Basal metabolic rate.

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Here is a comprehensive overview of Basal Metabolic Rate (BMR).

Basal Metabolic Rate (BMR)

Definition

BMR is the minimum energy expenditure required to maintain life — the functioning of the lungs, kidneys, brain, heart, maintenance of ionic gradients across membranes, and the reactions of biochemical pathways — measured in a person who is:

Mentally and bodily at rest
In a thermoneutral environment
12–18 hours after the last meal

The resting metabolic rate (RMR), measured after awakening (rather than during sleep), is numerically very similar but slightly higher than BMR, and is more practical to measure clinically.

BMR as a Component of Daily Energy Expenditure

BMR accounts for 50–70% of daily energy expenditure in sedentary individuals. The three major components of total daily energy expenditure are:

BMR / Resting energy expenditure (~60%) — sleeping + arousal cost
Thermic effect of food (~8%) — digestion, absorption, and metabolism of nutrients
Physical activity (~32%) — exercise and non-exercise activity thermogenesis

Daily energy expenditure components in a 70-kg person

Fig. 73.3 — Average daily energy expenditure in a 70-kg person ingesting ~3000 kcal/day (Guyton & Hall)

Standard Measurement Conditions

Strict criteria for measuring BMR (Guyton & Hall):

No food for at least 12 hours
After a night of restful sleep
No strenuous activity for at least 1 hour before the test
All psychic and physical sources of excitement eliminated
Ambient temperature 68–80°F (20–27°C)
No physical activity during the test

Oxygen utilization is the usual measure. BMR normally averages 65–70 Calories/hour in a 70-kg adult man.

Measurement Methods

Method	Description
Direct calorimetry	Measures heat production directly
Indirect calorimetry	Measures O₂ consumption and CO₂ production; calculates respiratory quotient (RQ)
Simplified O₂ only	Measures O₂ consumption alone; less accurate but more practical
Predictive equations	Estimates from height, weight, age, sex — accurate to ±10% in ~90% of adults with BMI 18.5–45

Respiratory quotient (RQ): CO₂ produced / O₂ consumed

Pure carbohydrate oxidation: RQ = 1.00
Pure protein oxidation: RQ = 0.83
Pure fat oxidation: RQ = 0.71

Estimating BMR: The Mifflin–St. Jeor Equation

The most widely validated predictive formula (Basic Medical Biochemistry, 6e):

Sex	Formula
Males	(10 × W) + (6.25 × H) − (5 × A) + 5
Females	(10 × W) + (6.25 × H) − (5 × A) − 161

W = weight in kg; H = height in cm; A = age in years; result in kcal/day

Quick estimate:

Men: ~1 kcal/kg/hr (24 kcal/kg/day)
Women: ~0.9 kcal/kg/hr (21.6 kcal/kg/day)

Factors Affecting BMR

Factor	Effect
Lean/fat-free mass	↑ muscle mass → ↑ BMR (muscle = 20–30% of BMR at rest)
Sex	Males > females (due to higher muscle, lower adipose %)
Age	BMR ↓ with age (muscle atrophy, fat deposition)
Thyroid hormones	Hyperthyroidism ↑ BMR 50–100%; hypothyroidism ↓ 40–50%
Testosterone	↑ BMR ~10–15% (anabolic effect on muscle)
Growth hormone	↑ BMR ~20% in GH-deficient adults on replacement
Fever	↑ ~10–12% per °C rise in body temperature
Sleep	↓ 10–15% (decreased muscle tone + reduced CNS activity)
Malnutrition	↓ 20–30% (starvation adaptation)
Pregnancy/lactation	↑ BMR
Cold environment	↑ slightly (thermogenesis activation)
Genetics	Accounts for up to ~10% of inter-individual variation

BMR vs. Age and Sex

Normal basal metabolic rates at different ages for each sex

Fig. 73.4 — Normal BMR vs. age for men and women. Both sexes show the highest BMR in early childhood (~53 Cal/m²/hr), declining steeply through adolescence and more gradually into old age. Men maintain a slightly higher BMR than women throughout life (Guyton & Hall)

Organ Contributions to BMR

Although skeletal muscle is the largest body mass contributor, at rest it accounts for only 20–30% of BMR. The major resting energy consumers are the brain, heart, kidneys, and liver. During maximal exercise, however, muscle can account for 80–90% of total energy expenditure.

Adipose tissue contributes very little — only ~3 kcal/kg of body fat per day. An exception is brown adipose tissue (BAT), which expresses uncoupling protein-1 (UCP-1), allowing mitochondrial proton leak that generates heat instead of ATP. While only present in small amounts in adults, it is a potential therapeutic target for obesity.

