The evolutionary origin of form and function

Spennende studie som nevner at gener har lite med utvikling av organismer å gjøre. Det er heller «the second law of thermodynamics» som styrer det.

In summary, we propose that the life process is based not on genetic variation, but on the second law of thermodynamics (hereinafter the second law) and the principle of least action, as proposed for thermodynamically open systems by De Maupertuis (Ville et al2008), which at the most fundamental level say the same thing.

Det som avgjør om en organisme er levedyktig eller ikke er dens evne til å hente energi fra omgivelsene. For oss kan dette peke på jo mer effektiv blodsirkulasjonens distribusjon av oksygen til cellene er, jo mer fri energi har vi tilgjengelig.

In this reformulation form and function, extant and extinct, are the consequence of natural selection acting primarily upon the ability of organisms to extract energy (nutrient) from their environment, as pointed out in 1835, prior to the publication of Origin, by Edward Blyth (Blyth, 1835).

De reformulerer også definisjonen på entropi, som vanligvis er sett på som kaos. Her sier de at det egentlig bør oppfattes som en organisert kompleksitet fordi den bundede energien i lavere livsformer er tilgjengelig som fri energi for høyere livsformer. Det er fullt mulig dette kan forståes i sammenheng med mitokondrias funksjon for oss. Energien som mitokondria skaper blir tilgjengelig som fri energi for oss.

Energy, in the form of nutrient, is consumed, thereby producing entropy, according to the second law in the most efficient way (least action) possible given the conditions. Under these circumstances, explicitly thermodynamically open systems, entropy is maximised in the form of organisation or complexity (Sharma & Annila, 2007) and not, as proposed by Boltzmann, disorder (Sharma & Annila2007). In terms of the food chain, the entropy (bound energy) of lower forms is available as free energy (nutrient) for higher forms.

Gener fungerer bare som en blueprint for de erfaringene molekyler og celler gjør seg med omgivelsene. Genene er notisblokken.

We predicate the current proposal on a metabolism-first origin of life (Baverstock, 2013), in which proteins, free of DNA, were a form of proto-life. Life appeared when these proto-life forms recruited nucleic acids in the form of DNA to act as a template for replication and to code for essential peptides (Annila & Baverstock, 2014) through the process of reverse translation making it possible for true replication to occur.

Sett i lys av dette kan vi innse at gen-mutasjon har lite å gjøre med evolusjon.

In other words mutation of existing coding sequences is unnecessary for evolution to have taken place – that is not to say that evolution has not taken advantage of mutational events, but that genetic variation is not rate limiting.

De forklarer også hvorfor f.eks. en mus og et menneske har nesten helt samme genuttykk, men helt forskjellig form og funksjon.

Thus, for example, mouse and man are phenotypically distinct organisms with closely similar genotypes (Baverstock, 2011), that is, a near identical complement of peptides, which give rise through dissipative information generating processes within the cell, to two distinct information outputs (phenotypes).

De konkluderer med at utvikling skjer ved at en organisme utvikler bedre måter å tilegne seg energi fra omgivelsene på. F.eks. kan mitokontria, respirasjonssystemet, sirkulasjonssystemet og nervesystemet være funksjoner som gir bedre tilegning og utnyttelse av energi i en organisme slik vi er. 

The evolution of multicellular organisms with complex forms and functional abilities can be accounted for based on a fundamental tenet underpinned by the second law of thermodynamics, with natural selection acting on the ability of the organism to transduct energy (nutrient) most efficiently from its ecosystem by deploying that form and those functions.

Systemic inflammation impairs respiratory chemoreflexes and plasticity

Denne studien beskriver hvordan systemisk betennelse påvirker pustefunksjonen og gjør at det blir vanskeligere å endre pustemønser, f.eks. å gjøre pusteøvelser, eller å tilpasse pusten til aktivitetsnivå. Spesielt den kjemiske og motoriske delen av pustefysiologien blir dårligere. Noe som viser seg i laver CO2 sensitivitet (kjemisk) og svakere pustemuskler (Motorisk).

Nevner spesielt at det er mikroglia celler i CNS som påvirkes av betennelse, og som kan oppretthodle betennelse siden de sender ut cytokiner, m.m. Astrosytter kan også bidra mye siden de aktiverer NFkB. Den gode nyheten her er at økt CO2 nedregulerer NFkB. TLR-4 (Toll-like receptor) aktiveres av patogener og problemer i cellene, og aktiverer NFkB, og nedreguleres av økt CO2.


