Affinity for oxygen and carbon dioxide in animals

Affinity for oxygen and carbon dioxide in animals

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I'm currently reading Treatise on Invertebrate Paleontology (A) and don't understand the following fragment:

The affinity for oxygen in lower animals is many times that in higher ones, whereas conditions are the reverse with regard to carbon dioxide.

What does it mean by "affinity"? Affinity of hemoglobins (in blood)? Affinity of oxygen-transporting molecules, whatever they are? Something else? The book doesn't provide a source of its claim here.

EDIT: @Will - I'm more interested in what kind of affinity they are speaking about in regards to carbon dioxide (I can't comment due to not having enough reputation yet.)

Transport of Carbon Dioxide in the Blood

Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Several properties of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body.

CO 2 + H 2 O ↔ H 2 CO 3 (carbonic acid) ↔ HCO 3 + H + (bicarbonate)

The benefit of the bicarbonate buffer system is that carbon dioxide is “soaked up” into the blood with little change to the pH of the system. This is important because it takes only a small change in the overall pH of the body for severe injury or death to result. The presence of this bicarbonate buffer system also allows for people to travel and live at high altitudes: When the partial pressure of oxygen and carbon dioxide change at high altitudes, the bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body.

Outline-4, BIO 3360, Respiration V – Oxygen and Carbon Dioxide Transport in Blood

—Affinity is the oxygen being attracted to hemoglobin and once they attach to each other, we use the term saturation and say that hemoglobin is saturated with oxygen. Oxygen will saturate hemoglobin where oxygen levels are high – such as in the lungs.

—Dissociation is oxygen pulling away from the hemoglobin and that way it is not attached and free to go into a tissue that requires oxygen. Oxygen will dissociate from hemoglobin in tissues were oxygen levels are low – such as an exercising leg muscle.

1. Partial pressure of oxygen is the greatest determining factor on oxygen and hemoglobin dissociation (letting go of each other ) or saturation (attaching to each other)

2. Tissues needing the oxygen get it, while oxygen and hemoglobin attach where tissues do not need the oxygen

3. Shifting the curve to the left –higher affinity between oxygen and hemoglobin higher pH, lower carbon dioxide level, lower temperature

4. Bohr Effect: Shifting the curve to the right –lower affinity between oxygen and hemoglobin lower pH, higher carbon dioxide levels, higher temperature, higher levels of metabolic byproducts (such as DPG)

5. Root Effect:increase in carbon dioxide plus decrease in pH not only cause a Bohr Effect but also the Root Effect when there is a reduction in the oxygen carrying capacity of the respiratory pigment (hemoglobin). This can release oxygen into solution and is the mechanism important in filling swim bladders with oxygen. Seen in some fish, cephalopods and crustaceans

II. Carbon Dioxide Transport in Body Fluids

A. Carbon dioxide diffuses from an area of higher pressure to lower pressure, but pressure gradient is not as great as with oxygen. Frogs compensate for this by having a huge skin surface area for carbon dioxide exchange.

70%) carried as bicarbonate ions in the plasma (some free in plasma, and some attached to the globin portion of hemoglobin)

CO 2 +H 2O <–> H 2CO3 <–> H + + HCO3

Enzyme for the formation of carbonic acid is carbonic anhydrase.

D. Carbon Dioxide equilibrium curve –rapid increase in CO2 content at relatively low PCO2in blood and a continued, but slower increase as PCO2 rises. Blood does not become saturated with CO2as it does with O2

E. Haldane Effect –Deoxygenated blood can carry more CO2 than oxygenated blood. The Haldane effect is that deoxygenation of hemoglobin at the tissues promotes CO2 uptake by the blood whereas oxygenation of the hemoglobin at the respiratory surface promotes CO2 unloading. Deoxygenated hemoglobin pushes the equation shown above in B. to the right and increases the amount of carbon dioxide that can be carried in the form of bicarbonate ions. Oxygenated hemoglobin pushes the equation shown above in B. to the left so that carbon dioxide is produced and may be exhaled.

Factors That Affect Oxygen Binding

The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to ( ext

_ ext_2), other environmental factors and diseases can affect oxygen carrying capacity and delivery.

Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity (Figure (PageIndex<2>)). When carbon dioxide is in the blood, it reacts with water to form bicarbonate (( ext_3^-)) and hydrogen ions (H + ). As the level of carbon dioxide in the blood increases, more H + is produced and the pH decreases. This increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.

Diseases like sickle cell anemia and thalassemia decrease the blood&rsquos ability to deliver oxygen to tissues and its oxygen-carrying capacity. In sickle cell anemia , the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen (Figure (PageIndex<3>)). In this form, red blood cells cannot pass through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygen-carrying capacity is diminished.

Figure (PageIndex<3>): Individuals with sickle cell anemia have crescent-shaped red blood cells. (credit: modification of work by Ed Uthman scale-bar data from Matt Russell)


Through the Bohr effect, more oxygen is released to those tissues with higher carbon dioxide concentrations. The sensitivity to these effects can be suppressed in chronic diseases, leading to decreased oxygenation of peripheral tissues. Chronic conditions such as asthma, cystic fibrosis, or even diabetes mellitus can lead to a chronic state of hyperventilation to maintain adequate tissue oxygenation. These states can have ventilation of up to 15 L per minute compared to the average normal minute ventilation of 6 L per minute. This hyperventilation minimizes the potential of the Bohr effect through excess exhalation of carbon dioxide resulting in hypocapnia, causing a left shift the oxygen dissociation and unnecessarily increased oxygen-hemoglobin binding affinity with impaired oxygen release to peripheral tissues, including our most vital organs (brain, heart, liver, kidney). Thus, the Bohr effect is essential in maximizing oxygen transport capabilities of hemoglobin and functionally dynamic oxygen-binding/release secondary to carbon dioxide equilibrium. [5]

Carbon Monoxide Effect

While the presence of carbon dioxide leads to the greater unloading of oxygen, carbon monoxide has the opposite effect. Carbon monoxide (CO) has a 200-times greater affinity for hemoglobin than oxygen, out-competing oxygen for available binding sites in a nearly irreversible fashion (reversible, but very minimally). Carbon monoxide further decreases oxygen delivery through the stabilization of hemoglobin in the R-form. Counter-intuitively, although this facilitates oxygen loading to the remaining binding sites, hemoglobin becomes resistant to environmental influences that would normally encourage conformational changes into taut-form, limiting the potential for unloading of oxygen. Under the influence of carbon monoxide, the oxy-hemoglobin dissociation curve significantly shifts left in addition to the reduction of the sigmoidal curve shape as a result of blunted positive cooperativity response of hemoglobin. In the presence of significant carbon monoxide inhalation, tissue hypoxia occurs despite normal pO2 levels, as carbon monoxide competitively binds hemoglobin while inhibiting the release of oxygen from the remaining binding sites. Carbon monoxide poisoning is treated with hyperbaric oxygen therapy, delivering 100% O2 at increased atmospheric pressures to facilitate hemoglobin oxygen binding in the presence of highly competitive carbon monoxide.[6]

Double Bohr effect is seen in the fetus. In the placenta, maternal and fetal circulation meets. The umbilical arteries carry de-oxygenated blood with high CO2 content from the fetus to the placenta. In the placenta, CO2 from fetal blood diffuses into maternal blood down its concentration gradient. As CO2 content of fetal blood decrease, this makes fetal blood relatively alkaline and shift the oxygen dissociation curve toward left, facilitating more oxygen uptake by fetal Hb.On the maternal side, this CO2 diffusion from the fetal side makes maternal blood in the placenta more acidic. This shift ODC towards the right and more oxygen is released from maternal Hb. Thus in the placenta, the Bohr effect occurs twice, one on the fetal side and another on the maternal side. This is known as the double Bohr effect. The clinical significance of the double Bohr effect is that it facilitating oxygen transfer across the placenta from mother to fetus and thus increase fetal oxygenation. Fetal Hb also has more affinity for oxygen than adult Hb. P50 ( partial pressure at which the hemoglobin molecule is half saturated with O2) for fetal Hb is 19 whereas P50 of adult Hb is 27. This Low P50 of fetal Hb also favors more oxygen transfer to the fetus.

For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist and gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which “breathe” through diffusion across the outer membrane. The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen.

