John H. Frenster, M.D., The Rockefeller Insititute, New York, NY 10021. 

It is now evident that the subunits of an individual organism - its organs, its cells, its sub-cellular particles, and its enzymes - are similarly engaged in a constant interaction with their immediate micro-environments within the organism (3-6). As a result of such interaction, these organs, cell, particles, and enzymes undergo changes in response to their micro-environments, and in turn produce changes in those environments. One of the most interesting of such interactions is the ready adjustment of the work capacity of a body process to the work demands presented to that process (7). The mechanisms mediating such ready responsiveness are of interest because of what they reveal about normal useful growth and its control in the organism.

To study such mechanisms, it is useful to analyze the individual body process under scrutiny into its components. Any single body process, whether of the organ, cell, particle, or enzyme level, can be viewed as the functional interaction of an imposed load presented to the process for action, the available capacity of the process to accomplish its action, and the resistances opposing the completion of the action (8). Such an analysis permits the quantitation of each of the components determining the action of the process.

I. Loads

The nature of the imposed load presented to a body process is as diverse as the nature or the level of the process examined. For example, at the organ level, the load imposed upon the heart is the quantity of blood presented to the heart for propulsion (8). The load imposed upon the liver includes the quantity of bilirubin presented to the liver for biliary excretion (9). At the cellular level, the load imposed upon each body cell includes the quantity of glucose presented to the cell membrane for transport into the interior of the cell (10). The load imposed upon a renal tubular cell includes the quantity of glucose presented to the lumen surface of the cell for renal reabsorption (11). At the sub-cellular particle level, the load imposed upon a mitochondrion includes the quantity of phosphorylated carbohydrate presented to the mitochondrial membrane for further oxidative phosphorylation (12). The load imposed upon a cell nucleus includes the quantity of amino acids presented to the nuclear membrane for transport into the interior of the cell nucleus (13). At the enzyme level, the imposed load is the quantity of specific substrate presented to each enzyme molecule for its action (14).

In biosynthetic processes which are forming new molecules, new particles, or new cells, thermodynamic and specific stimuli are presented to the process which tend to promote the new synthesis (15). The load imposed upon such biosynthetic processes is equal to the magnitude and intensity of such thermodynamic and specific stimuli imposed upon the process.

II. Resistances

The natures of resistances opposing the successful action of a body process are also diverse. Biophysical resistances include such factors as inertial resistance to propulsion, frictional and pressure resistances to air and liquid flow, gravitational resistance to elevation, elastic resistance to deformation, and viscous resistance to shearing (16). The most common type of biochemical resistance is the thermodynamic requirement for some minimal activation of the chemical reactants in a process before their energy level permits the reaction to proceed (14). Other biochemical resistances include the resistance offered to a reaction by the wide distribution of its reactants at some distance from the active site of the reaction. Their dispersion is often overcome by active trapping (17), pumping (18), or concentrating (19) mechanisms, as displayed by the thyroid in concentrating iodide ion for further thyroxin synthesis (17). In addition, enzyme reactions are resisted by the requirement for some minimum time period to be spent by the enzyme in complex union with its substrate before completing its action on the substrate (14).

In processes of several successive enzyme reactions, the resistance offered is higher than in single enzyme reactions, since the total time spent by the substrate in complex union with successive enzymes is the sum of such times of each enzyme in the process (20), and since the successive enzymes may not be arranged spatially in a successive manner corresponding to their sequence of function in the over-all reaction process (21), so that dispersion and concentration of substrate between enzymic steps resists smooth completion of the process.

The presence of end-products of an enzyme reaction tends to resist the net action of the enzyme on the substrate load, both by inhibiting the kinetics of existing enzyme molecules (22), and by inhibiting the formation of new enzyme molecules (23).

III. Hypertrophy and Atrophy

A body process, over an extended period of time, may often undergo a marked increase in its capacity for work when presented with sustained increased loads to be acted upon or with increased resistances to overcome in the accomplishment of its action (7). Conversely, such a process may undergo a marked decrease in capacity when the imposed load or the opposing resistance is decreased over an extended period of time. Such secondary hypertrophy or atrophy of capacity implies a constant close responsiveness of the process to the levels of load and resistance which are usually presented to it and can be considered as a steady compensatory adaptation by the process to its immediate environment.