Many lung and central nervous system disorders require robust and appropriate physiological responses to assure adequate breathing. Factors undermining the efficacy of ventilatory control will diminish the ability to compensate for pathology, threatening life itself. Although most of these same disorders are associated with systemic and/or neuroinflammation, and inflammation affects neural function, we are only beginning to understand interactions between inflammation and any aspect of ventilatory control (e.g. sensory receptors, rhythm generation, chemoreflexes, plasticity). Here we review available evidence, and present limited new data suggesting that systemic (or neural) inflammation impairs two key elements of ventilatory control: chemoreflexes and respiratory motor (vs. sensory) plasticity. Achieving an understanding of mechanisms whereby inflammation undermines ventilatory control is fundamental since inflammation may diminish the capacity for natural, compensatory responses during pathological states, and the ability to harness respiratory plasticity as a therapeutic strategy in the treatment of devastating breathing disorders, such as during cervical spinal injury or motor neuron disease.

Most lung and CNS disorders are associated with systemic and/or neural inflammation, including chronic lung diseases (Stockley, 2009), traumatic, ischemic and degenerative neural disorders (Teeling and Perry, 2009) and obstructive sleep apnea.

Systemic inflammation affects sensory receptors that modulate breathing, but can also trigger inflammatory responses in the central nervous system (CNS) through complex mechanisms. The primary CNS cells affected during systemic inflammation are microglia, the resident immune cells of the CNS, and astrocytes (Lehnardt, 2010).

Even when in their “resting state,” microglia are highly active, surveying their environment (Raivich, 2005,Parkhurst and Gan, 2010). When confronted with pathological conditions, such as neuronal injury/degeneration or bacterial/viral/fungal infection, they become “activated,” shifting from a stellate, ramified phenotype to an amoeboid shape (Kreutzberg, 1996). Activated microglia can be phagocytic, or they can release toxic and protective factors, including cytokines, prostaglandins, nitric oxide or neurotrophic factors (e.g. BDNF) (Kreutzberg, 1996Graeber, 2010). Despite the importance of microglia in immune function, they are diffuse in the CNS (~70-90% of CNS cells are glia; microglia are ~5-10% of those cells).

Astrocytes, on the other hand, contribute to the overall inflammatory response since they release cytokines, triggering nuclear factor-kappa B (NFκB) signaling elsewhere in the CNS. Further, they express many TLRs, including TLR-4, capable of eliciting an inflammatory response (Li and Stark, 2002Farina et al., 2007,Johann et al., 2008). Given their relative abundance, astrocytes may play a key role in CNS inflammatory responses.

TLR-4 receptors are cytokine family receptors that activate transcription factors, such as NFκB (Lu et al., 2008). NFκB regulates the expression of many inflammatory genes, including: IL-1β, -6 and -18, TNFα, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) (Ricciardolo et al., 2004Nam, 2006). Endogenous molecules known to activate TLR-4 receptors include (but are not limited to) heat shock proteins (specifically HSP60, Ohashi et al., 2000Lehnardt et al., 2008), fibrinogen, surfactant protein-A, fibronectin extra domain A, heparin sulfate, soluble hyaluronan, β-defensin 2 and HMGB1 (Chen et al., 2007).

The role of inflammation (and specifically microglia) in chronic pain has been studied extensively (reviewed in Woolf and Salter, 2000Trang et al., 2006Mika, 2008Abbadie et al., 2009Baumbauer et al., 2009). A remarkable story has emerged, demonstrating the interplay between neurons, microglia, inflammation and plasticity in this spinal sensory system. In short, inflammation induces both peripheral and central sensitization, leading to allodynia (hypersensitivity to otherwise non-painful stimuli) and hyperalgesia (exaggerated or prolonged responses to a noxious stimulus) (Mika, 2008).

An important aspect of ventilatory control susceptible to inflammatory modulation is the chemoreflex control of breathing. Chemoreflexes are critical for maintaining homeostasis of arterial blood gases viaclassical negative feedback (Mitchell et al., 2009), or acting as “teachers” that induce plasticity in the respiratory control system (Mitchell and Johnson, 2003). Major chemoreflexes include the hypoxic (Powell et al., 1998) and hypercapnic ventilatory responses (Nattie, 2001), arising predominantly from the peripheral arterial and central chemoreceptors (Lahiri and Forster, 2003).

To date, no studies have reported the impact of systemic inflammation on hypercapnic responses. However, increased CO2 suppresses NFκB activation, possibly suppressing inflammatory gene expression (Taylor and Cummins, 2011). In fact, hypercapnia has been used to treat ischemia/reperfusion injury to decrease inflammation and reduce lung tissue damage (Laffey et al., 2000O’Croinin et al., 2005Curley et al., 2010Li et al., 2010).

