Blood is the fluid tissue that feeds our cells. Half of the blood is a plasma – a nutrient-rich broth of salts, proteins, and sugars. The other half of blood is made up of red blood cells. These are the most abundant cells in the body, accounting for over 80% of the 30 trillion cells in the body.

Why this is important…Blood is vital to life! In medical practice, blood transfusions save lives. Medicines and nutrients can also be directly infused into the blood via intravenous lines. Diagnostically, blood tests monitor the overall condition of the body since blood picks up chemicals from every organ it flows through.

Foundations

Blood has long been recognized as a vital force for the body, sometimes with other significance attached to it. People once thought blood was purified to pass on traits to kids – which is why we hear about “bloodlines.” Blood was also one of the four humors – a theory about regaining health by balancing four types of body fluids (humors). This was often accomplished by “bloodletting” – literally draining blood out of people. Obviously this was misguided, but it was practiced for almost 2000 years up until the late 1800’s.

Blood is a living, complex, liquid tissue, made of cells floating in a nutrient-rich fluid. It wasn’t until about 50 years ago that microscopes allowed scientists to discover that blood was made up of living cells. The cells, the water, and the nutrients in blood each have roles to play in sustaining our bodies. In the approximately 5 liters of blood in your body, the composition is as follows:

Just over half is plasma – the yellowish fluid with many chemicals dissolved in water
Just under half is red blood cells – these carry oxygen and carbon dioxide
And under 1% is white blood cells (immune cells) and platelets (cell fragments that help blood clot)

Key terms

Solvent – liquid in which other substances can dissolve

Solute – a substance that dissolves in a liquid

Hydrogen bonding – holding together adjacent molecules by weak attractions created by opposite charges on polar covalent bonds. Note: The term “hydrogen bond” is often used to describe the attraction, but since it is an attractive force rather than a chemical bond, the term is avoided here for clarity.

Electrolyte – salt that dissolves in water to form ions; often applied to the ions themselves

Details

Plasma is mostly water, but contains a rich mix of other chemicals. The nutrients dissolved in the water include proteins, glucose, amino acids, lipids, electrolytes and nitrogen wastes. For now, let’s concentrate on the chemical properties of water that make it necessary to life.

The chemical makeup of water makes it polar. The oxygen pulls electrons toward itself, creating a lopsided sharing called a polar covalent bond (Figure 6-3). We have discussed polar bonds before, but did not discuss all of the reasons they are special.

Water molecules with hydrogen bonding between molecules. Oxygen on one molecule is attracted to hydrogen on another.

FIGURE 6-3. Hydrogen bonding in water. Dashed lines represent attractive forces (hydrogen bonding) between partially negative oxygen atoms (red) and partially positive hydrogen atoms (white). ("Hydrogen bonds in water" by Michal Maňas is in the Public Domain, CC0)

First, those partial charges allow water to “get along” with ions and other molecules with polar bonds. Therefore, water dissolves many substances well, such as glucose, amino acids, and salts. Another way to say this is that water is a good polar solvent.

Second, oppositely charged ends of polar molecules are attracted to one another. This attraction results in hydrogen bonding between water molecules. This is similar to the ionic bond studied earlier, but is much weaker since only partial charges are involved. The term “hydrogen bonding” can be misleading, since it is not a chemical bond like covalent or ionic bonds. Instead, it is a fairly weak attraction between molecules. This attraction holds water molecules together in a liquid state.

VIDEO 6a: Hydrogen bonding

After watching the video, you should be able to:

describe when hydrogen bonding occurs and what causes it
explain how it is different from other chemical bonds
give examples of hydrogen bonding in living systems

Electrolytes dissolve easily in the watery blood plasma. These play a crucial role in all parts of the body, including the blood. Common electrolytes dissolved in blood plasma include Na⁺_, Ca²⁺, and Cl^-. Cells have additional electrolytes, mainly K⁺, H⁺, bicarbonate (HCO₃^-), and inorganic phosphates (PO₄^3- for example). These play significant roles in water balance, nerve and muscle function, bone building, and pH regulation.

Proteins are abundant in blood plasma. These are a mix of free amino acids and small polypeptides (albumin and globulins) produced by the liver. These play several important roles, which we will return later.

Lipids require special care because they do not dissolve in the watery plasma. After lipids are digested, they are transported to the bloodstream in small bundles of triglycerides surrounded by phospholipids. Enzymes in blood vessels further break down some of these lipids. Free fatty acids produced this way attach to albumin proteins to circulate in the blood.

Stop & Think #3: Why is circulation of lipids more complex than other types of biomolecules?

Red blood cells have two simple jobs – carry oxygen to cells and carbon dioxide away from cells. They must be very small to perform this job, so they do something that other cells do not – they eject their nucleus. Recall that blood cells are produced in the bone marrow. To make red blood cells, stem cells multiply by mitosis, making many new “pre-red blood cells” or precursor cells. These precursor cells use the genetic instructions to make lots of hemoglobin – the protein that binds oxygen. They also make many of the other proteins necessary to function, including an enzyme (carbonic anhydrase) that helps the cells carry carbon dioxide. Once the proteins are made, the cells eject their nucleus and leave for the bloodstream.

Stop & Think #4: Since mature red blood cells lack a nucleus, what would they be unable to do?

Learning objectives

Describe properties of water and explain how they help sustain life.
Define electrolytes and give examples.
Describe what substances dissolve in water.
Describe hydrogen bonding and give several examples of its significance in living systems.

Stop & Think Questions Explained

3. Since blood is mostly water, which is polar, lipids pose a challenge since they are nonpolar.

4. Without a nucleus, red blood cells cannot make proteins to perform repairs. For this reason, they only live a few months before getting broken down.