oxygen transport in blood - 22 and 23

Amyl nitrite

administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of cyanide and the formation of non-toxic cyanomethemoglobin

Polycythemia and anemia

In polycythemia, *total O2 will increase* but dissolved O2 will remain the same (so PO2 will not be affected) In anemia, *total O2 will decrease* but dissolved O2 will remain the same (so PO2 will not be affected)

Remember 2,3 BPG from Biochem

*Cell metabolism increases 2,3 BPG*

Myoglobin pathologies

*Damaged muscle tissue (rhabdomyolysis), *Released myoglobin (filtered by the kidneys) causes acute renal failure. *Myoglobin is a sensitive marker for muscle injury (can be marker for heart attack in patients with chest pain). *Myoglobin has low specificity for acute myocardial infarction (AMI) and thus CK-MB, cTnT, ECG, and clinical signs should of course be taken into account to make the diagnosis

Methemoglobin

*Form of the oxygen-carrying protein hemoglobin, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. *Normally only 1% in normal person *Methemoglobin cannot carry oxygen. It is a bluish chocolate-brown in color (term cyanosis-blue color). *The NADH-dependent enzyme methemoglobin reductase is responsible for converting methemoglobin back to hemoglobin.

Hemoglobin

- Oxygen transport protein in the RBC's (Life depends on it!) - Blood chemistry affects hemoglobin oxygen binding properties - Oxygen is NOT very soluble, so we bind it to hemoglobin to transport it through the bloodstream. - Has 4 chains, each chain can bind one 02 (so each Hb can bind 4 02)

Myoglobin

- Stores oxygen in muscle for emergency use - Reserve for O2 - Higher in slow twitch muscles *Not normal in circulation (lysis of cells) can lead to kidney damage (myoglobin can damage kidneys)* - Bind 1 02

Bound O2

O2 bound to Hb =O2 capacity • gm of Hb • % sat. of Hb 1.39 ml/gm • gm of Hb • % sat. of Hb 1.39 ml/gm • 15 gm Hb/dl • 0.98 *~20.4 ml O2/dl or 204 ml O2/L*

2,3-Bisphosphoglyceric acid (2,3-Bisphosphoglycerate or 2,3-BPG) is

3 carbon isomer of the glycolytic intermediate 1,3-bisphosphoglyceric acid (1,3-BPG). 2,3-BPG is present in human red blood cells and binds with *greater affinity to deoxygenated hemoglobin than it does to oxygenated hemoglobin* It interacts with deoxygenated hemoglobin Beta subunits by decreasing their affinity for oxygen, so it *allosterically* promotes the release of the remaining oxygen molecules bound to the hemoglobin, thus enhancing the ability of RBCs to release oxygen near tissues that need it most. That's the reason why 2,3-BPG works as an allosteric effector.

sickle cell anemia

A single amino acid change on the surface of Hb β chain (Hb S) changes shape of Hb Causes Hb (S) to stick to Hb (A) and form large aggregates (clumps). *Happens more so in low pH* As Hb falls out of solution inside red blood cell cell takes on a sickle shape. Aggregated Hb is a poor O2 transporter. Sickled cells clog small capillaries, blocking blood flow in tissues.

Oxygen transport in blood in 2 forms

About 20% volume or 20 ml of O2 per 100 ml of blood (exits in two forms) *Form 1- 0.3%* - dissolved in blood is related to Pp of O2 and solubility (solubility of O2 in blood is 0.003 ml O2/100 ml blood) *Form 2- Amount carried by Hb 99.7%* - Depends on concentration and saturation - Normal Hb concentration 13-15 g/100 ml blood *Is increased polycythemia does not affect PO2 *Is decreased anemia

We want to keep in mind the function of the lungs is to take oxygen from the atmosphere as a gas and put it into a liquid environment (blood). Also, we need to take the CO2 (in the blood) from cellular metabolism and transport it back into the atmosphere. This is done by three main principles, *changes in pressure, change in concentration, and solubility.*

As we go through oxygen/C02 transport, we are really talking about gas transport. It always will occur by *diffusion and down a concentration gradient*. Some gases diffuse easier than others (smaller ones diffuse easier) some gases are more soluble than others *(CO2 is more soluble than oxygen)*. *Helium has a very low solubility* that is why when you inhale a balloon with helium you speak funny, it doesn't get into the blood and is less dense than 02 so your vocal cords vibrate differently. Also this is the basis for some dive canisters for scuba divers-we will talk about this later. Each gas exerts a partial pressure on the overall atmosphere. We hopefully learned (back in high school) that the total barometric pressure at sea level is 760 mmHg and gases each exert a partial pressure to make that total of 760 mm Hg. *once a gas is bound to a substance (like oxygen being bound to Hb) it does not exert a partial pressure anymore*

