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By the end of this section, you will be able to:
  • Describe how oxygen is bound to hemoglobin and transported to body tissues
  • Explain how carbon dioxide is transported from body tissues to the lungs

Once the oxygen diffuses across the alveoli, it enters the bloodstream and is transported to the tissues where it is unloaded, and carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body. Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by different mechanisms.

Transport of oxygen in the blood

Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of oxygen in the blood is dissolved directly into the blood itself. Most oxygen—98.5 percent—is bound to a protein called hemoglobin and carried to the tissues.

Hemoglobin

Hemoglobin , or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits ( [link] ). Each subunit surrounds a central heme group    that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is deoxygenated is darker red.

Part a shows disc-shaped red blood cells. An arrow points from a red blood cell to the hemoglobin in part b. Hemoglobin is made up of coiled helices. The left, right, bottom, and top parts of the molecule are symmetrical. Four small heme groups are associated with hemoglobin. Oxygen is bound to the heme.
The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the lungs is (b) hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.

It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph—an oxygen dissociation curve    —is sigmoidal, or S-shaped ( [link] ). As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.

Art connection

The graph plots percent oxygen saturation of hemoglobin as a function of oxygen partial pressure. Oxygen saturation increases in an S-shaped curve, from 0 to 100 percent. The curve shifts to the left under conditions of low carbon dioxide, high pH, and low temperature, and to the right in conditions of high carbon dioxide, low pH, or high temperature.
The oxygen dissociation curve demonstrates that, as the partial pressure of oxygen increases, more oxygen binds hemoglobin. However, the affinity of hemoglobin for oxygen may shift to the left or the right depending on environmental conditions.

The kidneys are responsible for removing excess H+ ions from the blood. If the kidneys fail, what would happen to blood pH and to hemoglobin affinity for oxygen?

Factors that affect oxygen binding

The oxygen-carrying capacity    of hemoglobin determines how much oxygen is carried in the blood. In addition to P O 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 ( [link] ). When carbon dioxide is in the blood, it reacts with water to form bicarbonate (HCO 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.

Questions & Answers

if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
Syamthanda Reply
hey , can you please explain oxidation reaction & redox ?
Boitumelo Reply
hey , can you please explain oxidation reaction and redox ?
Boitumelo
for grade 12 or grade 11?
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the value of V1 and V2
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advantages of electrons in a circuit
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we're do you find electromagnetism past papers
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what a normal force
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it is the force or component of the force that the surface exert on an object incontact with it and which acts perpendicular to the surface
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what is physics?
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what is the half reaction of Potassium and chlorine
Anna Reply
how to calculate coefficient of static friction
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how to calculate static friction
Lisa
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How to calculate force
Monambi
a structure of a thermocouple used to measure inner temperature
Anna Reply
a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4
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what is acceleration
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a rate of change in velocity of an object whith respect to time
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how can we find the moment of torque of a circular object
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Acceleration is a rate of change in velocity.
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t =r×f
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hi
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use fnet method. how many obects are being calculated ?
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Lungile Reply
you use Fnet equals ma , newtoms second law formula
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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