The time period required for the secondary hypertrophy or atrophy reaction to occur suggests the mechanism involved. Hypertrophy or atrophy reactions can involve changes in either the number or in the activity of the cells, subcellular particles, or enzymes of the affected process. Immediate changes in capacity probably result from changes in the activity of existing cells, particles, or enzymes, whereas gradual changes in capacity probably result from changes in the rate of formation and in the total number of cells, particles, or enzymes of the adapting body process (23).

Mechanisms are known which adjust the rate of enzyme synthesis (24) or of enzyme activity (25-27) to the quantity of substrate presented to the process. Analogous mechanisms are suspected which adjust the rate of cellular proliferation (28) or of cell activity (29) to the load presented to the affected organ. A change in cell activity within an organ can result from either a change in the activity of each cell in the organ, or a change in the fraction of cells active within the organ (30).

IV. Failure and Tolerance

The output of a body process is determined by the functional interaction of the available capacity of the process with its imposed load and its opposing resistances, and this output is equal to that fraction of the imposed load that is successfully acted upon by the process. When load and resistances exceed capacity, the process fails to accomplish its task completely, and evidence of such failure is found in the residual fraction of the imposed load which is not acted upon by the process. The magnitude of such residual load is a quantitative expression of the "disease" of the failing process. Conversely, when capacity exceeds load and resistances, the process has a degree of reserve or tolerance for higher loads or resistances. The magnitude of such tolerance is a quantitative expression of the "health" of the process.

Interestingly enough, the capacity of a body process, and thus its tolerance, can be progressively increased by the intermittant increase of imposed load or of opposed resistances - that is, by training (31). The benefits of such training, however, can soon be lost if the process is not further challenged periodically by high loads or resistances. The attainment of a state of training utilizes the inertia shown by an intermittantly challenged process in decreasing its capacity, probably a function of finite time required to degrade existing enzymes, subcellular particles, or cells (32).

V. Diseases of High-Output Failure

Although some body processes can increase their capacity up to 50 to 100 times above normal values, such increases are ultimately limited by spatial and thermodynamic factors. Thus, any body process is inherently capable of failure under sufficiently high levels of load and resistance, a state of so-called "high-output failure" (7). It is of interest, however, that in certain disease states the limits to hypertrophy of capacity are reached somewhat prematurely in the hypertrophy response. Under such pathologic limits to hypertrophy of the process, failure becomes evident at much lower levels of load and resistance than when the hypertrophy response has only normal limits (7).

Such pathologic limits to hypertrophy are usually secondary features of the basic disease state and are difficult in themselves to correct by therapy. By contrast, reductions of the pathologically increased loads or increased resistances in the failing process are more easliy achieved and are the essence of successful therapy in high-output failure disease states (7).

In those disease characterized by normal or low levels of imposed load or of opposed resistances in the affected body process, further marked reductions of the already-low levels of load and resistance are usually not feasible, and the main focus of therapy is to restore the diminished capacity of the process.

VI. Antagonism of Load and Resistance

It seems useful to distinguish that fraction of the total work capacity of a process which acts upon the imposed load from that fraction which acts to overcome the opposed resistances. In those body processes in which a finite capacity is available for the total work needs of the process, increases in the opposed resistance result in a decline in the fraction of capacity available for action upon the imposed load, and the process suffers a decline in the efficiency of accomplishing its action. For example, elevation of the aortic pressure markedly reduces the cardiac efficiency in the "external" action of propelling the quantity of blood presented to the heart, but no decline of cardiac efficiency in the "internal" action of developing an intraventricular pressure-time product (33). This discrepancy has given rise to the concepts of an "external" and an "internal" efficiency of the cardiac action and illustrates the necessity for defining both the level and the components of a body process under study (33).

