Physiology of the Blood Vessels and Blood

Physiology of the Blood Vessels

Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit. Systemic arteries provide blood rich in oxygen to the body’s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation.

Oxgenated and deoxygenated blood flow through the major organs. Image description available.
Figure 9.2 Cardiovascular Circulation. The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration. From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]

Blood Pressure

Blood pressure is the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the general term “blood pressure” refers to the pressure of blood flowing in the arteries of the systemic circulation. Blood pressure is one of the critical parameters measured on virtually every patient in every health care setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as Korotkoff sounds. Blood pressure is measured in mm Hg and is usually obtained from the brachial artery using a sphygmomanometer and a stethoscope. Blood pressure is recorded as systolic pressure over diastolic pressure.

The Composition (Anatomy) of Blood and the Functions of the Components

Blood is a connective tissue made up of cellular elements and an extracellular matrix. The cellular elements are referred to as the formed elements and include red blood cells (RBCs), white blood cells (WBCs), and platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.

In the laboratory, blood samples are often centrifuged in order to separate the components of blood from one another (see the figure below). Erythrocytes are the heaviest elements in blood and settle at the very bottom of the tube. Above the erythrocyte layer, we see the buffy coat, a pale, thin layer of leukocytes and thrombocytes, which together make up less than 1% of the sample of whole blood. Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid, which constitutes the remainder of the sample.

In normal blood, about 45 percent of a sample is erythrocytes, referred to as the hematocrit. The hematocrit of any one sample can vary significantly, however, by about 36–50 percent. One of the factors that determines hematocrit is testosterone levels, and so cisgender men tend to have higher hematocrits than cisgender women. Taking synthetic testosterone can elevate it as well. Not counting the buffy coat, which makes up less than 1% of the blood, we can estimate the mean plasma percentage to be the percentage of blood that is not erythrocytes: approximately 55%.


This figure shows three test tubes with a red and yellow liquid in them. The left panel shows normal blood, the center panel shows anemic blood and the right panel shows polycythemic blood. Image description available.
Figure 9.3 Composition of Blood. The cellular elements of blood include a vast number of erythrocytes and comparatively fewer leukocytes and platelets. Plasma is the fluid in which the formed elements are suspended. A sample of blood spun in a centrifuge reveals that plasma is the lightest component. It floats at the top of the tube separated from the heaviest elements, the erythrocytes, by a buffy coat of leukocytes and platelets. Hematocrit is the percentage of the total sample that is composed of erythrocytes. Depressed and elevated hematocrit levels are shown for comparison. From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]

Blood Plasma

Like other fluids in the body, plasma is composed primarily of water. In fact, it is about 92% water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins.


The most abundant formed elements in blood, erythrocytes are basically sacs packed with an oxygen-carrying compound called hemoglobin. Production of erythrocytes in the red bone marrow occurs at the staggering rate of more than 2 million cells per second. For this production to occur, raw materials including iron, copper, zinc, B vitamins, glucose, lipids, and amino acids must be present in adequate amounts. Erythrocytes live only 120 days on average and thus must be continually replaced. Worn-out erythrocytes are phagocytized by macrophages, and their hemoglobin is broken down. The breakdown products are recycled or removed as wastes.


This photograph shows a few red blood cells.
Figure 9.4 Shape of Red Blood Cells. Erythrocytes are biconcave discs with very shallow centers. This shape optimizes the ratio of surface area to volume, facilitating gas exchange. It also enables them to fold up as they move through narrow blood vessels. From Betts, et al., 2013. Licensed under CC BY 4.0.


Leukocytes protect the body against invading microorganisms and body cells with mutated DNA, and they clean up debris; thus they are a major component of the body’s defenses against disease.


This figure shows micrographs of the different types of leukocytes. From left to right, the order of leukocytes shown are: basophil, eosinophil, neutrophil, monocyte, and lymphocyte.
Figure 9.5 Leukocytes. (Micrographs provided by the Regents of University of Michigan Medical School © 2012.) From Betts, et al., 2013. Licensed under CC BY 4.0.


After entering the circulation, approximately one-third of the newly formed platelets migrate to the spleen for storage for later release in response to any rupture in a blood vessel. They then become activated to perform their primary function, which is to limit blood loss. Platelets remain only about 10 days, then are phagocytized by macrophages.

Platelets are key players in hemostasis, the process by which the body seals a ruptured blood vessel and prevents further loss of blood. Although rupture of larger vessels usually requires medical intervention, hemostasis is quite effective in dealing with small, simple wounds. There are three steps to the process: vascular spasm, the formation of a platelet plug, and coagulation (blood clotting). Failure of any of these steps will result in hemorrhage.

Physiology of Blood

Although carrying oxygen and nutrients to cells and removing wastes from cells is the main function of blood, it is important to realize that blood also serves in defense, distribution of heat, and maintenance of homeostasis.


  • Nutrients from the foods you eat are absorbed in the digestive tract. Most of these travel in the bloodstream directly to the liver, where they are processed and released back into the bloodstream for delivery to body cells.
  • Oxygen from the air you breathe diffuses into the blood, which moves from the lungs to the heart, which then pumps it out to the rest of the body.
  • Endocrine glands scattered throughout the body release their products, called hormones, into the bloodstream, which carries them to distant target cells.
  • Blood also picks up cellular wastes and byproducts and transports them to various organs for removal. For instance, blood moves carbon dioxide to the lungs for exhalation from the body, and various waste products are transported to the kidneys and liver for excretion from the body in the form of urine or bile.


  • Leukocytes protect the organism from disease-causing bacteria, cells with mutated DNA that could multiply to become cancerous, or body cells infected with viruses.
  • When damage to the vessels results in bleeding, blood platelets and certain proteins dissolved in the plasma interact to block the ruptured areas of the blood vessels involved. This protects the body from further blood loss.


  • If you were exercising on a warm day, your rising core body temperature would trigger several homeostatic mechanisms, including increased transport of blood from your core to your body periphery, which is typically cooler. As blood passes through the vessels of the skin, heat would be dissipated to the environment, and the blood returning to your body core would be cooler. In contrast, on a cold day, blood is diverted away from the skin to maintain a warmer body core. In extreme cases, this may result in frostbite.
  • Blood helps to regulate the water content of body cells.
  • Blood also helps to maintain the chemical balance of the body. Proteins and other compounds in the blood act as buffers, which thereby help to regulate the pH of body tissues. The pH of blood ranges from 7.35 to 7.45.

Image Descriptions

Figure 9.2 image description: This diagram shows how oxygenated and deoxygenated blood flows through the major organs in the body. Pulmonary circulation involves the lungs, pulmonary artery and vein, vena cava, and aorta. Systemic circulation involves the upper body, hepatic vein, renal vein, aorta, liver, hepatic artery, hepatic portal vein, stomach, intestines, renal artery, kidneys, and lower body. [Return to Figure 9.2].

Figure 9.3 image description: This figure shows three test tubes with a red and yellow liquid in them. The left panel shows normal blood, the center panel shows anemic blood and the right panel shows polycythemic blood. Labels indicate plasma (water, proteins, nutrients, hormones, etc.), buffy coat (white blood cells, platelets), and hematocrit (red blood cells). [Return to Figure 9.3].


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Medical Terminology: An Interactive Approach Copyright © 2022 by LOUIS: The Louisiana Library Network is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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