Energy Balance and Temperature Regulation

The body converts most food energy into heat. EEnergy input the energy supplied from ingested food EEnergy released from the broken bonds in food molecules is used, in part, to make ATP. EEnergy from ingested nutrients is used either immediately to perform work or stored in molecules in the body for future use. EEnergy output has two categories. EExternal work is expended when skeletal muscles move external objects. IInternal work refers to the expenditure of energy that does not accomplish mechanical work outside the body.

 ~ 50% of energy supplied from nutrients transferred to ATP.  The remaining energy from nutrients is lost as heat during the transfer of the energy to ATP  Heat energy cannot be used to perform work. Food energy Metabolic pool in body Internal work Thermal energy (heat) External work Energy storage Energy Input Energy Output

The metabolic rate is the rate of energy use. mmetabolic rate = energy expenditure/unit of time MMost of the energy expenditure of the body eventually appears as heat. TTherefore this rate is expressed as the rate of heat production in kilocalories per hour. AA person’s metabolic rate is measured under standardized basal conditions. TThis is the basal metabolic rate (BMR). TThe person is at physical and mental rest, and performing at comfortable room temperature. TThe person should not have eaten in the last 12 hours to avoid diet-induced thermogenesis.

DDirect calorimetry is one way to measure the BMR. TThis method is cumbersome as one must measure all heat release from the body BBy indirect calorimetry the person’s oxygen uptake per unit time is measured. TThe energy equivalent of oxygen is 4.8 kilocalories per liter of oxygen consumed. AAfter the rate of heat production is determined under basal conditions, it is compared to the normal values of people.

A neutral energy balance is maintained when energy input equals energy output. eenergy input = energy output when: ffood consumed = external work plus internal heat production plus/or minus stored energy AA neutral energy balance occurs if the energy in food intake equals the amount of energy expended by the muscles performing external work plus the basal internal energy expenditure that appears as body heat.

AA positive energy balance occurs if the amount of energy in food intake is greater than the amount of energy expended NNegative energy balance occurs when the energy from food intake is less than the body’s immediate energy requirements. IIn a negative energy balance, the body must use stored energy to supply energy needs. AAfter several weeks, eating more or less can produce small changes in metabolism. MMetabolism decreases as food intake becomes limited. AAn increase in metabolism also goes with satiety

Food intake is controlled primarily by the hypothalamus.  The correlation between total caloric intake and total energy output (energy homeostasis) is excellent over long periods of time.  The hypothalamus has two feeding (appetite) centers and a satiety center.  These centers communicate with multiple, highly- integrated pathways and respond to numerous chemical changes in the blood, such as the concentration of glucose.  Adipocytes are cells that store triglyceride fat.  They also secrete leptin, a hormone essential for normal body weight regulation.  Its level in the blood increases as more fat is stored.

Various signals control food intake.  The arcuate nucleus in the hypothalamus has receptors to detect leptin  In response to increased leptin the arcuate nucleus releases melanocortins  The paraventricular nucleus (PVN) responds to melanocortins by releasing corticotropin releasing hormone (CRH)  CRH promotes appetite suppression.  In response to decreased leptin, the arcuate nucleus releases neuropeptide Y (NPY)  NPY stimulates the lateral hypothalmic area (LHA) to release orexins  Orexins activate appetite stimulating pathways.

 Other controls include  Increased use of glucose can signal satiety.  Insulin in the blood signals satiety.  CCK is an important satiety signal.  The nucleus tractus solitarius (NTS) processes signals that terminate a meal (receives signals from the digestive tract).  Psychosocial and environmental influences (e.g., habits, stress) are also factors.

Body fat stores Leptin Neuropeptide YMelanocortins OrexinCorticotropin Appetite- enhancing pathway Appetite- suppressing pathway (from arcuate nucleus)* (from lateral hypothalamic area)* (from arcuate nucleus)* (from lateral paraventricular area)* Satiety signals important in short-term control of the size of meals Adipose tissue-related signals important in long-term matching of intake Psychosocial and environmental factors that influence food intake *Other chemicals are also released from this area that exert similar functions.

Satiety signals important in short-term control of the size of meals Adipose tissue-related signals important in long-term matching of intake Psychosocial and environmental factors that influence food intake Gastro- intestinal distension Insulin (from pancreas) Glucose use CCK (from duodenum) Food intake Smell, taste texture of food Stress, Anxiety, depression, boredom Amount of food available Appetite- enhancing pathway Appetite- suppressing pathway

Obesity occurs when more kilocalories are consumed that are burned up.  Obesity is excessive fat content in the adipose tissue.  Several factors can cause obesity.  There may be disturbances in the leptin signaling pathway.  The hypothalamus may set a higher level to maintain homeostasis in obese people.  There is a change in the hypothalamus to leptin resistance.