It is not yet apparent whether changes in imposed load equal to changes in opposed resistance always produce reciprocal effects on the output of a process - that is, effects equal in magnitude but opposite in sign. Cardiac studies comparing the effects of increasing aortic pressure to those of increasing cardiac inflow load disclose non-reciprocal effects on "internal" work rate, oxygen consumption, and efficiency (34).

VII. Conclusion

It appears probable that most body processes are capable within limits of adapting their work capacity to the work demands presented to them by their imposed load and opposing resistance. Such adaptations are apparently useful and usually appropriate in magnitude to the changed conditions initiating them. Adaptations can occur among the cells of an adapting organ, among the subcellular particles of an adapting cell, and among the enzymes of an adapting particle or cell.

It remains to be determined whether the changes characterizing those body processes involved by pathology such as neoplasia (35, 36) or hypersensitivity (37, 38) also occur at all of these levels within the organism, and whether these changes are merely adaptive to some unusual imposed load or opposing resistance or, instead, are truly inappropriate responses by the affected process to otherwise normal loads and resistances.

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1. "Load-Tolerance as a Quantitative Estimate of Health".

2. "Editorial: Medical Feudalism".

3. "Human Throughput Systems".

4. "Analysis of Queueing and Renewal within Human Systems".

5. "Medicine 275: Systems Analysis of Latent Disease".

6. Related articles from PubMed:
http://www.ncbi.nlm.nih.gov/sites/entrez

PubMed (Select 13701767).

