Principal Metabolic Organs

The metabolic reactions of every organ contribute to the body’s ability to function normally and maintain health. Metabolic reactions also use or release energy and therefore affect body weight, with consequences for health.

Of particular concern to metabolism are the digestive organs, liver, pancreas, circulatory system, and kidneys. Together, they perform much of the work of breaking down compounds, making new ones, transporting nutrients and oxygen throughout the body, and removing the wastes generated by metabolic processes.

The Digestive Organs

The digestive system transports foods through the gastrointestinal (GI) tract, produces digestive juices and enzymes, absorbs nutrients, provides transport proteins to carry lipids and vitamins to other sites in the body, and reabsorbs salts and fluids. The digestive system also possesses the body’s most rapidly multiplying cells: when healthy, they replace themselves every few days. Disorders affecting the GI tract interfere with the ingestion, digestion, absorption, and metabolism of nutrients.

The Liver

Nutrients absorbed into the bloodstream are taken first to the liver. The liver is one of the body’s most active metabolic factories. It receives nutrients and metabolizes, packages, stores, or ships them out for use by other organs. It manufactures bile, which the body uses to emulsify fat for digestion and absorption. It metabolizes and detoxifies drugs, prepares waste products for excretion, and participates in iron recycling and blood cell manufacture. It also makes many proteins necessary for health, including immune factors, transport proteins, and clotting factors. When liver disorders disrupt metabolism, they profoundly affect both nutrition and health status.

The Pancreas

The pancreas not only contributes digestive juices to the GI tract but also has another metabolic function: it produces the hormones insulin and glucagon that regulate the body’s use of glucose. After a meal, as blood glucose rises, the pancreas secretes insulin. Insulin prompts cells to take up glucose and use it as fuel; insulin also prompts liver cells to store glucose as glycogen. When blood glucose falls (as occurs between meals), the pancreas responds by secreting glucagon into the blood.

Glucagon raises blood glucose by signaling the liver to dismantle its glycogen stores and release glucose into the blood for use by all the other body cells. Glucose is an indispensable fuel for brain cells, nerve cells, and red blood cells. Its availability is therefore crucial to normal nervous system activity and blood chemistry. Abnormalities are associated with the digestive functions of the pancreas, and those associated with its hormonal functions.

The Heart and Blood Vessels

The heart and blood vessels conduct blood, with its cargo of nutrients and oxygen, to all the other body cells and carry wastes away from them. Diseases of the heart and arteries, therefore, affect the health of the whole body. Metabolic reactions that affect the heart and blood vessels include, most importantly, the making and transport of lipoproteins, which are the carriers of cholesterol and other lipids from the liver to the tissues and back again.

High blood levels of low-density lipoproteins (LDL) and very-low-density lipoproteins (VLDL) promote atherosclerosis, which increases the risk of disability or death from heart attacks and strokes. 

The Kidneys

The kidneys are also active metabolic organs. Unceasingly, for 24 hours of every day, they filter waste products from the blood to be excreted in the urine and reabsorb needed nutrients, thereby maintaining the blood’s delicate chemical balances.

The kidneys’ cells also produce compounds that help to regulate blood pressure and convert a precursor compound to active vitamin D, thereby helping to maintain the bones. Thus, disorders of the kidneys nearly always involve the heart and the skeleton.

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