Haldane effect

Binding of O2 with Hemoglobin tends to displace carbon dioxide from the blood. This effect is called the

PO2 pressure of O2 in blood

Blood gets oxygenated in pulmonary capillaries Some blood is shunted (small % doesn't see O2) PO2 remains high till systemic capillaries

Factors that Modify Hb O2 binding affinity

Blood pH CO2 levels 2,3-bisphosphoglycerate Temperature Hb chain composition (HbF has higher affinity for O2) this helps the fetus get more oxygen from the mother.

H+ affects Hb O2 affinity

Bohr effect: pH decreases due to more H+, the more H+ present the decreased affinity O2 has for hb, so curve shifts to the right!!!! The key here is when protons are more plentiful, they bind to hemoglobin and reduce its affinity for oxygen. This is good for tissues: metabolism produces protons (lower pH), and metabolizing tissue needs more oxygen.

1. Diffusion limited (depends on Graham's law, solubility) e.g. carbon monoxide (CO)

CO forms strong bond with Hb => increases in CO content result in very minimal increase in partial pressure => partial pressure difference still exists (ie. equilibrium is not reached) when blood finishes its passage through the alveoli => transfer of CO is limited by the rate of diffusion, not the amount of blood available. *once CO crossed capillary- immediately taken up by Hb, so always Pp pressure gradient exists(

With C0 poisoning, curve shifted to left, Hb wants to really hold onto the last two 02 bound

CO poisoning, the actual amount of Hb is not reduced, but functionally there is less O2 binding sites, so the O2 carrying capacity of the blood is reduced. Note that CO participates in cooperative binding of O2. *In fact, carboxyhemoglobin causes the Hb to have an increased affinity for O2* Therefore, the O2 bound to carboxyhemoglobin is more difficult to release in the tissue. The saturation of Hb would be nearly 100%, but you do not unload any of your O2 so it is the same as having no oxygen delivered to your tissues.

Hemoglobin and the Blood Buffer System

CO2 / HCO3- move in and out based on environmental conditions Allows for transport of CO2 to lungs for expiration. Also connects cellular respiration to oxygen delivery In RBC you have lots of CA. Leads to increase HCO3- The HCO3- is transported out of cell in exchange for Cl- ≈ *80% of CO2 transported as bicarb*

Hemoglobin and CO2

CO2 affects the blood buffer system and Hb O2 binding. CO2 can bind covalently to the N-termini of Hb chains Estimated that* 15% of CO2 in blood is carried by Hb in this way* Called *Carbaminohemoglobin*

Summary

CO2 is carried in the body three ways: ≈ 5% is dissolved in plasma ≈15% is bound to the carbamino compound ≈ 80% is transported as bicarbonate (chloride shift)

O2 gas (PAO2)

Capital A=alveoli

Summary of Bohr Effect

Carbon dioxide binding to Hb diminishes oxygen binding Hydration of CO2 in tissues and extremities leads to proton production (due to carbonic anhydrase) These protons are taken up by Hb as oxygen dissociates The reverse occurs in the lungs: CO2 is low, so H+ is low, so O2 affinity is increased

Board alert 2

Carbon dioxide binds reversibly to the N-terminus of the alpha and beta globin chains forming carbamino Hb, which has a reduced affinity for O2. The binding of carbon dioxide to hemoglobin can act a pH buffer. (Because C02 in the presence of carbonic anhydrase can form bicarb and H+).

board alert 3

Carbon monoxide (CO) binds to heme groups more avidly than O2, forming carboxyhemoglobin. (Actually 200 X more). And, when bound wont release the other oxygens bound to hemoglobin.

Measuring diffusion capacity

Carbon monoxide is used because the transfer of CO is diffusion limited. DLCO = VCO/PACO *Single breath method* Single aspiration of CO mixture Measurement of CO concentration in inspired and expired gas (with infrared analyzer) 10 seconds of breath holding Helium added to mixture to measure lung volume and thus dilutional effect.

The Bohr Effect

Competition between oxygen and H+ Binding of protons diminishes oxygen binding In exercising muscle you are producing CO2→HCO3- and H+ this can help unload O2. Important physiological significance (hemoglobin can act as a physiologic buffer)

What increases RBC's?