Further work concerning the influence of systemic inflammation on hypercapnic ventilatory responses is warranted, particularly since impaired CO2 chemoreflexes would allow greater hypercapnia and minimize the ongoing inflammation; in this sense, impaired hypercapnic ventilatory responses during inflammation may (in part) be adaptive.

Differential blood flow responses to CO2 in human internal and external carotid and vertebral arteries

Denne viser hvordan CO2-responsen er litt forskjellige i forskjellige blodkar. Den er sterkere i blodkar inni hjernen enn i blodkar i kraniet, ansiktet og ryggraden. Blodkar i ryggraden har større respons enn blodkar i ansiktet, men mindre respons enn blodkar i hjernen.

Because of methodological limitations, almost all previous studies have evaluated the response of mean blood flow velocity (Vmean) in the middle cerebral artery (MCA) to changes in CO2 as a measure of CO2 reactivity across the whole brain (Aaslid et al. 1989Ainslie & Duffin, 2009Ainslie & Ogoh, 2009).


ICA, VA and BA CO2 reactivity was significantly higher during hypercapnia than during hypocapnia (ICA, P < 0.01; VA, P < 0.05; BA, P < 0.05), but ECA and MCA were not significantly different.

The major finding from the present study was that cerebral CO2 reactivity was significantly lower in the VA and its distal artery (BA) than in the ICA and its distal artery (MCA). These findings indicate that vertebro-basilar circulation has lower CO2 reactivity than internal carotid circulation. Our second major finding was that ECA blood flow was unresponsive to hypocapnia and hypercapnia, suggesting that CO2 reactivity of the external carotid circulation is markedly diminished compared to that of the cerebral circulation. These findings suggest that different CO2 reactivity may explain differences in CBF responses to physiological conditions (i.e. dynamic exercise and orthostatic stress) across areas in the brain and/or head.

Hypercapnic cerebral CO2 reactivity in global CBF was greater than the hypocapnic reactivity (Ide et al. 2003) (Table 3). The mechanisms underlying this greater reactivity to hypercapnia compared with hypocapnia may be related to a greater influence of vasodilator mediators on intracranial vascular tone compared with vasoconstrictive mediators (Toda & Okamura, 1998Ainslie & Duffin, 2009). In humans, Peebles et al.(2008) recently reported that, during hypercapnia, there is a large release of nitric oxide (NO) from the brain, whereas this response was absent during hypocapnia.

The difference in CO2 reactivity between vertebro-basilar territories (VA and BA) and the cerebral cortex (ICA and MCA) may be due to diverse characteristics of vasculature, e.g. regional microvascular density (Sato et al. 1984), basal vascular tone (Ackerman, 1973Haubrich et al. 2004Reinhard et al. 2008), autonomic innervation (Edvinsson et al. 1976Hamel et al. 1988) and regional heterogeneity in ion channels or production of NO (Iadecola & Zhang, 1994Gotoh et al. 2001).

Interestingly, the response of the ECA to changes in CO2 may be similar to other peripheral arteries. It has long been appreciated that the vasodilatory effect of hypercapnia is much more profound in cerebral than in peripheral vasculature, particularly leg (Lennox & Gibbs, 1932Ainslie et al. 2005) and brachial arteries (Miyazaki, 1973). These findings suggest that control of CO2 is particularly important in the cerebral circulation. The high resting metabolic requirements of the brain, compared with that of other vasculature, might be one reason why this circulatory arrangement is desirable (Ainslie et al. 2005). Specifically, high CO2 reactivity may be a way for the brain to match metabolism with flow (Ainslie et al. 2005).

Lower CO2reactivity in the vertebro-basilar system may be important for maintaining central respiratory function because Graphic in central chemoreceptors is regulated by Graphic and blood flow to maintain breathing stability.

In summary, our study shows that cerebral CO2 reactivity in the vertebro-basilar circulation is lower than that in the internal carotid circulation, while CO2 reactivity in the external carotid circulation is much lower compared with two other cerebral arteries. These findings indicate a difference in cerebral CO2 reactivity between different circulatory areas in the brain and head, which may explain different CBF responses to physiological stress. Lower CO2 reactivity in the vertebro-basilar system may be beneficial for preserving blood flow to the medulla oblongata to maintain vital systemic functions, while higher CO2 reactivity in the internal carotid system may imply a larger tolerance for varied blood flow in the cerebral cortex.