This flatworm’s process of respiration works by diffusion across the outer membrane. (credit: Stephen Childs)

Oxygen and carbon dioxide transport in the blood of the muskrat (Ondatra zibethica)

We have investigated the oxygen and carbon dioxide transport properties of a small diving mammal, the muskrat (Ondatra zibethica), where the hemoglobin primary structure has been established by Duffy et al. (1978). While whole blood oxygen capacity, the Haldane effect and the buffer capacity are not different compared to non-diving mammals of similar size, the Bohr effect and the oxygen affinity are increased. The oxygen half saturation pressure (P50) was 26.1 mm Hg (3.5 kPa) at pH 7.4, and the Bohr effect -0.66 (related to plasma pH) and -1.07 related to cell pH. The high affinity of muskrat blood is caused by a comparatively small effect of 2,3 DPG and CO2 on muskrat hemoglobin, that is accentuated through a relatively low concentration of 2,3-DPG in the muskrat red cell. The increased Bohr effect is caused primarily through the pronounced pH dependence of oxygen-linked binding of 2,3-DPG. The weak interaction of muskrat hemoglobin with 2,3-DPG is not caused by substitutions at the binding site.

How Carbon Dioxide and Oxygen is Transported through Blood?

In normal condition an animal body requires enormous amount of oxygen to satisfy the demands of the energy yielding oxidative phosphorylation reactions. Various respiratory pigments serve to carry the necessary oxygen from the atmosphere to different parts of the body. Of the several pig­ments, haemoglobin is the most efficient in transporting respiratory gases.

Haemoglobin molecule has high affinity for oxygen and the attachment of oxygen to haeme is readily reversible. Here transport mechanism of oxygen and carbon dioxide of mammalian system will be discussed. In the alveoli of lungs, haemoglobin comes to direct contact with a rich supply of oxygen (100 torr) and is converted to oxyhaemoglobin.

These oxyhaemoglobin is carried by the arterial circulation to the cells in which there is a low oxygen concentration (40 torr) and a relatively high concentration of carbon dioxide (60 torr) prevails. The oxygen is given up to these cells the resulting haemoglobin carries some of the carbon dioxide back to the lungs to be expelled, and more oxyhaemoglobin is formed.

Oxyhaemoglobin does not transfer all of its oxygen to the tissue cells. In human, normally, every 100 ml of arterial blood combines with about 19 ml of oxygen. In the resting individual, the venous blood carries about 12 ml of oxygen per 100 ml of blood.

Therefore, more than 60% of the haemo­globin in venous blood is still remaining combined with oxygen. If a person is engaged in strenuous exercise, his oxygen demand becomes high. In this case the per­centage of oxyhaemoglobin in the venous blood may fall as low as 25%.

Atmospheric oxygen is taken into the lungs, where a difference in pressure exists between the alveoli and the capillaries (Fig. 8.6). Different steps of O2 and CO2 trans­port in lungs and cells are represented in Fig. 8.6 as 1 to 17. These steps are described here. Oxygen diffuses into the red blood cell (1), where it combines with haemoglobin (HHb), to form oxyhaemoglobin (HbO − 2)(2):

Two factors tend to shift this equilibrium to the right.

These factors are:

(1) High oxy­gen concentration in the lungs,

(2) The neutralization of hydrogen ions by bicarbo­nate ions present in the RBC

(3) The oxy­haemoglobin is then carried to the tissues, where CO2 is being produced as a result of cellular metabolism

(4) The CO2 diffuses into the RBC

(5) Where the enzyme carbonic anhydrase catalyses its combination with water to form carbonic acid

(6) Carbonic acid subsequently dissociates into bicarbonate ions and hydrogen ions (7):

The major factor that shifts this equili­brium to the right is the neutralisation of the hydrogen ions by oxyhaemoglobin

(8) Which is concomitantly split into oxygen and haemoglobin. Because the oxygen pressure is greater in the capillaries than in the tissues, the oxygen diffuses into the tissue cells

(9) To be utilized in the oxidative reactions of metabolism. As the bicarbonate ion concentration increases, the ions diffuse out of the RBC into the plasma

