An Overview of Blood

1. Welcome to Anatomy & Physiology II

Welcome to Anatomy & Physiology II, an in-depth exploration of the human body, its components, functions, diseases, and conditions. This course and the adjoining lab course (Anatomy & Physiology II Lab) are follow-ups to the Anatomy & Physiology I courses.

IMPORTANT: Students are strongly recommended to complete Anatomy & Physiology I before starting Anatomy & Physiology II. This course will not review terminology and concepts presented in Anatomy & Physiology I; this course assumes that information was previously learned. The Anatomy and Physiology courses are intended for those looking to work in a healthcare field or with a strong interest in learning about the human body.

In Anatomy & Physiology II (A&P II), you will learn about the cardiovascular, endocrine, lymphatic, respiratory, digestive, urinary, and reproductive systems as well as human development.

After completing this course, you are expected to understand and explain the basic organization of the body, anatomical terminology, how the included organ systems are organized and function to serve the body as a whole, and the foundations of various related diseases and conditions of the included organ systems.

try it

You have spent your whole life getting to know your own anatomy and physiology model: your body! With all this experience, there are more than a few things you already know about the body.

Let’s begin by seeing what you already know. Since this lesson is an overview of blood, try to guess the answers to the questions below. Don’t worry if there is a word you don’t yet recognize or understand—everything will be explained in the lessons in this course.

What color is blood?

Human blood is red.

Is blood a fluid, gas, or solid?

Blood is a fluid.

What are the functions of blood? (HINT: There are multiple)

Transport nutrients to and waste away from body cells, defense of the body, distribution of heat throughout the body, and maintenance of homeostasis.

What cells do we find in blood?

Blood contains erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

2. Course Organization

As you progress through this course, there are a few items to keep in mind.

• Content within the lesson:
  • Callouts: You will notice the use of callouts such as "Before You Start", "Did You Know?", "Try It", etc. The callout usage is intended to help convey a topic, start a brainstorm, ask a question, or give an example. You will notice the callouts are used selectively within the content.
  • Term(s) to Know: All identified terms to know will be listed out after each topic/subtopic. You will also be able to see these within the Glossary.
  • Term Pronunciation Table: Many terms in this course may be hard to pronounce; therefore, the term pronunciation table may appear right before the Term(s) to Know in each topic/subtopic. If this table is present, the contents will provide the difficult term, its pronunciation, and an audio file that, when clicked, pronounces the term spoken.
  • Assessments test the Content: All content shared within the lecture (lesson) can be found in the assessment(s).
• External content (links to videos, extra details, etc.)
  • External Content is Not Assessed: Any content that is linked out from the lessons—such as videos or “for more information”—is just that, more insight or details about a particular topic, and would not be included in the assessment(s). That said, these external links may help you better understand the content in the main lesson and we would recommend that you visit the links.

3. Functions of Blood

Blood is a tissue, a group of cells that work together to perform a specific function. More specifically, blood is a fluid connective tissue. It is a fluid because it assumes the shape of its container. Recall from A&P I that a connective tissue creates a connection between two or more other tissues. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements, also known collectively as formed elements, include erythrocytes (red blood cells, RBCs), leukocytes (white blood cells, WBCs), and cell fragments called thrombocytes (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.

The primary function of blood is to deliver nutrients to and remove wastes from body cells, but that is only the beginning of the story. The specific functions of blood also include defense and maintenance of homeostasis such as body temperature, electrolyte levels, and body pH.

3a. Transportation

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 from the lungs into the blood and is transported to the heart, which then pumps it out to the rest of the body. 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.

3b. Defense

Many types of leukocytes (white blood cells, WBCs) protect the body from external threats, such as disease-causing bacteria that have entered the bloodstream in a wound. Others seek out and destroy internal threats, such as cells with mutated DNA that could multiply to become cancerous, or body cells infected with viruses.

When damage to the vessels results in bleeding, thrombocytes (platelets) and certain proteins dissolved in the plasma, the fluid portion of the blood, interact to block the ruptured areas of the blood vessels involved by forming a blood clot. This protects the body from further blood loss.

3c. Maintenance of Homeostasis

reflect

You may recall from A&P I that homeostasis is the state of internal equilibrium or balance maintained by living things. In order to maintain this balance, the body uses various sensors to detect changes in the environment or stimuli. If a stimulus is significant, a signal is sent to the control center, either the spinal cord or the brain where it is processed. If a response is required to keep the body in balance, a signal is sent from the control center to an effector, such as a skeletal muscle, organ, gland, or other target where the action is performed. If the effect generated by the homeostatic reflex arc opposes or decreases the original stimulus, it is considered negative feedback. If the effect enhances or increases the stimulus, it is positive feedback.

