Physiology: metabolism and how the body works

Physiology is the branch of biology that studies how organisms, their organ systems and their cells function. Below is a brief but substantive overview of how your body actually works — from metabolism to hormones. This material is intended for sports enthusiasts and anyone planning to apply for a coaching qualification.

What physiology is and how it relates to other sciences

Physiology is an experimental science that grew out of the study of humans and animals. It examines metabolism, tissue oxygen consumption, body temperature, blood pressure, bioelectric potentials and other phenomena that characterize living processes.

The discipline has three branches:

• General physiology — examines the general principles of living processes, including excitability, energy conversion and homeostasis.
• Special physiology — examines the functions of different organisms and organ systems, including mammals, birds and insects, as well as circulation, digestion, respiration and other systems.
• Comparative physiology — examines how organisms at different evolutionary stages function.

Human physiology is further divided into sports physiology, work physiology, age-related physiology and psychophysiology. Evolutionary physiology studies the relationship between function and evolution, while pathological and clinical physiology examine how diseased organisms function.

Anatomy, biochemistry and biophysics have emerged as separate disciplines from physiology. Function cannot be studied without an understanding of structure — hence the close connection with anatomy. Psychology developed from the physiology of nerves and sensory organs. Research methods are predominantly chemical or physical.

The physiology of metabolism: anabolism and catabolism

Metabolism is the complex of biochemical processes through which an organism interacts with its environment. It enables growth, maintenance, renewal and reproduction. Metabolism involves two opposing but inseparable processes:

• Anabolism — the assimilation of nutrients to form the body’s components. In green plants, anabolism is based on photosynthesis (CO₂, H₂O, NH₃). In animals and fungi, it relies mainly on preformed organic matter obtained from food that is foreign to the organism. After an initial breakdown, this material is used to build substances specific to the organism.
• Catabolism — the breakdown of the body’s own substances or nutrients into simpler compounds, also known as dissimilation. The end products are CO₂, H₂O and NH₃; mineral compounds such as orthophosphate and hydrogen sulfide are also released. Breakdown can also stop at intermediate stages, leaving more complex compounds.

The body’s internal environment is maintained by regulating the osmotic pressure of blood plasma. A deviation in the extracellular or intracellular space causes water or electrolytes to shift between compartments. Oncotic pressure, also known as colloid osmotic pressure, is maintained by plasma proteins: a decrease in albumin concentration causes water retention inside cells.

Homeostasis is the set of processes through which the body keeps the conditions necessary for its functioning constant. Regulation occurs both within individual cells and throughout the body. Cellular metabolism comprises active, standby and maintenance metabolism; at the whole-body level, a distinction is made between resting and basal metabolism. If metabolism in the respiratory muscles or heart muscle falls to the standby level, their activity ceases and the entire organism dies. Regulation is primarily controlled by the nervous and hormonal systems, with hunger and thirst acting as triggers.

The physiology of proteins, carbohydrates and lipids

Proteins

Proteins form part of the structure of every cell, accelerate chemical reactions and act as regulators and antibodies. The exchange of water and dissolved substances between blood and tissues, the transport of O₂ and carbon dioxide, muscle contraction and many other processes depend on them.

The daily protein requirement at rest is 0.8 g of protein per 1 kg of body weight; during physical work, it is half as much again. Proteins provide energy equivalent to 11-13% of total energy expenditure. Amino acid composition is crucial: of the 20 known amino acids, 9 are essential — L-leucine, isoleucine, lysine, methionine, phenylalanine, threonine, L-tryptophan, valine and histidine. The more of these a protein contains, the higher its biological value. Essential amino acids are found mainly in animal proteins.

In the digestive tract, pancreatic enzymes break proteins down into polypeptides and oligopeptides, and then into amino acids, which are absorbed into the bloodstream from the small intestine. The liver’s main functions in protein metabolism are the transamination and deamination of amino acids; the synthesis of nonessential amino acids from glucose, glycogen and fatty acids; the synthesis of the liver’s structural proteins and enzymes, as well as blood plasma proteins; the detoxification of exogenous substances; and the formation of ammonia and urea. The liver also secretes bile, which is necessary for fat absorption in the intestine.

Amino acids processed in the liver are carried through the blood to the tissues, where ribosomes in the cells use them to synthesize tissue proteins. Excess amino acids are used for energy or converted into carbohydrates and lipids.

During starvation, the body first uses free carbohydrates, followed by liver glycogen, then stored fat and, in the final phase, muscle proteins and other body proteins.