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Abstract Intervertebral disc degeneration can predispose to anterior vertebral fractures in the thoracolumbar spine.
J Bone Miner Res. 2006 Sep;21(9):1409-16.
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Husa P, Slesinger P, Stroblova H, Svobodnik A.
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Abstract [The effect of patient's body weight, gender and baseline viral load on the efficacy of hepatitis C therapy]
Vnitr Lek. 2006 Jun;52(6):590-5. .
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Chow DH, Ng XH, Holmes AD, Cheng JC, Yao FY, Wong MS.
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Abstract Effects of backpack loading on the pulmonary capacities of normal schoolgirls and those with adolescent idiopathic scoliosis.
Spine. 2005 Nov 1;30(21):E649-54.
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Abe D, Yanagawa K, Niihata S.
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Abstract Effects of load carriage, load position, and walking speed on energy cost of walking.
Appl Ergon. 2004 Jul;35(4):329-35.
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Knapik J, Harman E, Reynolds K.
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Abstract Load carriage using packs: a review of physiological, biomechanical and medical aspects.
Appl Ergon. 1996 Jun;27(3):207-16.
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Floerkemeier T, Hurschler C, Witte F, Wellmann M, Thorey F, Vogt U, Windhagen H.
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Abstract Comparison of various types of stiffness as predictors of the load-bearing capacity of callus tissue.
J Bone Joint Surg Br. 2005 Dec;87(12):1694-9.
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Gray EM, Bradley TJ.
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Free Full Text Physiology of desiccation resistance in Anopheles gambiae and Anopheles arabiensis.
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No Abstract [Studies on skin resistance and capacity changes.]
Kiserl Orvostud. 1961 Mar;13:49-53. Hungarian. No abstract available.
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No Abstract Load tolerance as a quantitative estimate of health.
Ann Intern Med. 1962 Nov;57:788-94. No abstract available.
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GOLDBERGER E.
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Free in PMC Methods for measuring the resistance at any point on the body and for equalizing inequalities of skin resistance.
Br Heart J. 1951 Oct;13(4):549-53. No abstract available.
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Carlson ED, Chamberlain RM.
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Abstract Allostatic load and health disparities: a theoretical orientation.
Res Nurs Health. 2005 Aug;28(4):306-15. Review.
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Bangsberg DR, Acosta EP, Gupta R, Guzman D, Riley ED, Harrigan PR, Parkin N, Deeks SG.
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Abstract Adherence-resistance relationships for protease and non-nucleoside reverse transcriptase inhibitors explained by virological fitness.
AIDS. 2006 Jan 9;20(2):223-31.
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Loeschcke V, Sorensen JG, Kristensen TN.
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Free Full Text Ecologically relevant stress resistance: from microarrays and quantitative trait loci to candidate genes - a research plan and preliminary results using Drosophila as a model organism and climatic and genetic stress as model stresses.
J Biosci. 2004 Dec;29(4):503-11.
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Gronkvist MJ, Bergsten E, Eiken O, Gustafsson PM.
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Abstract Contributions of lower limb and abdominal compression to ventilation inhomogeneity in hypergravity.
Respir Physiol Neurobiol. 2005 Aug 25;148(1-2):113-23.
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No Abstract [The IV All-Russian Symposium on Slow Oscillatory Processes in the Human Body and in the II School/Seminar on Nonlinear Dynamics in Physiology and Medicine (Novokuznetsk, Russia, May 24-27, 2005)]
Fiziol Cheloveka. 2006 Mar-Apr;32(2):141-4. Russian. No abstract available.
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LUETHY E.
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No Abstract [PRESSURE AND VOLUME LOAD IN THE HUMAN HEART.]
Verh Dtsch Ges Inn Med. 1964;70:76-81. German. No abstract available.
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66:
Alvarez GA, Cavanagh P.
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Abstract The capacity of visual short-term memory is set both by visual information load and by number of objects.
Psychol Sci. 2004 Feb;15(2):106-11.
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BOUISSET S.
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No Abstract [EFFECT ON THE ELECTROMYOGRAM OF VARIATION OF FACTORS DEFINING THE MOMENT OF DISPLACED LOAD.]
J Physiol (Paris). 1964 May-Jun;56:303-4. French. No abstract available.
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68:
LEJHANCOVA G.
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No Abstract [Evaluation of alkali resistance tests and neutralizing capacity of the skin in occupational dermatology.]
Cesk Dermatol. 1959 Apr;34:97-100. . No abstract available.
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Kim SH, Yoo JC, Wang JH, Choi KW, Bae TS, Lee CY.
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Abstract Arthroscopic sliding knot: how many additional half-hitches are really needed?
Arthroscopy. 2005 Apr;21(4):405-11.
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Lawlor DA, Riddoch CJ, Page AS, Anderssen SA, Froberg K, Harro M, Stansbie D, Smith GD.
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Abstract The association of birthweight and contemporary size with insulin resistance among children from Estonia and Denmark: findings from the European Youth Heart Study.
Diabet Med. 2005 Jul;22(7):921-30.