 A lack of exercise is another factor resulting in fewer calories expended.  People who are less likely to carry out nonexercise activity thermogenesis expend less calories.  Differences exist in people in their ability to extract energy from food.  Other factors are heredity, excessive numbers of fat cells, certain endocrine disorders, emotional disturbances, and a possible virus link.  People with anorexia nervosa have a pathologic fear of gaining weight. Other factors contributing to obesity

Internal human body temperature must be maintained at an optimal level  Humans generate heat to maintain body temperature in cool outside environments.  Heat production depends on the oxidation of metabolic fuels for food.  Internal core temperature is maintained at 100 o F though it varies slightly over 24 hours.  The central core is the abdominal and thoracic organs, the CNS, & the skeletal muscles.  Skin and subcutaenous fat constitute the outer shell  Body temperature can be monitored at several sites externally.  It increases during exercise due to heat production.  External temperature extremes can affect it.

A stable core temperature is maintained when heat input is balanced by heat output.  Heat input occurs by heat gain from the external environment and internal heat production.  Most energy expenditures in the body appear as heat.  Heat loss occurs by the loss of heat from exposed body surfaces to the external environment.  The balance between input and output can be disturbed by factors such as exercise and temperature changes in the external environment.  If the core body temperature falls, heat production is increased in the body.  If this temperature rises, mechanisms for heat loss compensate.

Internal heat production Heat input Total body heat content Core temperature Heat output External environment Heat input and output Heat gainHeat loss

Heat exchange occurs in several ways. 1) Radiation  The body uses four mechanisms.  Radiation is the emission of heat from a surface as electromagnetic waves.  The human body emits and absorbs radiant energy.  The body loses half of its heat through radiation.

2) Conduction Snowball Heating pad  Conduction is the transfer of heat between objects of different temperatures when the objects make contact.  Heat moves down its thermal gradient.  The rate of heat transfer depends on the temperature difference & the thermal conductivity.  Only a small amount of heat transfer occurs by conduction in the human body.

3) Convection Convection current  Convection is the transfer of heat by air currents.  This process combines with conduction to dissipate heat from the body.

 By evaporation when water evaporates from the skin, the required heat for this change comes from the body.  This is a cooling process.  Sweating is an active evaporation heat-loss process under sympathetic control.  The relative humidity of the surround air determines the extent of evaporation of sweat. Liquid converted to gaseous vapor 4) Evaporation

The hypothalamus integrates a multitude of thermosensory inputs.  The hypothalamus is the thermostat of the body  The posterior region of the hypothalamus detects cold temperatures.  The anterior region detects warm temperatures.  Peripheral thermoreceptors send afferent information about the surrounding temperature to this the hypothalamus.  It can respond to extremely small changes in blood temperature.  Central thermoreceptors in the hypothalamus also detect temperature changes.

 Shivering is the main involuntary means of increasing heat production.  Ongoing metabolic activities of the thoracic and abdominal organs generate heat.  Oscillating skeletal muscle activity, shivering, adds to this heat.  A reduction in heat-producing skeletal muscle activity has the opposite effect.  Nonshivering thermogenesis also produces heat.  Heat loss is adjusted by the flow of blood through the skin.  The dilation of skin vessels eliminates heat.  Their constriction holds heat in the body. Effector mechanisms for maintaining body temperature.

The hypothalamus coordinates heat production and heat loss mechanisms.  The coordinated responses to cold exposure are shivering for heat production and skin vasoconstriction to minimize heat loss.  Behavioral adaptations also contribute to these responses (curling up).  The main, coordinated responses to heat exposure are decreased shivering and skin vasodilation to increase heat loss.  Also, sweating is a means of heat loss.  Skin vasomotor activity is highly effective in controlling heat loss in environmental temperatures between the upper 60s and mid 80s, the thermoneutral zone.  Above this zone sweating is the dominant factor.

Behavioral adaptations Motor neurons Skeletal muscles Control of heat production Muscle tone, shivering Sympathetic nervous system Skin blood vessels Control of heat loss Sympathetic nervous system Sweat glands Sweating Control of heat loss Skin temperature Peripheral thermoreceptors Core temperature Central thermoreceptors Hypothalamic thermoregulatory center (in hypothalamus, other areas of CNS, & abdominal organs) (in skin) Control of heat production or heat loss Skin vaso- constriction & vasodilation

Hyperthermia Cooling mechansims kick in and prevent and addition rise in the core body temperature Resting set-point temperature Core temperature Adjustment to exercise Increased heat generation increases core body temp several degrees above resting set point

 White blood cells produce endogenous pyrogens.  In response to this the hypothalamus maintains body temperature at a net set level that is higher.  Hyperthermia can occur unrelated to infection.  It can be induced by exercise, endocrine dysfunctions, and malfunctions of the hypothalamus. Infection of inflammation Prostaglandins Neutrophils Hypothalamic set point Endogenous pyrogen Heat production; heat loss Initiation of “cold response” Body temperature to new set point = Fever During fever the hypothalmic thermostat is “reset” at an elevated temperature.