The end products of protein metabolism are nitrogen-containing compounds: creatinine, ammonia, urea and uric acid. Most are excreted as urea. Creatinine comes from protein metabolism in the muscles; the amount produced daily depends on muscle mass, and its concentration in plasma remains relatively constant at 9 mg/l. Creatinine is eliminated by glomerular filtration. Ammonium and ammonia are excreted through the renal tubules: in the tubular cells, glutamine is deaminated to glutamate and then to oxoglutarate, producing one molecule of ammonium while simultaneously releasing one molecule of bicarbonate. The more acidic the urine, the more ammonium is excreted.

Hormonal regulation involves several hormones. Growth hormone from the anterior pituitary increases cell membrane permeability to amino acids and raises the rate of protein synthesis. The thyroid hormones thyroxine and triiodothyronine stimulate synthesis and promote tissue differentiation. Adrenal glucocorticoids (hydrocortisone, cortisone) increase protein breakdown, particularly in the muscles, while increasing the rate of synthesis in the liver. Male sex hormones have anabolic effects. Insulin and growth hormone enable amino acids to enter cells; insulin increases DNA transcription in the cell nucleus and accelerates protein synthesis.

Carbohydrates

Carbohydrates are the primary energy source for animal organisms. They account for ~60% of daily energy expenditure. They are readily oxidized, with carbon dioxide and water as the end products; oxidizing 1 g of carbohydrates releases 4.0 kcal.

Cellulose is a polysaccharide that humans cannot digest, but it adds bulk to food, stimulates intestinal motility, speeds up intestinal transit and keeps stools soft. The recommended intake is 30 g/day.

Carbohydrates are broken down in the digestive tract into monosaccharides—mainly glucose, but also galactose and fructose—which are absorbed into the blood from the small intestine. In the liver, they are converted into glycogen for storage through glycogenesis. Glycogen can also be formed from lactic acid and the products of protein and lipid metabolism—a process referred to as gluconeogenesis. Glycogen is stored in the liver and muscles. When needed, glycogenolysis takes place: liver glycogen is broken down and released into the blood as glucose. Excess carbohydrates are converted into lipids and deposited in fat stores.

Blood glucose is maintained at a relatively stable level within the range of 3.3…6.1 mmol/l. Changes in glucose levels are detected by glucose receptors in the liver, blood vessels and the ventrolateral nucleus of the hypothalamus. During starvation and intense physical exertion, blood sugar may fall—a condition called hypoglycemia. Eating too many sweets raises it above the normal range, causing hyperglycemia. If blood sugar falls below 0.5-0.2 g/l, hypoglycemic shock occurs, accompanied by impaired consciousness or coma. Glycosuria is the excretion of glucose in the urine; glucose is not normally present in urine.

Blood lactate concentration depends on how much lactate is produced by muscles working anaerobically and how quickly it is cleared. Lactate is broken down or processed further in inactive skeletal muscles, adipose tissue, the liver, the kidneys and the heart muscle. At rest, lactate concentration is 1 mmol/l; during strenuous physical work, it can reach 15 mmol/l (the maximum value). During prolonged strenuous physical work, lactate concentration falls again after an initial rise.

Regulation by the nervous system includes Bernard’s “sugar puncture”—a rise in blood glucose in a stressful situation (the pre-start state); cells in the central nervous system use glucose to replenish their large energy reserves independently of insulin. In terms of hormonal regulation, insulin from the B cells of the islets of Langerhans lowers blood sugar, increases glucose uptake into all body cells and enhances glycogenesis. Glucagon from the A cells stimulates glycogen breakdown in the liver, raises blood sugar, activates adenylate cyclase and increases cAMP production. Glucocorticoids stimulate gluconeogenesis in the liver and reduce glucose use in cells. Adrenaline stimulates glucagon production and raises blood sugar. Growth hormone reduces glycogen stores, inhibits glucose uptake into cells and raises blood sugar. Thyroid hormones increase the activity of carbohydrate-degrading enzymes and enhance carbohydrate utilization.

Lipids

Lipids make up ~10-20% of body weight. There is a substantial difference between the amount of lipids incorporated into cell structures and the amount of stored fat: the latter depends on diet, lifestyle and heredity. Lipids are rich in energy—oxidizing 1 g yields 9.0 kcal. They play an important role in energy balance, accounting for ~30% of daily energy expenditure.