(10) The loss of negative ions by the cell is then balanced by the migration of chloride ions into the cell from the plasma. The process, referred to as the chloride shift

(11) Brings about a re-establishment of electrolyte equilibrium. Most of the carbon dioxide is taken to the lungs as bicarbonate ions (

90%), but some combines with haemoglobin

(12) This compound is formed with free amino groups in the globin portion of the haemoglobin and referred to as carbaminohaemoglobin. When the RBCs return to the lungs carbaminohaemoglobin breaks to haemoglobin and CO2

(13) Chloride ions which entered during chloride shift return to the plasma

(14) Bicarbonate ions reduced to carbonic acids

(15) Carbonic acids broken down to H2O and CO2 by the activity of car­bonic anhydrase

(16) These CO2 are trans­ferred to the lungs

(17) To be expelled and the cyclic process repeats itself.

Some Details of the Gaseous Transport:

1. Conformational changes of haemo­globin apoprotein with oxygenation:

Each tetramer of the haemoglobin binds four oxygen molecules (one oxygen molecule in one subunit of haeme). The oxygen satu­ration curves are sigmoidal in nature (Fig. 8.7). Binding of O2 is accompanied by the rupture of all four subunits (Fig. 8.8). Subsequent oxygen binding is facilitated, since it involves rupture of fewer salt bonds.

This is why O2 binds to haemoglobin depends on whether other O2 molecules are present on the same tetramer. If O2 is already present, binding of subsequent O2 molecules occur more readily. This property is known as cooperative bind­ing kinetics of haemoglobin oxygenation. This property permits it to bind maximum quantity of oxygen at the respiratory organ and to deliver maximum at peripheral tissues.

With the rupture of salt bonds a pro­found change alter hemoglobin’s secondary, tertiary arid quaternary structures. One pair of α/β subunits rotate with respect to the other α/β pair, compacting the tetramer and increasing the affinity of the haemes for oxygens (Figs. 8.9 and 8.10).

The quaternary structure of partially oxygenated haemoglobin is described as the T (taut) state and that of oxygenated haemo­globin (HbO2) as the R (relaxed) state (Fig. 8.11).

De-Oxygenation and Changes in Haemoglobin: Bohr and Haldane Effect:

Haemoglobin can bind with carbon dioxide (CO2) directly when oxygen is released from it. About 15% of the CO2 carried in blood, is carried directly by the haemoglobin molecule. CO2 reacts with the amino terminal a-amino groups of the haemoglobin, forming a carbamate and releasing protons that contribute to the Bohr Effect:

Conversion of the amino terminal from positive to a negative charge favours salt bridge formation between the α and β chains. At the lungs, oxygenation of haemoglobin is accompanied by expulsion and subsequent expiration of CO2. As CO2 is absorbed in blood, the carbonic anhydrase of erythrocytes catalyzes the formation of car­bonic acid:

Carbonic acid rapidly dissociates into bicarbonate and a proton. This proton tends to increase acidity of the blood. To avoid increasing acidity of blood, there is a buffe­ring system, which absorbs these excess protons. Haemoglobin binds two protons for every four oxygen molecules released and thus contributes significantly to the buffering capacity of blood (Fig. 8.12).

In the lungs, the process is reversed, i.e., as oxygen binds to deoxygenated haemo­globin, protons are released and combine with bicarbonate, forming carbonic acid. Carbonic anhydrase releases CO2 from carbo­nic acid for exhalation. Thus, the binding of oxygen forces the exhalation of CO2.

This reversible phenomenon is called the Bohr and Haldane effect. This effect is a property of tetrameric haemoglobin and is dependent upon its haeme-haeme interaction or coope­rative effects.

Chloride Shift or Hamburger’s Phenomenon:

Chloride ions from the plasma enter the RBC when CO2 enters blood, while the base (Na) is left behind. When CO2 escapes from blood, chloride ions leave the cells enter the plasma and combines with base sodium again. Due to this alternate movement of chloride ion, this phenomenon is called chlo­ride shift or Hamburger’s phenomenon (Fig. 8.13).