Morning attenuation in cerebrovascular CO2 reactivity in healthy humans is associated with a lowered cerebral oxygenation and an augmented ventilatory response to CO2

Denne beskriver hvordan blodkarenes respons på CO2 er dårligere om morgenen, og det er derfor det skjer flere slag og slikt om morgenen. Den nevner mange interessante prinsipper. Bl.a. at lavere vasomotor respons (på CO2) gir mindre oksygen til hjernen. Og at i opptil 20 sekunder etter en 20 sekunder holdning av pust (etter utpust) øker fortsatt oksygenmengden og blodgjennomstrømningen i hjernen. Nevner også at siden blodkarene i hjernen reagerer dårligere på CO2 om morgenen blir det lett at pusten over- eller underkompenserer, så pustemønsteret blir uregelmessig om morgenen. Spesielt om man har underliggende faremomenter som hjerte/karsykdommer.


Furthermore, our results suggest that morning cerebral tissue oxygenation might be reduced as a result of a decreased cerebrovascular responsiveness to CO2 or other factors, leading to a higher level of desaturation.

Our data indicate that the cerebrovascular reactivity to CO2 in healthy subjects is significantly reduced in the morning and is strongly associated with an augmented ventilatory response to CO2. It is likely that this reduction in MCAV CO2 reactivity, by reducing blood flow through medullary respiratory control centers, increases both the arterial-brain tissue PCO2 difference and the H+ concentration presented to the central chemoreceptor(s) (1144). In effect, it appears the brain tissue is more susceptible in the morning to changes in arterial PCO2, which could increase the likelihood of ventilatory overshoots and undershoots.

However, as was the case with the hypercapnic challenge, subjects holding their breath in the morning experienced a significantly blunted increase in MCAV compared with evening, likely a result of a reduced cerebrovascular responsiveness to CO2.

In conclusion, our results suggest that early morning reductions in cerebrovascular CO2 reactivity strongly influence the magnitude of the ventilatory response to CO2. This may have significant implications for breathing stability, increasing the chances of periodic breathing in the morning in patients with additional risk factors. The early morning reduction in cerebral oxygenation with hypercapnic challenge, mild hypoxemia, or during apnea may be a contributing factor in the high prevalence of early morning stroke. Whether differences in the responses of CBF, oxygenation, or V̇E to CO2challenge are associated with other risk factors for stroke, such as gender or age, remains to be elucidated.

Behavioural modification of the cholinergic anti-inflammatory response to C-reactive protein in patients with hypertension

Denne beskriver hvordan regulering av pusten kan påvirke vagus nerven til å dempe betennelser og redusere CRP (en betennelsesmarkør) i blodet.

Objectives.  A central hypothesis of the cholinergic anti-inflammatory reflex model is that innate immune activity is inhibited by the efferent vagus. We evaluated whether changes in markers of tonic or reflex vagal heart rate modulation following behavioural intervention were associated inversely with changes in high-sensitivity C-reactive protein (hsCRP) or interleukin-6 (IL-6).

Design.  Subjects diagnosed with hypertension (= 45, age 35–64 years, 53% women) were randomized to an 8-week protocol of behavioural neurocardiac training (with heart rate variability biofeedback) or autogenic relaxation. Assessments before and after intervention included pro-inflammatory factors (hsCRP, IL-6), markers of vagal heart rate modulation [RR high-frequency (HF) power within 0.15–0.40 Hz, baroreflex sensitivity and RR interval], conventional measures of lipoprotein cholesterol and 24-h ambulatory systolic and diastolic blood pressure.

Results.  Changes in hsCRP and IL-6 were not associated with changes in lipoprotein cholesterol or blood pressure. After adjusting for anti-inflammatory drugs and confounding factors, changes in hsCRP related inversely to changes in HF power (β =−0.25±0.1, P = 0.02), baroreflex sensitivity (β = −0.33±0.7, P = 0.04) and RR interval (β = −0.001 ± 0.0004, P = 0.02). Statistically significant relationships were not observed for IL-6.

Conclusions.  Changes in hsCRP were consistent with the inhibitory effect of increased vagal efferent activity on pro-inflammatory factors predicted by the cholinergic anti-inflammatory reflex model. Clinical trials for patients with cardiovascular dysfunction are warranted to assess whether behavioural interventions can contribute independently to the chronic regulation of inflammatory activity and to improved clinical outcomes.