Recall that body temperature is regulated via a classic negative-feedback loop. 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 also helps to maintain the chemical balance of the body. Proteins and other compounds in blood act as buffers, which help to regulate the pH of blood and the body. Blood also helps to regulate the water content of body cells by allowing for fluid shift between intracellular and extracellular compartments.

terms to know

Blood

A fluid connective tissue which transports nutrients and waste is responsible for defending against potentially harmful microorganisms or molecules and works to maintain homeostasis in the body.

Formed Elements

The cellular component of blood.

Plasma

The liquid component of blood.

4. Composition of Blood

You have probably had blood drawn from a superficial vein in your arm, which was then sent to a lab for analysis. Some of the most common blood tests—for instance, those measuring lipid or glucose levels in plasma—determine which substances are present within blood and in what quantities. Other blood tests check for the composition of the blood itself, including the quantities and types of formed elements.

As you already learned, blood is composed of formed elements (cells) and plasma (fluid). When blood is drawn from the body, it contains both of these components and is known as whole blood. The formed elements are made up of three types of cells. Erythrocytes, commonly known as red blood cells (RBCs), work to transport oxygen (O₂) to and carbon dioxide (CO₂) away from body cells. Leukocytes, commonly known as white blood cells (WBCs), primarily work to identify and remove potentially harmful or infectious agents from the body. Thrombocytes, commonly known as platelets, are cell fragments that work to clot blood and prevent continued blood loss when a blood vessel is cut or broken open. As you can see in the histology image below, erythrocytes make up the vast majority of formed elements in blood.

After a vial of blood has been drawn, one test that can be performed will determine your hematocrit (also known as packed cell volume, PCV), the percentage of erythrocytes (red blood cells), in a blood sample. It is performed by spinning the blood sample in a specialized centrifuge, a process that causes the heavier elements suspended within the blood sample to separate from the lightweight, liquid plasma. Because the heaviest elements in blood are the erythrocytes, these cells settle at the very bottom of the hematocrit tube. Located above the erythrocytes is a pale, thin layer composed of the remaining formed elements of blood. These are the leukocytes and thrombocytes. This layer is referred to as the buffy coat because of its color; it normally constitutes less than 1% of a blood sample. Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid, which constitutes the remainder of the sample.

think about it

What is blood made up of?

In normal blood, about 45% of a sample is erythrocytes. The hematocrit of any one sample can vary significantly, however, about 36–50%, according to gender and other factors. Normal hematocrit values for females range from 37 to 47, with a mean value of 41; for males, hematocrit ranges from 42 to 52, with a mean of 47. The percentage of other formed elements, the WBCs and platelets, is extremely small so it is not normally considered with the hematocrit. Therefore, the mean plasma percentage is the percent of blood that is not erythrocytes: for females, it is approximately 59% (or 100 minus 41), and for males, it is approximately 53% (or 100 minus 47).

A person's hematocrit can vary for a number of reasons including hydration status, elevation (altitude above sea level), recent trauma, disease states, kidney function, and more. When a person becomes dehydrated, their plasma levels decrease which, in turn, increases their hematocrit. This would also be true if the body is producing too many erythrocytes. In reverse, if overhydrated, the plasma levels increase and hematocrit decreases. This would also be true if the number of erythrocytes removed from circulation increased dramatically due to disease or injury.

terms to know

Whole Blood

Blood containing all of its components, formed elements and plasma.

Erythrocytes

Blood cells that transport oxygen to and carbon dioxide away from body cells; red blood cells.

Leukocytes

Formed elements that primarily identify and remove potentially harmful agents; white blood cells.

Thrombocytes

Blood cell fragments that function to clot blood and reduce blood loss.

Hematocrit

The percentage by volume of red blood cells in a blood sample.

Buffy Coat

A thin, pale layer of leukocytes and platelets that separates the erythrocytes from the plasma in a sample of centrifuged blood.

5. Characteristics of Blood

When you think about blood, the first characteristic that probably comes to mind is its color. Blood that has just taken up oxygen in the lungs is bright red, and blood that has released oxygen in the tissues is a more dark red. This is because hemoglobin, the protein that allows for erythrocytes to carry oxygen, is a pigment that changes color, depending upon the degree of oxygen saturation.