The daily lipid requirement is 80-90 g, with 95% of dietary lipids absorbed. The diet must include both plant and animal fats. In the digestive tract, lipids are broken down into fatty acids and glycerol, which are absorbed from the small intestine partly into the blood, but mainly into the lymphatic vessels, and then carried into the bloodstream by the lymph. Cytoplasmic lipids are used to build tissues and meet energy needs; any excess is transported to subcutaneous adipose tissue. Carbohydrates can also be converted into lipids in the body.

The liver’s main roles in lipid metabolism: complete oxidation produces carbon dioxide and water as end products; incomplete oxidation produces ketone bodies, which are excreted in urine.

Hormonal regulation: lipid metabolism is controlled by the hypothalamus. Epinephrine, norepinephrine and glucagon stimulate lipolysis. Glucocorticoids break down triglycerides and increase fatty acid levels in the blood. Growth hormone reduces fat stores; insulin and somatotropin from the pituitary gland accelerate the uptake of free fatty acids into muscle tissue.

The physiology of water and minerals

Water makes up ~57-65% of the human body. You can survive only a very short time without water: metabolic waste is not excreted, the regulation of osmotic pressure and acid-base balance is disrupted, and chemical reactions that take place in aqueous solutions are impaired. The average person needs 2.2-2.8 l of water per day, of which 1.9-2.4 l comes from food and 0.3-0.4 l is produced within the body (primarily through lipid oxidation). Water is lost through urine, sweat, feces and exhaled air. Water intake and output must be equal; excessive water loss is dehydration.

Water in the body is distributed between two compartments: intracellular water, or water inside cells, accounts for 60%, and extracellular fluid, or fluid outside cells, accounts for 40%. Extracellular fluid carries nutrients, metabolic waste and regulatory substances into and out of cells. Extracellular water is further distributed as follows: 31% in interstitial fluid, 7% in blood plasma and 2% in transcellular fluid. This distribution is influenced by the electrolyte concentration in extracellular fluid, capillary blood pressure and the concentration of proteins in blood plasma.

Key major and trace minerals:

• Sodium and potassium (10-20 g of NaCl and 2-4 g per day of KCl are sufficient) — components of cells and tissue fluids that maintain osmotic pressure and help generate bioelectrical potentials across cell membranes.
• Calcium (1 g) — a building material for bone tissue; helps generate and propagate excitation, influences membrane permeability to K and Na, is required for excitation-contraction coupling in muscles, enables neurotransmitter release at synapses, participates in blood clotting, activates enzymes and acts as a second messenger.
• Phosphate salts (1 g) — essential for bone tissue formation and the synthesis of energy-rich compounds (ATP, cAMP, creatine phosphate, DNA, cGMP).
• Iron (15 mg for women, 10 mg for men) — needed for the synthesis of hemoglobin and myoglobin, as well as enzymes and proteins involved in oxidation. Very little iron is excreted; it is kept in circulation and reused.
• Zinc — involved in blood cell formation and carbohydrate, lipid and protein metabolism; cobalt, iodine (thyroid hormone synthesis); and small amounts of manganese, magnesium, copper and fluorine.

The liver’s main roles in water and mineral metabolism are to act as a water reservoir and regulate calcium metabolism through vitamin D.

The nervous system regulates water metabolism through receptors in the oral mucosa, baroreceptors in the stomach, osmoreceptors in tissues, volume receptors in the heart and blood vessels, the hypothalamus, and the coordinated function of the sweat glands and kidneys. The hormones involved include renin, angiotensin II, antidiuretic hormone and aldosterone. Calcium metabolism is regulated by calcitonin and parathyroid hormone.

Frequently asked questions about physiology

What is the difference between anabolism and catabolism?

Anabolism is a building process — nutrients are used to form the body’s components. Catabolism is a breakdown process that produces simpler compounds (CO₂, H₂O, NH₃) and releases energy. Both occur continuously and together.

Why is your blood glucose level so important?

Glucose is the main energy source for the brain and many tissues. If its level falls below 0.5-0.2 g/l, hypoglycemic shock occurs, with clouding of consciousness. The normal range is 3.3-6.1 mmol/l and is maintained by several hormones — insulin lowers it, while glucagon, adrenaline, glucocorticoids and growth hormone raise it.

How much protein should you get per day?

At rest, you need 0.8 g of protein per 1 kg of body mass. During physical work, you need about twice as much. The amount is not the only thing that matters; so does the amino acid composition: all 9 essential amino acids must come from your diet.

Author: EKFL

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