Chronic low-grade inflammation contributes to the development of experimental and clinical hypertension [1–3], and it increases the risk for myocardial infarction, stroke and sudden cardiac death [4]. C-reactive protein (CRP) is an established index of systemic inflammation. It is produced chiefly by hepatocytes under the regulation of a cascade of pro-inflammatory cytokines [tumour necrosis factor-α (TNF-α), interleukin-1ß [IL-1ß] and IL-6] that are expressed in response to conditions that include vascular injury and infection. In addition, CRP is produced by human coronary artery smooth muscle cells following exposure to pro-inflammatory cytokines [5], which suggests that it may contribute independently to endothelial dysfunction and atherogenesis [6].

Clinical trials that have attempted to modify vagal efferent activity by means of aerobic exercise [17, 18], resistance exercise [19] or device-guided vagal nerve stimulation [20–22] have yet to demonstrate consequent reduction in pro-inflammatory activity that is independent of confounding factors such as anti-inflammatory medications.

Subjects received four weekly and two biweekly 1-h sessions of behavioural neurocardiac training or autogenic relaxation, as described previously [23]. Home practice sessions complemented the laboratory-based training. All sessions began with a 10-min review of cognitive-behavioural guidelines for managing daily stress [25].

At the completion of each task, subjects were trained to cognitively disengage from negative or aroused affect and to focus attention on slowing respiration (within their comfort zone) to 10-s cycles (6 breaths min−1). During each countering exercise, subjects were guided by the use of biofeedback to increase RR spectral power at approximately 0.1 Hz, as shown on a biofeedback display of the RR power spectrum (0.003–0.5 Hz) and breaths min−1.

The major finding of this study is that following an 8-week protocol of behavioural neurocardiac training or autogenic relaxation amongst patients with hypertension, change in hsCRP was associated independently and inversely with changes in tonic and reflex vagal heart rate modulation as measured by RR high-frequency power (ms2 per Hz), baroreflex sensitivity (ms per mmHg) and lengthening of the RR interval (ms). A statistical trend in the data suggested a similar inverse association between changes in IL-6 and RR high-frequency power.

A central hypothesis of the cholinergic anti-inflammatory reflex model is that the innate immune response is regulated, in part, by rapid and localized efferent activity of the vagus nerve. Previous reviews have identified the functional anatomy and neural mechanisms of this model [10, 29, 30]. In brief, efferent fibres of the vagus nerve comprise a neural anti-inflammatory pathway that culminates in the release of acetylcholine in proximate sites where pro-inflammatory factors have been expressed. Acetylcholine has been shown to bind to subunit α7 of nicotinic acetylcholine receptors on cytokine-producing immune cells [30]. This inhibits the activation of NF-κB and the subsequent expression of a pro-inflammatory cascade that includes TNF-α, IL-6 and CRP [10].

To our knowledge, the present proof of principle study involving hypertensive patients provides the most direct evaluation of whether augmentation of tonic or reflex vagal heart rate modulation, in this instance by a behavioural intervention, attenuates independently pro-inflammatory activity as assessed by hsCRP and IL-6. It is noteworthy that the present findings were observed following only modest changes in markers of vagal HR modulation. Previous behavioural trials of heart rate variability biofeedback or relaxation [32–34] have reported a small but statistically significant increase in vagal HR modulation. Similarly, behavioural training is associated with a modest, but statistically significant decrease in proinflammatory factors, including hsCRP and IL-6 [35], although heart rate variability biofeedback failed to reduce other inflammatory factors following experimental administration of an endotoxin (lipopolysaccharide) [36].

In sum, the present findings support the model of a cholinergic anti-inflammatory reflex when pro-inflammatory activity is measured by hsCRP. Clinical trial evidence has demonstrated that behavioural interventions can significantly augment vagal heart rate modulation or cardiovagal baroreflex gain through the use of relaxation training and biofeedback [32–34].

Understanding the rhythm of breathing: so near yet so far

Nevner mange interessante prinsipper om pusten og hvordan dens rytmiske egenskaper regulerer kroppsfunksjoner.

