15.1 Essential ideas

15.1.6 Transport of the respiratory gases (HL)

  • Hemoglobin is the primary vehicle for the transport of oxygen in the bloodstream to tissues.
  • Each molecule of hemoglobin is composed of:
    • four globular protein sub-units (two a– and two b– proteins)
    • four heme groups, each of which is capable of binding to one molecule of oxygen.

Oxygen dissociation curves show affinity of hemoglobin for oxygen

  • Affinity for oxygen refers to how readily hemoglobin binds and holds oxygen molecules.
  • When the partial pressure of oxygen in the gas surrounding tissues is high (i.e. at alveoli), oxygen saturation of hemoglobin is high. When the partial pressure of oxygen is low (i.e. at tissues), oxygen saturation of hemoglobin is low.

normal curveFigure 15.1.6a – Generalised oxygen dissociation curve

  • The curve has a sigmoid shape because binding of the first oxygen molecule causes conformational changes that make it easier for subsequent molecules of oxygen to bind.
  • This principle is called cooperativity.

Fetal hemoglobin is different from adult hemoglobin

  • Fetal hemoglobin has four heme groups, but in place of b– sub-units, there are two g– sub-units, which have a slightly different amino acid structure.
  • As a result of the structural difference, fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin. It is saturated at lower partial pressures of oxygen.
  • At the placenta, oxygen must be transferred between maternal and fetal blood, yet the partial pressure of oxygen is low.
  • If the affinity of hemoglobin for oxygen were equal in both bloodstreams, there would be no net movement.

fetal hemoglobinFigure 15.1.6b – Fetal hemoglobin
The dissociation curve is shifted to the left because fetal hemoglobin has a higher affinity for oxygen.

Three ways to transport carbon dioxide in the blood

  • Dissolved in blood plasma as gaseous CO2 (less than 10%).
  • Bound to hemoglobin – in a complex called carbaminohemoglobin. As more carbon dioxide is bound to hemoglobin, more oxygen is released. At the lungs, the carbon dioxide is exhaled (about 20%).
  • As hydrogen carbonate ions (HCO3-) – hydrogen carbonate is less toxic and more soluble than carbon dioxide (about 70%).

formation of hydrogen carbonateFigure 15.1.6c – Transformation of hydrogen carbonate
Transformation of carbon dioxide to hydrogen carbonate ions inside a red blood cell.

  • Carbon dioxide reacts with water to form carbonic acid (H2CO3). The reaction involves the enzyme carbonic anhydrase (CA).
  • Carbonic acid dissociates to form hydrogen (H+) and hydrogen carbonate (HCO3-).
  • Hydrogen carbonate ions diffuse into the plasma.
  • Hydrogen ions bind to hemoglobin (HbH+), which now has a lower affinity for oxygen.
  • Blood pH is buffered by hydrogen ions, which are (reversibly) released and bound to hemoglobin.

The Bohr shift explains increased release of oxygen in tissues

  • When the partial pressure of carbon dioxide is high (i.e. in respiring tissues), blood pH decreases due to the increase in carbonic acid.
  • The affinity of hemoglobin for oxygen decreases in acidic conditions – oxygen is less likely to stay bound to hemoglobin and more likely to enter the tissues when the tissues need it most.
  • The phenomenon is known as the Bohr shift.

bohr effectFigure 15.1.6d – Bohr effect
The Bohr shift (to the right) occurs because acidic conditions lower the affinity of hemoglobin for oxygen.

Chemoreceptors and the rate of ventilation

  • During exercise, the rate of ventilation changes in response to the amount of carbon dioxide in the blood.
  • Chemoreceptors, located in the medulla oblongata, aorta and carotid arteries, are sensitive to changes in blood pH.

ventilation rateFigure 15.1.6e – Respiration rate
The rate of respiration is controlled by the respiratory centre in the medulla oblongata.

  • When the pH drops because of increased concentration of dissociated carbonic acid, chemoreceptors respond by sending nervous signals to the respiratory centre of the medulla oblongata.
  • The respiratory centre responds by sending nervous signals to effector muscles in the ribcage and diaphragm. The rate of ventilation increases.
  • More carbon dioxide is exhaled and blood pH returns to normal.

hemoglobinFigure 15.1.6f – Hemoglobin
Each molecule of hemoglobin has four oxygen-binding heme groups, but oxygen saturation is often less than 100%.

Essential idea

Red blood cells are vital in the transport of both respiratory gases.

Key questions

  • State three factors that affect the affinity of hemoglobin for oxygen.
  • Explain the shape of oxygen dissociation curves shown in Figure 15.1.6d.
  • Outline the processes involved in maintaining blood pH.
  • Explain the effect of exercise on blood composition and respiration rate.

composition of airFigure 15.1.6g – Percentage composition of air

Concept help

  • Oxygen makes up 21 per cent of air. According to Dalton’s law, the pressure exerted by one gas in a mixture of gases is proportional to its percentage composition of the mixture. At standard atmospheric pressure (101.32kPa), the partial pressure of oxygen is 21.27kPa (101.32kPa x 21% = 21.27kPa).
  • The structure of fetal hemoglobin is an adaptation to low oxygen conditions.

types of hemoglobin proteinsFigure 15.1.6h – Fetal hemoglobin
Fetal hemoglobin (HbF) proteins are replaced completely by adult hemoglobin (HbA) by the age of six months.

Science and social responsibility (Aim 8)

Some sports, such as high-altitude mountain climbing and scuba diving, may push the limits of the human body beyond endurance and cause damage. Should such sports be controlled or banned?