Decreased 02 delivery to the kidneys Renal cortex produced EPO EPO induces differentiation of proerythroblasts Proerythroblats turn into RBC's *Also remember someone in kidney failure-decreased hct*

Haldane effect

Deoxygenation of the blood increases its ability to carry carbon dioxide Conversely, oxygenated blood has a reduced capacity for carbon dioxide.

Hb Subunits Change During Development

Different subunits have slightly different structure, slightly different O2 binding properties, based on developmental stage of fetus and O2 demands α always present γ major second subunit during development (HbF α2γ2), quickly replaced by b after birth (HbA α2β2). HbF has higher affinity for oxygen than HbA (this is how fetus can get O2)

Diffusion of gas Fick's law

Diffusion is directly proportional to the driving force, diffusion coefficient, and the surface area. It is inversely proportional to the thickness of the membrane.

Diffusion of gas Fick's law (2)

Driving force (partial pressure differences of the gasses) Diffusion coefficient- depends on molecular weight and solubility. Thickness of the membrane- more mucus means longer distance to travel through

Where Red blood cells come from

First couple weeks Yolk sac Middle trimester Fetal liver (also spleen/lymph nodes) Last month From the bone marrow After puberty only from rib, sternum, and vertebra RBC production is induced by production of erythropoietin by the kidney

Uptake of N2O, CO, and O2

Gases that are perfusion limited have their partial pressures equilibrated with the alveolar pressure before exiting the capillary, the Pp of CO (diffusion limited) does not reach equilibrium

Diffusing capacity

Graph shows the diffusing capacities of O2, CO2 and CO. CO2 really diffuses easily *Diffusion limited* as long as a Pp gradient is maintained diffusion will occur *Perfusion limited* Pp gradient is not maintained, only way to increase amount of gas is to increase blood flow

Board alert 1

HbF (the fetal form of hemoglobin) binds 2,3-DPG much less avidly than HbA (the adult form of hemoglobin) with the result that HbF in fetuses of pregnant women binds oxygen with greater affinity than the mother's HbA, thus giving the fetus preferential access to oxygen carried by the mother's circulatory system.

Hemoglobin (Hb) and Myoglobin

Hemoglobin and myoglobin function very differently as oxygen transport and storage proteins Compare the oxygen binding curves for hemoglobin and myoglobin (next slide) Myoglobin is monomeric Hemoglobin is tetrameric (Hb: two "α" chains of 141 residues, 2 "β" chains of 146) Mb: 153 aa, 17,200 MW (Hb about 4X bigger

Final Summary

Hemoglobin has a sigmoid O2 binding curve Binding of O2 to Hb can be altered by different factors Hb can be altered by genetics (e.g. sickle cell) Oxygen is transported bound and unbound in the body and is dependent on partial pressures Total oxygen in the body is a sum of bound and unbound Carbon monoxide is diffusion limited while oxygen is perfusion limited-there are consequences to both

anemia, polycythemia, CO poisoning

In anemia, Hb is saturation is normal but arterial but arterial blood oxygen content is depressed because of the reduced concentration of Hb In polycythemia, arterial blood concentration is above normal because of increased Hb. In CO poisoning, arterial PO2 is normal but O2 saturation of Hb is decreased.

Surface area of the lungs can increase or decrease, we can recruit more of the lungs with greater blood flow (cardiac output) we breathe deeper.

In emphysema, the surface of the lungs is increased but the "springyness (aka they become too compliant)" of the lungs is compromised so we do not generate the proper pressure gradients for gases to flow. Also we destroy some of the cells (type 1 pneumocytes) that allow diffusion.

2,3-Bisphosphoglycerate curve

In the absence of 2,3-BPG, oxygen binding to Hb follows a rectangular hyperbola! The sigmoid binding curve is only observed in the presence of 2,3-BPG Since 2,3-BPG binds at a site distant from the Fe where oxygen binds, it is called an allosteric effector With fetal Hb you DO NOT have 2,3 BPG acting as allosteric effector so fetal Hb has higher affinity to O2.