Clinical Relevance

Caloric restriction can reduce BMR disproportionately to fat-free mass loss (mediated by ↓ T3 conversion from T4 and ↓ sympathetic drive) — a key mechanism of weight-loss plateau
Overfeeding transiently raises BMR slightly above expected for lean mass
BMR ranges from <1200 to >3000 kcal/day among sedentary adults; ~80% of this variability is explained by lean vs. fat tissue composition
Clinical measurement: Portable indirect calorimetry is used in hospitalized patients requiring precise energy needs (e.g., ICU nutrition planning)

Sources: Guyton and Hall Textbook of Medical Physiology; Basic Medical Biochemistry: A Clinical Approach, 6e; Goldman-Cecil Medicine

Basal metabolic rate. Pleural Cavity & intrapleural pressure. total body water, avenues by which water leaves and enters body, types of body fluids general principles of fluid balance how body fluids maintain Homeostasis, Electrolytes & ions Function of electrolytes, how electrolyte imbalance leads to fluid imbalance. Different in each topics

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Topic 1 — Basal Metabolic Rate (BMR)

Definition

BMR is the minimum energy expenditure required to sustain life at complete rest — covering the energy cost of cardiac pumping, respiration, renal filtration, neuronal activity, membrane ion gradients, and biochemical pathway function. It is measured under strict conditions: awake but completely at rest, thermoneutral environment (68–80°F), after 12–18 hours of fasting, following a night of sleep, with no strenuous prior activity.

The Resting Metabolic Rate (RMR) or Resting Energy Expenditure (REE) is a practical clinical proxy — measured after waking rather than during sleep. It is numerically slightly higher than BMR. They are often used interchangeably.

BMR Within Total Daily Energy Expenditure

Fig. 73.3 — Components of daily energy expenditure in a 70-kg person (~3000 kcal/day). Guyton & Hall, Medical Physiology

Component	Contribution
BMR / Resting energy	50–70% (sedentary)
Thermic effect of food	~8%
Physical activity	~32%

For most sedentary adults, BMR ranges from <1,200 to >3,000 kcal/day. About 80% of this variability is explained by lean vs. fat tissue composition.

Standard Measurement Conditions

No food for ≥12 hours
After a restful night's sleep
No strenuous activity ≥1 hour before
Psychological rest — no excitement
Room temperature 68–80°F (20–27°C)
No physical activity during the test

Normal value: ~65–70 Cal/hour in a 70-kg adult man.

How BMR Is Measured

Method	Detail
Direct calorimetry	Measures heat produced directly
Indirect calorimetry	Measures O₂ consumed + CO₂ produced → respiratory quotient (RQ)
O₂ consumption only	Simpler, portable, slightly less accurate; used in ICU
Predictive equations	Height, weight, age, sex → accurate ±10% in ~90% of adults with BMI 18.5–45

Respiratory Quotient (RQ) = CO₂ produced ÷ O₂ consumed:

Carbohydrate oxidation: RQ = 1.00
Protein oxidation: RQ = 0.83
Fat oxidation: RQ = 0.71

Estimating BMR — Mifflin–St. Jeor Equation

Sex	Formula
Males	(10 × W) + (6.25 × H) − (5 × A) + 5
Females	(10 × W) + (6.25 × H) − (5 × A) − 161

W = kg, H = cm, A = years → result in kcal/day

Quick estimate:

Men: ~1 kcal/kg/hr
Women: ~0.9 kcal/kg/hr

Fat-free mass (FFM)-based formula: BMR = 186 + (23.6 × FFM in kg) kcal/day — eliminates sex and age differences due to adiposity.

Factors That Alter BMR

Factor	Effect	Mechanism
Lean/skeletal muscle mass	↑ BMR	Muscle = 20–30% of resting BMR
Male sex	↑ ~10% vs females	Higher muscle, lower adipose
Age	↓ progressively	Muscle loss, fat gain
Thyroid hormones ↑	↑ BMR 50–100%	Uncoupling futile cycles
Thyroid hormones ↓	↓ BMR 40–50%	Reduced cellular metabolism
Testosterone	↑ 10–15%	Anabolic effect on muscle
Growth hormone	↑ ~20% (GH-deficient adults)	Cellular metabolism + muscle mass
Fever	↑ ~10–12% per °C	Accelerated chemical reactions
Sleep	↓ 10–15%	↓ muscle tone + CNS activity
Malnutrition	↓ 20–30%	Paucity of cellular substrates
Pregnancy/lactation	↑	Increased metabolic demands
Cold environment	↑ slightly	Thermogenesis activated
Genetics	±10% variation	Inter-individual differences
Energy-restricted diet	↓ disproportionately	↓ T3 production, ↓ sympathetic drive

