10.1: Overview of Blood and Metabolism

Learning Objectives

• List the components of blood.
• Describe the major role of blood.
• Define metabolism, catabolism, and anabolism.
• Identify functions of various catabolic and anabolic pathways.

Blood

Blood is a connective tissue of the circulatory system, transporting nutrients to cells and waste products from cells. It supports cellular metabolism by transporting synthesized macromolecules and waste products. Additionally, it transports molecules, such as hormones, allowing for communication between organs. Blood is made up of red blood cells (erythrocytes), white blood cells (leukocytes), platelets, and plasma (Figure \(\PageIndex{1}\)).

The liquid part of blood is called plasma and it is mostly water (~90%), but also contains proteins, ions, glucose, lipids, vitamins, minerals, waste products, gases, enzymes, and hormones. Plasma makes up ~55% of total blood.

The cellular components of blood include red blood cells, white blood cells, and platelets. Red blood cells (erythrocytes) are the most numerous of the components and make up ~45% of total blood. The most vital duty of red blood cells is to transport oxygen from the lungs to all cells in the body so that they can make energy via aerobic metabolism. The white blood cells (leukocytes) that circulate in blood are part of the immune system and they survey the entire body looking for foreign invaders to destroy. Platelets are fragments of cells that are always circulating in the blood in case of an emergency. When blood vessels are injured, platelets rush to the site of injury to help form blood clots to stop bleeding at the wound site. White blood cells and platelets make up ~1% of total blood.

Figure \(\PageIndex{1}\): Components of blood include erythrocytes, leukocytes, platelets, and plasma. (CC BY 3.0; by KnuteKnudsen via Wikimedia Commons)

Once absorbed from the small intestine, all nutrients require transport to cells in need of their support. Additionally, molecules manufactured in other cells sometimes require delivery to other organ systems. Blood transports nutrients and molecules to all cells. Water-soluble molecules, such as some vitamins, minerals, sugars, and many proteins, move independently in blood. Fat-soluble vitamins, triglycerides, cholesterol, and other lipids are packaged into lipoproteins that allow for transport in the watery environment of blood. Many proteins, drugs, and hormones are dependent on transport carriers, primarily albumin. In addition to transporting all of these molecules, blood must transfer the oxygen breathed in by the lungs to all cells in the body. For additional information about the composition of blood and how that composition relates to anemia, watch this video from the Khan Academy:

"What's Inside of Blood?" (CC BY-NC-SA 4.0 by Khan Academy)

 

In the metabolism of macronutrients to energy, cells produce the waste products carbon dioxide and water. As blood travels through smaller and smaller vessels, the rate of blood flow is dramatically reduced allowing for efficient exchange of nutrients and oxygen for cellular waste products. The kidneys remove any excess water in the blood, and blood delivers the carbon dioxide to the lungs where it is exhaled. Also, the liver produces the waste product urea from the breakdown of amino acids, and detoxifies many harmful substances, all of which require transport in the blood to the kidneys for excretion.

Microcytic vs. Macrocytic Anemia

Many people have heard of anemia, but did you know that there are different types of anemia? Microcytic anemia occurs when the red blood cells are smaller than normal (micro = small). Microcytic anemia can develop with iron, copper, or vitamin B₆ deficiencies. When iron, copper, or vitamin B6 is inadequate, microcytic anemia develops because there is not enough hemoglobin synthesis to make normal red blood cells. On the other hand, macrocytic anemia is when the red blood cells are larger than normal (macro = large). Macrocytic anemia can develop with folate or vitamin B₁₂ deficiencies. When folate or vitamin B12 is inadequate, macrocytic anemia develops because the red blood cells cannot mature and divide appropriately.

Figure \(\PageIndex{2}\): Comparison of normal red blood cells to microcytic and macrocytic cells. (by Health Testing Centers)

Metabolism

Metabolism is defined as the sum of all chemical reactions required to support cellular function. Metabolism is either categorized as catabolism, referring to all metabolic processes involved in molecule breakdown, or anabolism, which includes all metabolic processes involved in building bigger molecules. Generally, catabolic processes release energy and anabolic processes consume energy. The overall goals of metabolism are energy transfer and matter transport. Energy is transformed from food macronutrients into cellular energy, which is used to perform cellular work. Metabolism transforms the matter of macronutrients into substances a cell can use to grow and reproduce and also into waste products.

In Chapter 6, you learned that enzymes are proteins and that their job is to catalyze (speed up) chemical reactions. Without enzymes, chemical reactions would not happen at a fast enough rate and would use up too much energy for life to exist. A metabolic pathway is a series of enzymatic reactions that transform the starting material (known as a substrate) into intermediates, which are the substrates for the next enzymatic reactions in the pathway, until, finally, an end product is synthesized by the last enzymatic reaction in the pathway.

Food is broken down into nutrients during digestion. Once absorbed, blood transports the nutrients to cells. Cells requiring energy or building blocks take up the nutrients from the blood and process them in either catabolic or anabolic pathways.

Energy metabolism refers more specifically to the metabolic pathways that release or store energy. Some of these are catabolic pathways, like glycolysis (the splitting of glucose), β-oxidation (fatty-acid breakdown), and amino acid catabolism (Table \(\PageIndex{1}\)). Others are anabolic pathways, and include those involved in storing excess energy (such as glycogenesis), and synthesizing triglycerides (lipogenesis) (Table \(\PageIndex{2}\)). When energy levels are high, cells build molecules; when energy levels are low, catabolic pathways are initiated to make energy.

Table \(\PageIndex{1}\): Catabolic Pathways

Catabolic Pathways	Function
Glycolysis	Glucose breakdown
Glycogenolysis	Glycogen breakdown
β-oxidation	Fatty-acid breakdown
Proteolysis	Protein breakdown to amino acids

 

Table \(\PageIndex{2}\): Anabolic Pathways

Anabolic Pathways	Function
Gluconeogenesis	Synthesize glucose
Glycogenesis	Synthesize glycogen
Lipogenesis	Synthesize triglycerides
Amino-acid synthesis	Synthesize amino acids

Key Takeaways

• Blood is the fluid in the body that transports absorbed nutrients to cells and waste products from cells.
• The fluid part of blood, plasma, makes up the largest amount of blood volume and contains proteins, ions, glucose, lipids, vitamins, minerals, waste products, gases, enzymes, and hormones.
• The cellular components of blood include red blood cells, white blood cells, and platelets.
• As blood travels through smaller and smaller vessels the rate of blood flow is dramatically reduced, allowing for efficient exchange of nutrients and oxygen for cellular waste products.
• Metabolism is defined as the sum of all chemical reactions required to support cellular function. The overall goals of metabolism are energy transfer and matter transport.
• A metabolic pathway is a series of enzymatic reactions that transforms the starting material (known as a substrate) into intermediates, which are the substrates for the next enzymatic reactions in the pathway, until, finally, an end product is synthesized by the last enzymatic reaction in the pathway.
• Energy metabolism refers more specifically to the metabolic pathways that release or store energy. When energy levels are high cells build molecules, and when energy levels are low catabolic pathways are initiated to make energy.