Understanding the mechanisms leading from DNA to molecules to neurons to networks to behavior is a major goal for neuroscience, but largely out of reach for many fundamental and interesting behaviors. The neural control of breathing may be a rare exception, presenting a unique opportunity to understand how the nervous system functions normally, how it balances inherent robustness with a highly regulated lability, how it adapts to rapidly and slowly changing conditions, and how particular dysfunctions result in disease. Why can we assert this? First and foremost, the functions of breathing are clearly definable, starting with its regulatory job of maintaining blood (and brain) O2, CO2 and pH; failure is not an option. Breathing is also an essential component of many vocal and emotive behaviors including, e.g., crying, laughing, singing, and sniffing, and must be coordinated with such vital behaviors as suckling and swallowing, even at birth. Second, the regulated variables, O2, CO2 and pH (and temperature in non-primate mammals), are continuous and are readily and precisely quantifiable, as is ventilation itself along with the underlying rhythmic motor activity, i.e., respiratory muscle EMGs. Third, we breathe all the time, except for short breaks as during breath-holding (which can be especially long in diving or hibernating mammals) or sleep apnea. Mammals (including humans) breathe in all behavioral states, e.g., sleep-wake, rest, exercise, panic, or fear, during anesthesia and even following decerebration. Moreover, essential aspects of the neural mechanisms driving breathing, including rhythmicity, are present at levels of reduction down to a medullary slice. Fourth, the relevant circuits exhibit a remarkable combination of extraordinary reliability, starting ex utero with the first air breath – intermittent breathing movements actually start in utero during the third trimester – and continuing for as many as ~109 breaths, as well as considerable lability, responding rapidly (in less than one second) and with considerable precision, over an order of magnitude in metabolic demand for O2 (~0.25 to ~5 liters of O2/min). Breathing does indeed persist! Finally, breathing is genetically determined to work at birth, with a well-defined developmental program underlying a neuroanatomical organization with apparent segregation of function, i.e., rhythmogenesis is separate from motor pattern (burst shape and coordination) generation. Importantly, single human gene mutations can affect breathing, and several neurodegenerative disorders compromise breathing by direct effects on brainstem respiratory circuits (See below).


Evolution of Air Breathing: Oxygen Homeostasis and the Transitions from Water to Land and Sky

Omfattende studie som beskriver hvordan vi har tilpasset oss høyere nivåer av oksygen. Bekrefter alle innspill jeg har hatt om oksygen sin destruktive effekt og at beskyttelsen mot oksygenets skadevirkninger er viktigere enn å få mer oksygen inn i kroppen. Lunger, sirkulasjonssystem, hemoglobin, antioksidantsystem og det at mitokondriene er godt gjemt inni en annen celle som er godt beskyttet av en tett cellevegg er forsvars- og reguleringsmekanismer mot det livsfarlige men også livsnødvendige oksygenet.

Nevner at den opprinnelige atmosfæren bestod av veldig lite O2(1-2% eller 2,4 mmHg) og mer enn dobbelt så mye CO2. Dette er miljøet mitokondriene ble utviklet i for 2,7 billioner år siden, og som de fortsatt lever i inni cellene våre. Om oksygennivået økes tilmer enn dette blir mitokondriene dårligere og mister sin funksjon.

Nevner også at forsvarsmekanismene mot oksygen var tilstede helt fra starten. Og hemoglobin (blodcelle i dyr) og klorofyll (i planter) tilfredstiller alle de nødvendige beskyttende egenskapene vi trenger mot oksygen.

Nevner at CO2 var den første antioksidanten i evolusjonen.

Beskriver også det som skjer i mitokondria, at hypoxi (lavere O2 tilgjengelighet) gir mindre ROS og økt mitochondrial uncoupling (produksjon av varme istedet for ATP). Vi kan se dette som om mitokondriene går på tomgang med lavt turtall, mens ATP produksjon er høyt turtall og dermed også mer slitasje.

Nevner også evolusjonen av diafragma som den primære pustemuskelen.

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the “oxygen cascade”—step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism’s lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.


Oxygen, a vital gas and a lethal toxin, represents a trade-off with which all organisms have had a conflicted relationship. While aerobic respiration is essential for efficient metabolic energy production, a prerequisite for complex organisms, cumulative cellular oxygen stress has also made senescence and death inevitable. Harnessing the energy from oxidative phosphorylation while minimizing cellular stress and damage is an eternal struggle transcending specific organ systems or species, a conflict that shaped an assortment of gas-exchange systems.

The respiratory organ is the “gatekeeper” that determines the amount of oxygen available for distribution. Gas exchangers arose as simple air-blood diffusion interfaces that in active animals progressively gained in complexity in coordination with the cardiovascular system, leading to serial “step-downs” of oxygen tension to maintain homeostasis between uptake distribution and cellular protection.

While a comprehensive treatment of the evolutionary physiology of respiration is beyond the scope of any one article, here we focuses on the first step of the oxygen cascade—convection and diffusion in the gas-exchange organ—to provide an overview of the diversity of nature’s “solutions” to the dilemma of acquiring enough but not too much oxygen from the environment.