BMR vs. Age

Normal BMR at different ages for men and women

Fig. 73.4 — BMR declines steeply from early childhood (~53 Cal/m²/hr) through early adulthood, then gradually. Men consistently maintain higher BMR than women. Guyton & Hall

Brown Adipose Tissue — Thermogenic Special Case

Brown fat expresses uncoupling protein-1 (UCP-1), creating a proton leak across the inner mitochondrial membrane. This generates heat rather than ATP — a process called non-shivering thermogenesis. Though present in small amounts in adults, brown fat is detectable by ¹⁸F-FDG PET during cold exposure and is a potential therapeutic target for obesity.

Topic 2 — Pleural Cavity & Intrapleural Pressure

The Pleural Cavity

The pleural cavity (intrapleural space) is the thin, fluid-filled potential space between the visceral pleura (covering the lungs) and the parietal pleura (lining the chest wall and diaphragm). It contains only a small amount of serous fluid (~5–15 mL) that lubricates the surfaces, allowing the lung to slide freely during breathing.

Key structural points:

The visceral and parietal layers are continuous, forming a closed sac
The space is airtight; any breach (pneumothorax) disrupts the pressure dynamics required for breathing
The pleural fluid is a transudative filtrate of plasma maintained at near-zero net volume by lymphatic drainage

Intrapleural Pressure — The Driving Force of Breathing

Intrapleural pressure arises from the opposing recoil forces of two structures:

The lung has a natural tendency to collapse inward (elastic recoil of alveoli + surface tension)
The chest wall has a natural tendency to spring outward

These two opposing forces pull the pleural layers apart, creating a subatmospheric (negative) pressure within the intrapleural space.

Intrapleural Pressure Through the Breathing Cycle

Phase	Alveolar Pressure	Intrapleural Pressure	Transmural Pressure (Alveolar − Intrapleural)	Air Flow
Rest (FRC)	0 cm H₂O	−5 cm H₂O	+5 cm H₂O (lungs open)	None
Mid-inspiration	−1 cm H₂O	−6 to −8 cm H₂O	+5 to +7 cm H₂O	Into lungs
End-inspiration	0 cm H₂O	−8 cm H₂O	+8 cm H₂O	Ceases
Mid-expiration	+1 cm H₂O	−5 cm H₂O	+6 cm H₂O	Out of lungs

Transmural pressure = Alveolar pressure − Intrapleural pressure. As long as transmural pressure is positive (expanding), the alveoli and airways remain open.

Why Intrapleural Pressure Is Negative

Elastic recoil of the lung — alveolar walls and surfactant-depleted surfaces constantly pull inward
Chest wall recoil — ribs and intercostal muscles pull outward
Together, these create a virtual vacuum in the sealed intrapleural space

During inspiration:

Diaphragm contracts → thoracic volume ↑
Lung volume ↑ → elastic recoil ↑ → intrapleural pressure becomes more negative (−8 cm H₂O)
Alveolar pressure drops below atmospheric → air flows in

During expiration (passive):

Elastic recoil compresses alveolar air → alveolar pressure becomes +1 cm H₂O
Air flows out; intrapleural pressure returns to −5 cm H₂O

During forced expiration:

Expiratory muscles raise intrapleural pressure to +20 cm H₂O
In emphysema, loss of elastic recoil means lower alveolar pressure → transmural pressure across large airways becomes negative → dynamic airway collapse

Clinical Significance

Condition	Mechanism	Effect
Pneumothorax	Air enters intrapleural space	Intrapleural pressure → 0 (atmospheric); lung collapses
Tension pneumothorax	One-way valve effect — air only enters	Intrapleural pressure becomes positive; contralateral mediastinal shift
Pleural effusion	Fluid accumulates in pleural space	Compresses lung; reduces ventilation
PEEP (mechanical ventilation)	Positive airway pressure transmits into pleural space	Intrapleural pressure becomes less negative → ↓ venous return → ↓ cardiac output

In pneumothorax, the negative intrapleural pressure is lost: "Each breath can increase intrapleural pressure, especially if the diameter of the chest wall wound approaches that of the trachea" — Roberts & Hedges' Clinical Procedures.

