Organic Metabolism Definition

The period in which ingested nutrients enter the blood is called the absorptive state. In this state, some of the nutrients fulfill the energy need of the body while the extra nutrients remain stored. When the GI tract is empty of nutrients and required energy supplied by body storage, the state is called the post-absorptive state.

Absorptive State

Our body absorbs carbohydrates and proteins as monosaccharides and amino acids. These absorbed nutrients enter into the blood while the lymph absorbs fat as triacylglycerols.

• Absorbed carbohydrates: the primary source of energy for our body is glucose that is absorbed in the absorptive state. The glucose is stored as glycogen in the liver and skeletal muscles. Glucose is converted by adipose tissue into fat.

• Absorbed triacylglycerols: The capillaries of adipose tissue release fatty acids of plasma chylomicrons and from triacylglycerols.

• Absorbed amino acids: the major function of amino acids is to synthesize proteins. The excess amino acids are converted into carbohydrates and fat.

Postabsorptive State

The state includes net catabolism of glycogen, fat, and proteins. The glucose level in blood plasma is maintained by:

• Glycogenolysis: the process includes hydrolysis of glycogen.

• Lipolysis includes the conversion of triacylglycerols into glycerol and fatty acids.

• A catabolic reaction converts protein into glucose.

• Gluconeogenesis, synthesis of glucose.

• The glucose sparing, process includes an increase in fat utilization and a reduction in glucose metabolism.

Endocrine and Neural Control

• Insulin: The beta cells of islets of Langerhans in the pancreas secrete a hormone named insulin. The absorptive state increases the secretion of insulin while the secretion decreases during the post-absorptive state. The major functions of insulin include increase the movement of glucose into cells, increase facilitated diffusion, increase glycogen synthesis, and inhibit or decrease the rate of glycogen catabolism. The secretion of insulin secretion is stimulated by parasympathetic fibers.

• Glucagon: The alpha cells of the pancreas secrete glucagon. The action of glucagon is the opposite of insulin. It helps in increasing the rate of glycogen breakdown and gluconeogenesis. Glucagon increases the level of sugar in blood plasma in the condition of hypoglycemia. The glucagon secretion is stimulated by sympathetic nerves.

Diabetes Mellitus and Hypoglycemia

Deficiency of insulin or hyporesponsiveness of the hormone results in diabetes mellitus. It can be of two types: type I diabetes, and type II diabetes. Type I diabetes or insulin-dependent diabetes mellitus results in a low level of insulin in the body.

The cause of type I diabetes is mainly the destruction of beta cells. It results in an increasing amount of glucose in blood plasma. The problem can be treated by the administration of insulin. Type II diabetes or noninsulin-dependent diabetes mellitus causes hyporesponsiveness to insulin. In this condition, the insulin level is normal but the target cell does not respond to insulin. This condition of also known as insulin resistance is related to obesity.

The condition in which plasma glucose level decreases in the blood is called hypoglycemia. The increasing insulin or deficiency of glucagon arises the condition of hypoglycemia in the body.

Regulation of Plasma Cholesterol

Diet and synthesis by the liver are two major sources of cholesterol in our bodies. The plasma cholesterol is regulated by homeostatic control, which involves hepatic synthesis. Animal fats or saturated fats increase the level of plasma cholesterol while it is decreased by unsaturated fatty acids.

The cholesterol is delivered to cells by low-density lipoproteins while the excess cholesterol is removed by high-density lipoproteins. HDL delivers extra cholesterol to the liver for excretion. Thus, the ratio of HDL and LDL is important for the regulation of cholesterol in our bodies.

Growth Regulators

Two major growth regulators are (1) environmental factors, and (2) hormonal influences.

• Growth hormone and insulin-like growth factors: The growth hormone is the most important hormone for postnatal growth, which is secreted by the anterior pituitary. Gigantism is caused by an excess of GH, whereas deficiency of GH causes dwarfism. The increasing level of GH also results in bone thickening without lengthening, which is called acromegaly. GH stimulates the secretion of IGF-I from the liver that further promotes cell division and also stimulates protein synthesis.

• Thyroid hormone: It is an important hormone that controls metabolism. It also plays important role maintenance of the nervous system and normal development.

• Insulin: It inhibits protein degradation and promotes cell division and differentiation.

• Sex hormones: Levels of GH and IGF-I are stimulated by sex hormones. Testosterone also stimulates protein synthesis and exerts an anabolic effect.

• Cortisol: It stimulates protein catabolism and exerts antigrowth effects.

Energy Balance

i. Concepts of Energy Expenditure

Energy is liberated by the breakdown of organic molecules. The bodily cells use this energy to perform biological work, active transport, and molecular synthesis. Breakdown of an organic molecule liberates (∆E) energy that can either appear as heat (H) or can be used to perform work (W).