The membrane of the RBCs is not per­meable to basic ions (K + , Na + , etc.), but is permeable to anions (HCO – 3, Cl − , etc.). When CO2 enters the blood stream in the tissue capi­llaries, very little H2CO3 is formed in plasma but largely in the RBCs, because red cells are rich in carbonic anhydrase which is absent in plasma. H2CO3 reacts with KHb in the RBC, producing KHCO3 and HHb.

Thus the bicar­bonate content of the RBC increases and thereby the reactions of the cells tend to become alkaline. To maintain a constant pH, the chloride ion (CI − ) of NaCl from plasma enters the RBC and combines with KHCO3 forming KCl. The free HCO − 3 ion now tends to make the cell reaction acidic.

This is pre­vented by the migration of HCO − 3 from the RBC into the plasma. It combines with Na + of NaCl, which is left free by the shift of chlo­ride and forms NaHCO3 in the plasma. All these changes take place in the tissue capi­llaries. Due to this reaction, large amount of Na + of NaCl is made available for CO2 carriage.

In the lungs, these changes are reversed. Chloride comes out of the cells, reacts with NaHCO3 of plasma, forming NaCl and car­bonic acid. Carbonic acid thus liberated passes out through the lungs. In the tissue capillaries, chloride shifts from plasma to cells. Thereby, the osmotic pressure of the cell will rise, water will be drawn in and cell volume will increase. In the pulmonary capi­llaries, chloride shifts from the cells back to the plasma, and this will reduce the osmotic pressure of the cell and the cell will shrink.

Temperature dependence of haemoglobin–oxygen affinity in heterothermic vertebrates: mechanisms and biological significance

As demonstrated by August Krogh et al. a century ago, the oxygen-binding reaction of vertebrate haemoglobin is cooperative (described by sigmoid O2 equilibrium curves) and modulated by CO2 and protons (lowered pH) that – in conjunction with later discovered allosteric effectors (chloride, lactate and organic phosphate anions) – enhance O2 unloading from blood in relatively acidic and oxygen-poor tissues. Based on the exothermic nature of the oxygenation of the haem groups, haemoglobin–O2 affinity also decreases with rising temperature. This thermal sensitivity favours oxygen unloading in warm working muscles, but may become detrimental in regionally heterothermic animals, for example in cold-tolerant birds and mammals and warm-bodied fish, where it may perturb the balance between O2 unloading and O2 requirement in organs with substantially different temperatures than at the respiratory organs and thus commonly is reduced or obliterated. Given that the oxygenation of haemoglobin is linked with the endothermic release of allosteric effectors, increased effector interaction is an effective strategy that is widely exploited to achieve adaptive reductions in the temperature dependence of blood-O2 affinity. The molecular mechanisms implicated in heterothermic vertebrates from different taxonomic groups reveal remarkable variability, both as regards the effectors implicated (protons in tunas, organic phosphates in sharks and billfish, chloride ions in ruminants and chloride and phosphate anions in the extinct woolly mammoth, etc.) and binding sites for the same effectors, indicating multiple evolutionary origins, but convergent physiological functionality (reductions in temperature dependence of O2-binding affinity that safeguard tissue O2 supply).

Clinical Significance

A thorough history should be taken to gain an understanding of any factors that may have precipitated signs and symptoms of hypercapnia. Patients with hypercapnia can present with tachycardia, dyspnea, flushed skin, confusion, headaches, and dizziness. If the hypercapnia develops gradually over time, symptoms may be mild or may not be present at all. Other cases of hypercapnia may be more severe and lead to respiratory failure. In these cases, symptoms such as seizures, papilledema, depression, and muscle twitches can be seen. If a patient with COPD presents with signs and symptoms of hypercapnia, immediate medical attention should be attained before CO2 reaches life-threatening levels.[12][13]

Hypercapnia should be managed by addressing its underlying cause. A noninvasive positive pressure ventilator may provide support to patients who have inadequate respiratory drive. If a noninvasive ventilator is not efficient, intubation may be indicated. Bronchodilators may also be used in patients suffering from an obstructive airway disease.

In recent studies, the use of the esophageal balloon in managing hypercapnia in a patient with acute respiratory distress syndrome was also shown to be effective.

Watch the video: Bohr effect vs. Haldane effect. Human anatomy and physiology. Health u0026 Medicine. Khan Academy (September 2022).


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