Topic 3 — Total Body Water, Avenues of Water Entry & Exit, Types of Body Fluids

Total Body Water (TBW)

TBW is the total amount of water contained within the body. It is proportional to lean body mass (fat is largely anhydrous).

Population	TBW as % of Body Weight
Full-term neonate	~70%
Infant (1 year)	~65%
Adult male	~60%
Adult female	~55% (more adipose, less muscle)
Elderly	↓ further (↑ fat, ↓ muscle)
Fetus	~90%

In a 70-kg man: TBW ≈ 42 L total.

Distribution — The Body Fluid Compartments

Distribution of total body water compartments

Volume and distribution of total body water — Tietz Textbook of Laboratory Medicine, 7e

Compartment	% of TBW	% of Body Weight	Volume (70-kg man)
Intracellular (ICF)	~67% (2/3)	~40%	~28 L
Extracellular (ECF)	~33% (1/3)	~20%	~14 L
— Interstitial fluid	~75% of ECF (~25% TBW)	~15%	~10.5 L
— Intravascular (plasma)	~25% of ECF (~8% TBW)	~5%	~3.5 L
— Transcellular*	~4% TBW	~2%	~1–2 L

Transcellular fluid includes cerebrospinal fluid (CSF), synovial fluid, pleural fluid, peritoneal fluid, intraocular fluid, GI secretions.

The ICF and ECF are separated by cell plasma membranes; the intravascular and interstitial spaces are separated by capillary endothelium.

Types of Body Fluids in Detail

1. Intracellular Fluid (ICF)

2/3 of TBW
Dominant ions: K⁺ (140 mmol/L), Mg²⁺, organic phosphates, protein (16 g/dL)
Provides the environment for energy production (ATP), cell repair, replication, and specialized cell functions

2. Extracellular Fluid (ECF)

1/3 of TBW — a bidirectional delivery system
Delivers: glucose, fatty acids, amino acids, O₂, ions, hormones
Removes: CO₂, metabolic waste, toxic byproducts
Dominant ions: Na⁺ (140 mmol/L), Cl⁻ (100 mmol/L), HCO₃⁻ (27 mmol/L)

Sub-compartments of ECF:

Sub-compartment	Key Feature
Plasma	Contains proteins (mainly albumin) that exert oncotic pressure; 5 L blood volume (3 L plasma)
Interstitial fluid	Essentially protein-free ultrafiltrate of plasma; bathes cells
Transcellular fluid	Secreted by epithelial cells; includes CSF, synovial, pleural, peritoneal, aqueous humour, GI secretions

Ionic Composition of Body Fluid Compartments

Ion	Plasma (mEq/L)	Interstitial (mEq/L)	Intracellular (mEq/L)
Na⁺	140	146	12
K⁺	4	4	150
Ca²⁺	5	3	~10⁻⁷
Mg²⁺	2	1	7
Cl⁻	103	114	3
HCO₃⁻	27	~28	10
PO₄³⁻	~2	~2	60
Protein	16 g/dL	trace	16 g/dL

The large difference in Na⁺ and K⁺ between ECF and ICF is maintained by the Na⁺/K⁺-ATPase pump (3 Na⁺ out, 2 K⁺ in per cycle).

Avenues by Which Water Enters the Body

Route	Approximate Volume/Day
Drinking	1,200–1,500 mL
Water in food	700–1,000 mL
Metabolic water (oxidation of nutrients)	~200–300 mL
Total intake	~2,000–2,500 mL/day

Avenues by Which Water Leaves the Body

Route	Approximate Volume/Day	Notes
Urine	1,200–1,500 mL	Primary regulated route (kidney)
Insensible skin evaporation	~350–400 mL	Increases with fever, burns
Respiratory (expired air)	~300–400 mL	Increases with hyperventilation, exercise
Feces	~100–200 mL	↑↑ in diarrhea
Sweat	Variable (~100 mL at rest → >1,000 mL/hr in exercise)	Hypotonic fluid
Total output	~2,000–2,500 mL/day	Matches intake at balance

Minimum daily water requirement ≈ 1.5–2.0 L/day to compensate for renal + insensible losses.

Topic 4 — General Principles of Fluid Balance & How Body Fluids Maintain Homeostasis

The Principle of Fluid Balance

Fluid balance = Water in = Water out over time. Homeostasis requires that daily intake (drinking + food + metabolic water) equals daily losses (urine + insensible + sweat + feces).

The body does not maintain total body water directly — it primarily maintains osmolality (concentration) and intravascular volume via overlapping, redundant mechanisms.