∆E = H + W

This energy is incorporated into ATP, which is further used by cells.

ii. Metabolic Rate

The total energy expenditure per unit of time is defined as metabolic rate. The metabolic rate under certain standardized conditions is called the basal metabolic rate. The activity of the heart, brain, liver, and kidney contributes most of the activity to the basal metabolic rate.

iii. Thyroid Hormones

They are used as an important determinant of BMR. They increase oxygen consumption and heat-production in body tissues that increase BMR.

iv. Epinephrine

Epinephrine increases metabolic rate by calorigenic effect. The catabolism of glycogen and triacylglycerols is stimulated by Epinephrine.

v. Food-induced Thermogenesis

Body temperature is raised by ingestion of food. Proteins have the greatest effect in producing temperature changes.

vi. Muscle Activity

It increases muscle contraction. The examples of muscle contraction can be seen during exercise or shivering, which increases the metabolic rate.

Regulation of Body Energy

Energy stored = energy from food intake – (internal heat produced + External work)

i. Control of Food Intake

The adipose tissue synthesizes a hormone called leptin. The hormone is released in a proportion of fat in adipose tissue. It inhibits the release of neuropeptides that cause a reduction in food intake. The hormone also increases the metabolic rate, which results in control changes in energy expenditure. The duration and frequency of meals in short term are regulated by various signals such as insulin, body temperature, presence of food in the GI tract, etc.

ii. Overweight and Obesity

An increased amount of fat in the body is described by the term overweight. The weight has an index named body mass index (BMI) which is calculated by dividing a person’s weight by the square of their weight. The weight and body fat storage can be managed by exercise, which is an effective method.

iii. Eating Disorders

The consequent reduction of food intake and a pathological fear of gaining weight is termed Anorexia nervosa. Low blood pressure and altered secretion of many hormones can be caused due to this disorder. It is associated with self-induced vomiting, laxatives, diuretics, dieting, or exercise.

Regulation of Body Temperature

Homeothermy is the ability of an organism to maintain body temperatures within narrow limits. The net difference between heat production and heat loss is in the total heat content of the body. Radiation, conduction, convection are some methods to lose heat from the body surface.

i. Temperature-regulating Reflexes

Two thermoreceptors detect the changes in body temperature. The thermoreceptors include peripheral thermoreceptors and central thermoreceptors (present in the hypothalamus, spinal cord, etc). The essential negative feedback is provided by the central thermoreceptors whereas feedforward information is provided by peripheral thermoreceptors.

The overall integrator of reflexes is served by the hypothalamus that sends outputs via sympathetic nerves to sweat glands, skin arterioles, and adrenal medulla.

ii. Control of Heat Production

The major control in heat production is constituted by changes in muscle activity. A decrease in core body temperature leads to an increase in skeletal muscle contraction. Shivering does not perform external work and the liberated energy appears as internal heat, which is known as shivering thermogenesis. Heat is also produced by voluntary muscle contractions.

iii. Control of Heat Loss by Radiation and Conduction

Change in blood flow to the skin cells controls the skin’s effectiveness as an insulator. Two temperatures become closer by reaching more blood to the skin. The insulating capacity of skin can be reduced by the blood vessels by carrying heat to the surface to be lost to the external environment. Reducing surface area by curling up into a ball decreases heat loss by radiation and conduction.

iv. Control of Heat Loss by Evaporation

Heat loss can also be contributed by water loss through the skin, sweat, and respiratory tract.

Integration of effector mechanisms: The thermoneutral zone is the range of environmental temperature over which body temperature can be maintained by vasoconstriction or vasodilation. The body stimulates its heat production below this zone and increases heat loss above this zone.

v. Temperature Acclimatization

Adaptation to high temperature can be determined by changing sweating onset, volume, and composition. Increased reabsorption of sodium due to aldosterone secretion reduces sodium loss through sweat.

vi. Fever and Hyperthermia

Increasing body temperature is called fever, which causes due to a resetting of the thermostat in the hypothalamus. During fever, the body temperature is always maintained. The causes of fever include any infection or stress that results in the overall sensation of feeling cold. The body temperature is then driven up by vasoconstriction and shivering. The person starts feeling hot, vasodilation when the fever breaks.

Chemical messengers called endogenous pyrogen (EP) reset the thermostat. EP consists of interleukin 1 (IL-1) and IL6 that are released from macrophages. The defensive responses to infection can be stimulated by increased body temperature. Sometimes the fever is not due to infection, which is called hyperthermia that causes due to exercise and heat retention.

vii. Heat Exhaustion and Heat Stroke

A state of collapse is called heat exhaustion. It can occur due to (1) depletion of plasma volume secondary to sweating and (2) extreme dilation of ski n blood vessels which results in decreasing peripheral resistance. It is a complete breakdown of heat-regulating systems that increases the body temperature continuously. The symptoms of heatstroke include collapse, delirium, unconsciousness, and seizures. It also results in increasing metabolism and producing still further heat, thus it also works in a positive feedback manner.

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