which forms tiny fibers. These fibers trap red blood cells and initiate clot formation, preventing blood loss and blocking entry of microbes.

Blood Vessels

Blood circulates through blood vessels: arteries, veins and capillaries. Arteries branch into arterioles, which lead to capillaries; blood is then returned via venules, which join together to form veins that return blood to the heart.

Arteries

Arteries are vessels that carry blood from the heart throughout the body. They are elastic and adapt to withstand the pressure of blood during systole and diastole. Arteries have three layers:

• Tunica externa (outer connective tissue);
• Tunica media (muscle fiber layer — thick in arteries);
• Tunica intima (inner endothelial layer).

Veins

Veins are vessels that return blood from the capillaries to the heart. They are less elastic than arteries and contain valves (semilunar valves) to prevent backflow of blood. Veins also have three layers similar to arteries, but the tunica media is thinner.

Capillaries

Capillaries are the smallest vessels, positioned between arteries and veins. Blood flows very slowly through them, and because their walls are very thin (formed by endothelial cells), they allow the passage of nutrients into tissues and the movement of white blood cells for defense.