Ubiquity of Reactive Oxygen Species

As reviewed by Lane (407) and Maina (466), the primary atmosphere contained mainly nitrogen, carbon dioxide, and water vapor. Much of these were swept away by meteorite bombardment and replaced with a secondary atmosphere (416-418, 579, 590) consisting of hydrogen sulfide, cyanide, carbon monoxide, carbon doxide, methane, and more water vapor from volcanic eruptions. Only trace oxygen (<0.01% present atmospheric level) existed (418), originated from inorganic (photolysis and peroxy hydrolysis) (622) and organic (photosynthesis) sources.

Oxidative respiration is the reverse process as O2 accepts four electrons successively to form water. Many of these steps are catalyzed by transitional metal ions (e.g., iron, copper, and magnesium). Therefore, aerobic respiration, oxygen toxicity and radiation poisoning represent equivalent forms of oxidative stress (407).

Origin of Oxygen Sensing and Antioxidation—Metalloproteins

If oxidative stress was present from the beginning, early anaerobic organisms must have possessed effective antioxidant defenses, including mechanism(s) for controlled O2 sensing, storage, transport, and release as well as pathways for neutralizing ROS. The general class of compounds that fulfill these requirements is the metalloproteins that transfer electrons via transitional metals (766, 767), for example, heme proteins and chlorophyll (Fig. 2).

Hydrogen may have been the first electron donor and CO2 the first electron acceptor for synthesizing ATP by chemiosmosis (408).

Because of a high redox potential of O2 as the terminal electron acceptor in electron transport, aerobic respiration is far more efficient in energy production (36 moles of ATP per mole of glucose) than anaerobic respiration (~5 moles). Aerobic multicellular organisms arose approximately 1 Ga and more complex organisms such as marine molluscs thrived approximately 550 to 500 million years ago (Ma). Exposed to a still low O2 tension in the deep sea, these organisms uniformly possessed metalloprotein respiratory pigments with a characteristically high O2 affinity for efficient O2 storage and slow O2 release thereby avoiding flooding the cell with excessive ROS (783). Contemporary myoglobin continues to perform this regulatory function in muscle.

It is well recognized that embryos and undifferentiated cells grow better in a hypoxia (129, 153). A low O2 tension (1%-5%) is an important component of the embryonic and mesenchymal stem cell “niche” that maintains stem cell properties, minimizes oxidative stress, prevents chromosomal abnormalities, improves clonal survival, and perpetuates the undifferentiated characteristics (457). In addition, hypoxia stimulates endothelial cell proliferation, migration, tubulogenesis, and stress resistance (752, 850) as well as preferential growth and vascularization of many malignant tumor cells; the latter observation constitutes the basis for the use of adjuvant hyperoxia to enhance tumor killing during irradiation and chemotherapy (277,738). Collectively, these responses to O2 tension suggest that the pulmonary gas-exchange organs adapted in a direction toward controlled restriction of cellular exposure to O2.

Origin of the Oxygen Cascade

The oxygen cascade (Fig. 6) describes serial step-downs of O2 tension from ambient air through successive resistances across the pulmonary, cardiac, macrovascular and microvascular systems into the cell and mitochondria. These resistances adapt in a coordinated fashion in response to changes in ambient O2 availability or utilization (333). Traditional paradigm holds that the primary selection pressure in the evolution of O2 transport systems is the efficiency of O2 delivery to meet cellular metabolic demands. If this is the sole function of the cascade, why are there so many resistances? Once we accept the anaerobic origin of eukaryotes and their persistent preference for hypoxia, an alternative paradigm becomes plausible, namely, the entire oxygen cascade could be viewed as an elaborate gate-keeping mechanism the major function of which is to balance cellular O2 delivery against oxidative damage.