Blood is viscous and somewhat sticky to the touch. It has a viscosity approximately five times greater than water. Viscosity is a measure of a fluid’s thickness or resistance to flow, and is influenced by the presence of the plasma proteins and formed elements within the blood. The viscosity of blood has a dramatic impact on blood pressure and flow.

EXAMPLE

Consider the difference in flow between water and honey. The more viscous honey would demonstrate a greater resistance to flow than the less viscous water. The same principle applies to blood.

6. 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. There are literally hundreds of substances dissolved or suspended in the plasma, although many of them are found only in very small quantities.

6a. Plasma Proteins

About 7% of the volume of plasma—nearly all that is not water—is made of proteins. These include several plasma proteins (proteins that are unique to the plasma), plus a much smaller number of regulatory proteins, including enzymes and some hormones. The major components of plasma are summarized in the table below. The three major groups of plasma proteins are as follows:

• Albumin is the most abundant of the plasma proteins. Manufactured by the liver, albumin molecules serve as binding proteins—transport vehicles for hormones made from lipids. Recall that lipids are hydrophobic and would therefore not dissolve in blood which is mostly water. However, their binding to albumin enables their transport in the watery plasma. Albumin is also the most significant contributor to the osmotic pressure of blood; that is, its presence holds water inside the blood vessels and draws water from the tissues. This in turn helps to maintain both blood volume and blood pressure. Albumin normally accounts for approximately 54% of the total plasma protein content.
• The second most common plasma proteins are the globulins. A heterogeneous group, there are three main subgroups known as alpha, beta, and gamma globulins. The alpha and beta globulins transport iron, lipids, and the fat-soluble vitamins A, D, E, and K to the cells; like albumin, they also contribute to osmotic pressure. The gamma globulins are proteins involved in immunity and are better known as antibodies or immunoglobulins. Although other plasma proteins are produced by the liver, immunoglobulins are produced by specialized leukocytes. Globulins make up approximately 38% of the total plasma protein volume.
• Fibrinogen is the third of the three major groups of plasma proteins. Like albumin and the alpha and beta globulins, fibrinogen is produced by the liver. It is essential for blood clotting, a process described later in this chapter. Fibrinogen accounts for about 7% of the total plasma protein volume.

6b. Other Plasma Solutes

In addition to proteins, plasma contains a wide variety of other substances. These include electrolytes, such as sodium (Na), potassium (K), and calcium (Ca) ions; dissolved gasses, such as oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂); various organic nutrients, such as vitamins, lipids, glucose, and amino acids; and metabolic wastes. All of these nonprotein solutes combined contribute approximately 1% to the total volume of plasma.

IN CONTEXT

Career Connection
Phlebotomy and Medical Lab Technology
Phlebotomists are professionals trained to draw blood (phleb-, a blood vessel; -tomy, to cut). When more than a few drops of blood are required, phlebotomists perform a venipuncture, typically of a surface vein in the arm. They perform a capillary stick on a finger, an earlobe, or the heel of an infant when only a small quantity of blood is required. An arterial stick is collected from an artery and used to analyze blood gasses. After collection, the blood may be analyzed by medical laboratories or perhaps used for transfusions, donations, or research. While many allied health professionals practice phlebotomy, the American Society of Phlebotomy Technicians issues certificates to individuals passing a national examination, and some large labs and hospitals hire individuals expressly for their skill in phlebotomy.

Medical or clinical laboratories employ a variety of individuals in technical positions:

• Medical technologists (MT), also known as clinical laboratory technologists (CLT), typically hold a bachelor’s degree and certification from an accredited training program. They perform a wide variety of tests on various body fluids, including blood. The information they provide is essential to the primary care providers in determining a diagnosis and in monitoring the course of a disease and response to treatment.
• Medical laboratory technicians (MLT) typically have an associate’s degree but may perform duties similar to those of an MT.
• Medical laboratory assistants (MLA) spend the majority of their time processing samples and carrying out routine assignments within the lab. Clinical training is required, but a degree may not be essential to obtaining a position.

terms to know

Albumin

A class of plasma proteins that function as binding proteins in lipid transport and serve to support blood osmotic pressure.

Globulin

A class of plasma proteins that function in nutrient transport, supporting blood osmotic pressure, and immunity.

Fibrinogen

A class of plasma proteins that function in blood clotting.

summary

In this lesson, you learned about the general function and make up of blood. You first learned about the functions of blood, including how it provides transportation, plays a role in defense, and serves in the maintenance of homeostasis. You then learned about the composition of blood, what it is made of, and the key features or characteristics of blood. Lastly, you learned about blood plasma which is composed of various plasma proteins and other plasma solutes.