Osmosis and the Driving Force for Water Movement

Water moves across semipermeable membranes from areas of low solute concentration → high solute concentration (osmosis), driven by the osmotic pressure gradient. The body maintains equal osmolality (~285–295 mOsm/kg) across all fluid compartments.

When water is added to ECF:

ECF osmolality ↓ transiently → water moves into cells → ICF expands, ECF shrinks back When water is lost from ECF:
ECF osmolality ↑ → water moves from cells into ECF → ICF shrinks

Osmotic water shifts between ECF and ICF

A: Water added to ECF → osmolality falls → water enters cells. B: Water lost from ECF → osmolality rises → water exits cells. Mulholland & Greenfield's Surgery, 7e

Key rule: Because cell membranes are freely permeable to water, imbalances in TBW are initially reflected in the ECF — the ECF is the first compartment to expand or contract.

The Gibbs-Donnan Equilibrium

When impermeable large anions (proteins, phosphates) are confined within the ICF, they attract cations inward and repel anions outward. This creates an uneven but electrically neutral distribution of diffusible ions across the membrane — called the Gibbs-Donnan equilibrium. This is why:

ICF has high K⁺, Mg²⁺, protein, PO₄³⁻
ECF has high Na⁺, Cl⁻, HCO₃⁻
CSF has ~15% higher Cl⁻ than plasma (due to near-absence of protein in CSF)

Starling Forces — Fluid Exchange at the Capillary

Fluid movement between the intravascular and interstitial compartments is governed by Starling forces:

Force	Direction	Effect
Capillary hydrostatic pressure (Pc)	Out of vessel	Filtration → interstitium
Plasma oncotic pressure (πc)	Into vessel	Reabsorption → plasma
Interstitial hydrostatic pressure (Pi)	Into vessel	Opposes filtration
Interstitial oncotic pressure (πi)	Out of vessel	Promotes filtration

Net filtration pressure = (Pc − Pi) − (πc − πi)

At the arterial end of capillaries: hydrostatic pressure dominates → net filtration into interstitium At the venous end: oncotic pressure dominates → net reabsorption

Excess interstitial fluid is returned via the lymphatic system.

Homeostatic Regulatory Mechanisms

1. Antidiuretic Hormone (ADH / Vasopressin)

Released by posterior pituitary in response to: ↑ plasma osmolality (via hypothalamic osmoreceptors) or ↓ blood volume (via baroreceptors in aortic arch)
Action: ↑ water permeability of collecting ducts via aquaporins → water reabsorption → ↓ urine output → ↓ osmolality restored
Suppressed when ECF volume ↑ or osmolality ↓ → ↑ water excretion (diuresis)

2. Renin–Angiotensin–Aldosterone System (RAAS)

Triggered by: ↓ renal perfusion, ↓ Na⁺ delivery to macula densa, β-adrenergic stimulation
Renin (from juxtaglomerular cells) → Angiotensin II → Aldosterone (adrenal cortex)
Aldosterone: stimulates cortical collecting duct → ↑ Na⁺ reabsorption + ↑ K⁺ excretion → water follows Na⁺ passively → ↑ ECF volume

3. Thirst Mechanism

Hypothalamic osmoreceptors and baroreceptors stimulate thirst when plasma osmolality ↑ or blood volume ↓
The only behavioral mechanism for restoring water balance

4. Renal Tubular Handling of Na⁺/H₂O

Segment	Process
Proximal tubule	70–80% of filtered Na⁺ reabsorbed; Cl⁻ and H₂O follow passively
Descending Loop of Henle	Water reabsorption (not electrolytes) — driven by medullary hypertonicity
Ascending Loop of Henle	Cl⁻ actively reabsorbed; Na⁺ follows
Distal tubule + collecting duct	Aldosterone → Na⁺ reabsorption; ADH → H₂O reabsorption

5. Atrial Natriuretic Peptide (ANP)

Released by cardiac atria when stretched (↑ blood volume)
Opposes aldosterone: ↑ Na⁺ and water excretion → ↓ blood volume

Topic 5 — Electrolytes & Ions: Functions and How Imbalance Causes Fluid Imbalance

What Are Electrolytes?

Electrolytes are ions in body fluids that carry electrical charge and regulate fluid distribution, membrane potential, nerve conduction, muscle contraction, and acid-base balance.

Major cations: Na⁺, K⁺, Ca²⁺, Mg²⁺ Major anions: Cl⁻, HCO₃⁻, HPO₄²⁻, SO₄²⁻, protein