Mitochondria consume the majority of cellular O2, directly control intracellular O2 tension, and generate most of the cellular ROS (136). Intracellular O2 tension in turn regulates mitochondrial oxidative phosphorylation, ROS production, cell signaling, and gene expression. Via O2-dependent oxidative phosphorylation the mitochondria act as cellular O2 sensors in the regulation of diverse responses from local blood flow to electric activity (830). Earlier studies reported that hypoxia increases mitochondrial ROS generation (126, 782, 823) via several mechanisms: (i) O2 limitation at the terminal complex IV (cytochrome c oxidase) in the mitochondrial electron transport chain causes electrons to back up the chain with increased electron leak to form superoxide (•O2−). (ii) Hypoxia induces conformational changes in complex III (ubiquinol cytochrome c oxidoreductase) to enhance superoxide formation (88, 287). (iii) Oxidized cytochrome c scavenges superoxide (722). Hypoxia-induced O2 limitation at complex IV leads to cytochrome c reduction, limiting its ability to scavenge superoxide and enhancing mitochondrial ROS leakage. However, recent studies of isolated mitochondria show that hypoxia actually reduces mitochondrial ROS generation and causes mitochondrial uncoupling, suggesting extramitochondrial sources of ROS generation in hypoxia (330). These conflicting reports remain to be resolved. Nonetheless, moderate hypoxia rapidly and reversibly downregulates mitochondrial enzyme transcripts, in parallel with reductions in mitochondrial respiratory activity and O2 consumption (631).

As paleo-atmospheric O2 concentration increased and multicellular aerobic organisms arose, the endosymbiotic mitochondria-host relationship faced the challenge of balancing conflicting needs of aerobic energy generation for the host cell and anaerobic protection for its internal power generator. The host cell must finely control a constant supply of O2 to the mitochondria for oxidative phosphorylation while simultaneously protecting mitochondria against oxidative damage by maintaining a near-anoxic level of local O2 concentration. This trade-off may have led to the evolution of ever more elaborate physicochemical barriers that created and maintained successive O2 partial pressure gradients, by convection and diffusion in the lung, chemical binding to hemoglobin, distribution and release via cardiovascular delivery, dissociation from hemoglobin, and diffusion into peripheral cells with or without myoglobin facilitated transport. As a result, the primordial anoxic conditions of the Earth necessary for survival and optimal function of this proteobacterial remnant are preserved inside the host cell. In working human leg muscle O2 tension at the sarcoplasmic and mitochondrial boundaries has been estimated at approximately 2.4 mmHg (0.32 kPa) (835) and muscle mitochondrial O2 concentration at half-maximal metabolic rate 0.02 to 0.2 mmHg (834), that is, in the range of the ancient atmospheric level approximately 2 Ga. Raising O2 tension above these levels impairs mitochondrial activity (672). In this context, protection of mitochondria from O2 exposure likely constitutes a major selection factor in the evolution of complex aerobic life while the various forms of systemic O2 delivery systems are necessary but secondary functions that sustain the “gate-keeping” barrier apparatus to maintain adequate partial pressure gradients along the O2 transport cascade and preserve the near-anoxic intracellular conditions for the mitochondria. In parallel with physical barriers, cells also developed various biochemical scavenging and antioxidant pathways to counteract the toxic effects of ROS as ambient oxygenation increased.

Defense against the Dark Arts of Oxidation

To summarize, the evolution of life on Earth has adapted to a wide range of ambient O2 levels from 0% to 35%. Periods of relative hyperoxia promote biodiversity and gigantism but incur excess oxidative stress and mandating the upregulation of antioxidant defenses. Periods of relative hypoxia promote coordinated conservation of resources and downregulation of metabolic capacities to improve energy efficiency and channel some savings into compensatory growth of gas-exchange organs. The trajectory of early evolution is at least partly coupled to O2 content of the atmosphere and the deep ocean, and there is a plausible explanation for the coupling, namely, defense against the dark arts of oxidation. Oxygen is capable of giving and taking life. The anaerobic proteobacteria escaped the fate of annihilation by taking refuge inside another cell and in a brilliant evolutionary move coopted its own oxygen-detoxifying machinery to provide essential sustenance for the host cell in return for nourishment and physical protection from oxidation. As the threat of oxidation increased with rising environmental O2 concentration, selection pressure escalated for ever more sophisticated defense mechanisms against oxidative injury and in direct conflict with simultaneously escalating selection pressures to harness the energetic advantage of oxidative phosphorylation.

Trading off the above opposing demands shaped all known respiratory organs, from simple O2 diffusion across cell membranes to facilitated transport via O2 binding proteins to gas-exchange systems of varying complexity (skin, gills, tracheae, book lung, alveolar lung, and avian lung) (Sections 2-5). Concurrently evolving with a convective transport system, these increasingly elaborate respiratory organs not only increase O2 uptake but also maintain air-to-mitochondria O2 tension gradients and intracellular O2 fluxes at a hospitable ancestral level. This epic struggle began at the dawn of life and persisted to the present on a universal scale. The evolutionary trajectory of air breathing has continued contemporary significance to the understanding of oxygen-dependent metabolic homeostasis, especially in relation to maturation, senescence, and aging-related organ degeneration and disease.