Chloride shift vs reverse chloride shift

In venous blood RBC's have more Cl- in cell (this is known as chloride shift) In arterial blood (oxygenated blood) RBC's have less Cl- in cell due to reverse Cl- shift

Diffusion of O2 (2)

Made up of Type I and Type II pnuemocytes Type I cells gas exchange surface of the alveoli. Type II cells proliferate and differentiate into Type I cells to restore the damaged barrier. Type II produce surfactant: Type I more susceptible to toxins

Measuring diffusion capacity take home

Major point here is CO is diffusion limited-used to test fibrosis (increased barriers etc), since once it diffuses though lungs it binds Hb with great affinity. O2 is perfusion limited-depends on Cardiac output. (Solubility is quickly reached and saturation of Hb is quickly reached)

CO2 Transport

Most of the CO2 diffuses into RBC. RBC has high carbonic anhydrase Convert CO2 and H20 to bicarb (neg charge) Bicarb goes out of cell (neg ion) exchange for Cl- (neg ion) *Called chloride shift* At the lung the opposite takes place *(called reverse chloride shift)* so have Cl efflux the RBC

Why do we have hemoglobin?

Myoglobin is found in skeletal muscles. Hemoglobin in RBC's. O2 is pretty insoluble, so we need something to carry it in the bloodstream. If we just relied on the dissolved oxygen to meet all our needs we will run out pretty quickly since the solubilty of 02 is so low. Therefore we have hemoglobin.

Uptake of N2O, CO, and O2 concept

N2O does not bind any Hb so it reaches equilibrium pretty quick, because it diffuses pretty easily so the only way to get more N2O in the blood is to remove the saturated blood (so we say N2O is perfusion limited). O2 normally is perfusion limited due to Hb getting saturated pretty quickly. Now CO is diffusion limited since once it get across it quickly binds the Hb more effectively than O2 and will never reach saturation until all the Hb is bound. In this case, you are probably dead.

2. Perfusion limited e.g. nitrous oxide (N2O)

N2O doesn't form bond with Hb => increase in N2O content results in rapid rise in partial pressure  (equilibrium within 0.075 second) => equilibrium is reached very early on => transfer of N2O is limited by the amount of blood available. *O2 only becomes diffusion limited in Fibrotic diseases and strenuous exercise*

oxygen tranport concept

Note that O2 is not much dissolved in blood. Most of O2 in body is bound to Hb. What I mean here is that if you did not have Hb you would not carry very much O2 in the body. You would quickly hit your saturation point of O2 (dissolved gas) in the blood. When it binds to Hb it does not show a partial pressure so you can have much more total O2 in the body.

summary of O2 binding

Now for increased temperature, you just have to remember when things get hotter they come apart. So when you exercise you can create heat and increase unloading O2 to working muscles.

O2 Bound to Hemoglobin (Hb)

O2 bound to Hb =Hb O2 binding capacity (1.39 ml O2/gm Hb theoretical) • gm of Hb • % sat. of Hb (15 gm Hb/dL) 99% saturated 20 ml O2/dL

Forms of O2 in Blood: (Can be dissolved or bound to hemoglobin)

O2 dissolved = PO2 • O2 solubility O2 dissolved = PO2 • 0.03 ml/(L • mm Hg) The amount of dissolved O2 in the blood is a linear function of the PO2 in the blood O2 is carried in the blood in two forms: dissolved and bound to hemoglobin (Hb). The amount of O2 that can be dissolved in aqueous solution is related to the partial pressure of O2 in the compartment, and solubility of O2 in the liquid. The amount of dissolved O2 in the blood is therefore a linear function of the PO2 in the blood. Dissolved O2 account for only about 3 ml of O2 per liter of blood at 100 mm Hg PO2.

Dissolved O2

O2 dissolved = PO2 • solubility O2 dissolved = PO2 • 0.0304 ml/(L • mm Hg) *~0.3 ml O2/dl or 3 ml O2/L plasma @ PO2=100 mm Hg*

Forms of O2 in blood concept

O2 is carried in the blood in two forms: dissolved and bound to hemoglobin (Hb). The amount of O2 that can be dissolved in aqueous solution is related to the partial pressure of O2 in the compartment, and solubility of O2 in the liquid. The amount of dissolved O2 in the blood is therefore a linear function of the PO2 in the blood. Dissolved O2 account for only about 3 ml of O2 per liter of blood at 100 mm Hg PO2.

The hemoglobin molecule contains four heme units and four globulin chains. There are four different types of globin chains: alpha, beta, gamma, and delta.

Of the four chains in a hemoglobin molecule, two are always alpha, and the other two are either beta (in hemoglobin A, the normal adult form), delta (in hemoglobin A2, a minor form), or gamma (in hemoglobin F, the fetal form).

Cyanide makes the cells of an organism unable to use oxygen, primarily through the inhibition of cytochrome c oxidase.

One final point on cyanide- we will discuss hypoxemia and hypoxia later- Cyanide poisoning leads to hypoxia but not hypoxemia. What's the difference? This will be known as histotoxic hypoxia-tissue can't use the oxygen.

