The Blood

The heart pumps blood through a vast network of arteries and veins. Blood is a living fluid. It transports oxygen and other essential substances throughout the body, fights sickness, and performs other vital functions. 

1. Blood Is Fluid Connective Tissue

Blood is composed of 55% plasma and 45% “formed elements,” including red blood cells, white blood cells, and platelets. Because of these living cells suspended in the plasma, blood is considered a fluid connective tissue (not a fluid). It is the only fluid tissue in the body.

2. Blood Provides the Body's Cells with Oxygen and Removes Carbon Dioxide

Blood absorbs oxygen from air in the lungs. It transports the oxygen to cells throughout the body, and it removes waste carbon dioxide from the cells. In the lungs, the carbon dioxide moves from the blood to the air and is exhaled.

3. Blood Transports Nutrients and Hormones

Blood plays a large role in digestion and endocrine system functions. Digested nutrients are absorbed into the bloodstream through capillaries in the villi that line the small intestine. These nutrients include glucose, amino acids, vitamins, minerals, and fatty acids. Blood also transports some hormones secreted by endocrine system glands to target organs and tissues.

4. Blood Regulates Body Temperature

Blood absorbs and distributes heat throughout the body. It helps to maintain homeostasis through the release or conservation of warmth. Blood vessels expand and contract when they react to outside organisms, such as bacteria, and to internal hormone and chemical changes. These actions move blood and heat closer to or farther from the skin surface, where heat is lost.

5. Platelets Clot Blood at Sites of Injury

When a blood vessel tears, platelets and plasma proteins work together to stop blood loss. Platelets, also called thrombocytes, clump and form a plug in the damaged area. The proteins form threads called fibrins to complete the platelet plug, or clot.

6. Blood Brings Waste Products to the Kidneys and Liver

Blood transports waste substances to the organs that remove and process them for elimination. Blood flows into the kidneys through the renal arteries and out through the renal veins. The kidneys filter substances such as urea, uric acid, and creatinine out of the blood plasma and into the ureters. The liver also removes toxins from blood. During digestion, it cleans blood that has been enriched with vitamins before sending it back out to the rest of the body.

7. Red Blood Cells Are the Most Numerous Living Cells in Blood

Blood is 55% plasma and 45% formed elements. Red blood cells, also called erythrocytes, make up most of that 45%. Their primary function is to transport oxygen from the lungs to the cells of the body. Red blood cells are disc-shaped. They are flexible and bioconcave—flat and round with depressed centers.

8. White Blood Cells Protect the Body from Pathogens

White blood cells, also called leukocytes, are the disease-fighting components of blood. They account for just 1% of circulating blood but multiply during infection or inflammation. There are five types of white blood cells: neutrophils, eosinophils, basophils, lymphocytes, and monocytes. Neutrophils are the most abundant, comprising 60% to 70% of all white blood cells.

How Platelets Halt Bleeding? 

Platelets are activated when they come into contact with collagen, a protein that is found in the walls of blood vessels. Once activated, platelets release a number of substances that help to form a clot.

In other words, Platelets clump and form a plug in the damaged area of a torn blood vessel to stop blood loss.

These substances include:

• Adenosine diphosphate (ADP): ADP attracts more platelets to the site of injury and helps them to stick together.

• Thromboxane A2: Thromboxane A2 causes platelets to change shape and become more sticky.

• Fibrinogen: Fibrinogen is a protein that is converted to fibrin by the enzyme thrombin. Fibrin forms a mesh that traps other platelets and blood cells to form a clot.