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Willi Y, Van Buskirk J, Fischer M.
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Free in PMC A threefold genetic allee effect: population size affects cross-compatibility, inbreeding depression and drift load in the self-incompatible Ranunculus reptans.
Genetics. 2005 Apr;169(4):2255-65. Epub 2005 Feb 3.
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Lau C, Faerch K, Glumer C, Tetens I, Pedersen O, Carstensen B, Jorgensen T, Borch-Johnsen K; Inter99 study.
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Free Full Text Dietary glycemic index, glycemic load, fiber, simple sugars, and insulin resistance: the Inter99 study.
Diabetes Care. 2005 Jun;28(6):1397-403. Erratum in: Diabetes Care. 2005 Sep;28(9):2340-1.
PMID: 15920058 [PubMed - indexed for MEDLINE]
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Jabs DA, Martin BK, Forman MS, Ricks MO; Cytomegalovirus Retinitis and Viral Resistance Research Group.
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Abstract Cytomegalovirus (CMV) blood DNA load, CMV retinitis progression, and occurrence of resistant CMV in patients with CMV retinitis.
J Infect Dis. 2005 Aug 15;192(4):640-9. Epub 2005 Jul 11. Erratum in: J Infect Dis. 2005 Oct 1;192(7):1310.
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Adje-Toure CA, Cheingsong R, Garcia-Lerma JG, Eholie S, Borget MY, Bouchez JM, Otten RA, Maurice C, Sassan-Morokro M, Ekpini RE, Nolan M, Chorba T, Heneine W, Nkengasong JN.
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Abstract Antiretroviral therapy in HIV-2-infected patients: changes in plasma viral load, CD4+ cell counts, and drug resistance profiles of patients treated in Abidjan, Cote d'Ivoire.
AIDS. 2003 Jul;17 Suppl 3:S49-54.
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Berwaerts K, Van Dyck H.
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Abstract Take-off performance under optimal and suboptimal thermal conditions in the butterfly Pararge aegeria.
Oecologia. 2004 Nov;141(3):536-45. Epub 2004 Aug 7.
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Linden DE, Bittner RA, Muckli L, Waltz JA, Kriegeskorte N, Goebel R, Singer W, Munk MH.
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Abstract Cortical capacity constraints for visual working memory: dissociation of fMRI load effects in a fronto-parietal network.
Neuroimage. 2003 Nov;20(3):1518-30.
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Kiviniemi TO, Snapir A, Saraste M, Toikka JO, Raitakari OT, Ahotupa M, Hartiala JJ, Scheinin M, Koskenvuo JW.
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Abstract Determinants of coronary flow velocity reserve in healthy young men.
Am J Physiol Heart Circ Physiol. 2006 Aug;291(2):H564-9. Epub 2006 Feb 24.
PMID: 16501023 [PubMed - indexed for MEDLINE]
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Bamman MM, Ragan RC, Kim JS, Cross JM, Hill VJ, Tuggle SC, Allman RM.
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Free Full Text Myogenic protein expression before and after resistance loading in 26- and 64-yr-old men and women.
J Appl Physiol. 2004 Oct;97(4):1329-37. Epub 2004 May 21.
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Pereira MA, Swain J, Goldfine AB, Rifai N, Ludwig DS.
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Free Full Text Effects of a low-glycemic load diet on resting energy expenditure and heart disease risk factors during weight loss.
JAMA. 2004 Nov 24;292(20):2482-90.
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Scarpace PJ, Matheny M, Tumer N, Cheng KY, Zhang Y.
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Abstract Leptin resistance exacerbates diet-induced obesity and is associated with diminished maximal leptin signalling capacity in rats.
Diabetologia. 2005 Jun;48(6):1075-83. Epub 2005 Apr 30.
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81:
Wiseman SB, Singer TD.
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Abstract Applications of DNA and protein microarrays in comparative physiology.
Biotechnol Adv. 2002 Dec;20(5-6):379-89.
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Sokolov EI, Ozerova IN, Perova NV, Olfer'ev AM, Metel'skaia VA, Serdiuk AP, Gorbacheva OI, Shchukina GN.
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Abstract [Lowering of antiatherogenic significance of high density lipoproteins in obese persons]
Kardiologiia. 2004;44(2):45-50. Russian.
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83:
Ebben WP, Kindler AG, Chirdon KA, Jenkins NC, Polichnowski AJ, Ng AV.
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Abstract The effect of high-load vs. high-repetition training on endurance performance.
J Strength Cond Res. 2004 Aug;18(3):513-7.
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84:
LEWIS JA, NICHOLLS DM, TROOP VL, KIM OS.
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No Abstract CATION LOAD AND RENAL FUNCTION IN HYPERTENSION.
Med Serv J Can. 1965 Jan;21:59-63. No abstract available.
PMID: 14266598 [PubMed - indexed for MEDLINE]
85:
ERONIN FT.
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No Abstract [EFFECT OF VARIOUS DRINKING PATTERNS ON DIURESIS UNDER THE CONDITION OF HIGH AIR TEMPERATURE AND PHYSICAL LOAD.]
Fiziol Zh SSSR Im I M Sechenova. 1963 Oct;49:1249-53. Russian. No abstract available.
PMID: 14148912 [PubMed - indexed for MEDLINE]
86:
Williams L, Frenneaux M.
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Abstract Diastolic ventricular interaction: from physiology to clinical practice.
Nat Clin Pract Cardiovasc Med. 2006 Jul;3(7):368-76. Review.
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Jones CY, Wilson IB, Greenberg AS, Shevitz A, Knox TA, Gorbach SL, Spiegelman D, Jacobson DL, Wanke C.
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Abstract Insulin resistance in HIV-infected men and women in the nutrition for healthy living cohort.
J Acquir Immune Defic Syndr. 2005 Oct 1;40(2):202-11.
PMID: 16186739 [PubMed - indexed for MEDLINE]
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