Uptake of O2 by Pulmonary Capillaries

Oxygen and CO2 exchange in the lung Is perfusion limited During exercise, the limiting factor for oxygen deliver to the muscles will be cardiac output

We will get into partial pressures more in the next several lectures, for now, you need to know that

PA refers to the alveolus and Pa refers to dissolved blood in the arterial system. Now in real life you will not always see A and a after the pressure, when you don't see this, you will assume the question refers to arterial blood. Now, O2 is not very soluble in plasma itself. It will quickly reach it saturation partial pressure. Now once it binds hemoglobin, it takes the partial pressure exerted by dissolved O2 out of the equation.

Clinical relevance points

Red blood cells are packed with the protein hemoglobin (hb) Oxygen binding to hemoglobin is needed to transport oxygen to tissues *(oxygen has limited solubility in liquid so needs hb)* Hemoglobin variations (thalassemia) or hematocrit (% RBC) -too high or too low is problematic Different types of hemoglobin (hb) (i.e. fetal, meth-hb) have different affinity for oxygen (need to bind and release at proper times) Carbon monoxide kills.... Oxygen moves in and out of cells/body by *diffusion* (down a concentration/pressure gradient) Barometric pressure is important in climbing a mountain and trying to breathe

Arterial O2 saturation (SaO2)

Relative measurement of Hb saturation by a colorimetric transcutaneous measurement

Dissolved O2 (PaO2)

Small a = arterial blood

Ectopic production of RBC's can occur in renal cell carcinoma and hepatocellular carcinoma

So in renal cancer, often erythropoietin (EPO) can increase and this will increase Hct. Conversely,* in kidney failure (or renal insufficiency), you will see anemia (decreased RBC production)* Boards love to test this little fact. In Costanzo text, it talks about hypoxia induced factor 1alpha acting on fibroblast in the renal cortex and medulla to produce EPO. Really just remember (for now) that *hypoxia (lack of oxygen to the tissues) causes EPO to be produced (from the kidney). EPO induces RBC production from the bone marrow*

Diffusion of O2

The alveolar capillary network has Large surface area. Very thin barrier (less than 1 micron).

Forms of O2 in Blood: bound to Hb

The amount of O2 bound to Hb can be calculated by multiplying the capacity of Hb to bind O2 (1.39 ml/gm Hb)

Ficks Law

The diffusion of a gas across a sheet to tissue is directly related to the surface area diffusion constant of the specific gas and the partial pressure difference.

Gas movement to respiratory system

The flow rates for CO2 and O2 are shown for 1 L of blood. The CO2 production to O2 consumption is the *respiratory exchange ratio* R, which at rest is approx 0.8

Hemoglobin

The majority of O2 is carried bound to Hb. Each molecule of Hb can bind 4 O2 molecules. When PO2 is high, O2 binds Hb and when PO2 is low O2 dissociates from Hb. So we call this a percent saturation of Hb (Hb saturation %). This is your O2 saturation index This curve is non-linear (cooperative binding

Graham's law

The rate of diffusion is directly proportional to the solubility coefficient of the gas and inversely proportional to the square root of it molecular weight CO2 diffuses 20 x faster than O2

Structural features

V=flow A=surface area for exchange - increase in exercise - decrease in emphysema T=thickness of the membrane - Increases in fibrosis D= diffusion constant P1-P2= partial pressure gradient

Factors affecting O2

With polycythemia, hemoglobin concentration increases but saturation remains the same (Saturation is not affected). Anemia, Hb concentration is affected, not the PO2 or Hb saturation.

Hb or O2 saturation

amount of O2 bound to Hb as a percentage of the total O2 binding capacity of Hb in a blood sample

Dissolved O2

amount of O2 in solution as defined by the PO2 and O2 solubility

Now respiratory exchange ratio is important for dealing with partial pressure,

but you only need to know that it is 0.8.

What must you know about myoglobin?

it can induce acute renal failure

Bohr effect

states that CO2 (carbon dioxide in water solution acts as a weak acid and produces hydrogen ions) shifts the oxygen-hemoglobin curve to the right. How: Carbon dioxide in the presence of carbonic anhydrase (in the cells) can form bicarb and H+. Hb is negatively charged (as with most proteins) and can pick up the H+ but in doing so it has less affinity for O2

O2 content (or concentration)

total amount (or concentration) of O2 in the blood, including free and bound forms

O2 content = Dissolved + Bound

~21 ml/dl or 210 ml/L blood