#45 The components of blood, O2 and CO2 transport

The body contains approximately 5 litres of blood and this is a mixture made up of red blood cells, white blood cells and platelets all suspended in a liquid called plasma.










1. Blood components

Red blood cells

• Transport O₂ from lungs --> respiring tissues, carry CO₂ away from cells
• Very small and have the shape of a biconcave disc --> ↑surface area/volume ratio --> rapid diffusion of O₂ into and out of them. 
• Contain Hb, which combines with O₂ to form oxyhaemoglobin (HbO₂) in areas of high concentration (lungs) and releases O₂ in areas of low concentration (respiring tissues).
• Have no nucleus or mitochondria. No nucleus --> more surface area to carry Hb and hence O_2.

White Blood Cells (Leukocytes)

Defend the body against infection and disease
2 main types: Lymphocytes + Phagocytes

    - Lymphocytes

• Recognise virus/bacteria as being foreign and make antibodies to attack and destroy them or destroy them directly.
•  Each lymphocyte can recognise one particular pathogen and respond to it by secreting one particular type of antibody or by attacking it. 

 - Phagocytes

• Destroy unwanted cells (damaged body cells or pathogens like virus, bacteria) by engulfing them in a process known as phagocytosis. They take the germ into the cell then digest and destroy it.
• Larger than red blood cells and often have a lobed nucleus. 
• Have a flexible shape so that they can engulf microorganisms

Platelets (Thrombocytes)

• Fragments of larger cells
• Help blood to clot by clumping together and forming a plug. 
• Protect the body by stopping bleeding
• No nucleus.

2. Tissue fluid and lymph

• Capillaries have tiny gaps between the cells in their walls. Near the arteriole end of capillary, there is relatively high pressure inside the capillary, and plasma leaks out through these gaps to fill the spaces between the body cells. This leaked plasma is called tissue fluid.

• Tissue fluid is therefore very similar to blood plasma. However, very large molecules such as albumin (a protein carried in solution in blood plasma) and other plasma proteins cannot get through the pores and so remain in the blood plasma.

• The tissue fluid bathes the body cells. Substances such as oxygen, glucose or urea can move between the blood plasma and the cells by diffusing through the tissue fluid.

• Some tissue fluid moves back into the capillaries, becoming part of the blood plasma once more. This happens especially at the venule end of the capillary, where bloodpressure is lower, producing a pressure gradient down which the tissue fluid can flow. However, some of the tissue fluid collects into blind-ending vessels called lymphatic vessels. It is then called lymph.

• Lymphatic vessels have valves that allow fluid to flow into them and along them but not back out again. They carry the lymph towards the subclavian veins (near the collarbone) where it is returned to the blood. 

• The lymph passes through lymphatic glands where white blood cells accumulate. Lymph therefore tends to carry higher densities of white blood cells than are found in blood plasma or tissue fluid.

Lymphatic vessels pick up excess tissue fluid, purify it in the lymph nodes,
 and then return it to the circulatory system.

2. The differences between blood, tissue fluid and lymph

• Blood is a suspension of red and white cells and platelets in plasma. When left to settle or spun in a centrifuge, blood separates into these 3 components.
• Tissue fluid is a colourless fluid that is formed from blood plasma by pressure filtration through capillary walls. It surrounds all the cells of the body and all exchanges between blood and cells occur through it.
• Lymph is tissue fluid that has drained into lymphatic vessels. It passes through lymph nodes where it gains white cells and antibodies.  Lymphatic vessels absorb hormones from some endocrines glands and fat in the small intestine.

Components	Blood	Tissue fluid	Lymph
Red blood cells	(+)	(- )	(- )
White blood cells	(+)	some	some
Water	(+)	(+)	(+)
Plasma proteins	(+)	very few	very few
Na ions	(+)	(+)	(+)
Glucose	(+)	(+)	very little
Antibodies	(+)	(+)	(+)
Fats	(+)	some	(+) especilly after meal

3. Haemoglobin and O₂ transport

• Hb is a protein with quaternary structure. A Hb molecule is made up of 4 polypeptide chains, each of which has a haem group at its centre.
• Each haem group contains an Fe2+ ion which is able to combine reversibiy with O₂, forming HbO₂. 
• Each iron ion can combine with 2 oxygen atoms, so one Hb molecule can combine with 8 oxygen atoms. 

Hb + 4O₂ → HbO₈

• O₂ concentration can be measured as partial pressure (pO₂), in kilopascals (kPa).
• Hb combines with more O₂ at high pO₂ than it does at low (pO₂). At high pO₂, all the Hb will be combined with O₂, and we say that it is 100% saturated with O₂. 

Dissociation curve - A graph showing the relationship between pO₂ and the % saturation of Hb with O₂.

Dissociation curves show how efficient Hb is at absorbing O₂ in the lungs and dilivering O₂ to tissue.

• In the lungs, pO_2 = 12 kPa. You can see from the graph that the Hb will be about 98% saturated.
• In a respiring muscle, pO₂₌ 2kPa --> Hb will be about 23% saturated.

--> When Hb from the lungs arrives at a respiring muscle it gives up more than 70% of the O₂ it is carrying.

The effect of pH - The Bohr effect 

• The amount of O₂ carried and released by Hb depends not only on the pO₂ but also on pH.
• An acidic environment causes HbO₂ to dissociate (unload) to release the O₂ to the tissues. Just a small decrease in the pH results in a large decrease in the percentage saturation of the blood with O₂.
• Acidity depends on the concentration of hydrogen ions.
• The presence of CO₂  ↑concentration of H⁺ ions

--> H⁺ displaces O₂ from the HbO₂, thus increasing the O₂ available to the respiring tissues.

H⁺ + HbO₂ → HHb + O₂ 

HHb is called haemoglobinic acid. 

• This means that the Hb mops up free H⁺. That way the Hb helps to maintain the almost neutral pH of the blood. Hb acts as a buffer.
• In areas of high CO₂ concentration, Hb is less saturated with O₂ than it would be if there was no CO₂  present. This release of O₂ when the pH is low (even if the pO₂ is relatively high) is called the Bohr effect.
• It enables Hb to unload more of its O₂ in tissues where respiration (which produces CO₂) is taking place.

The Bohr effect causing a shift to the right 
in the oxyhemoglobin dissociation curve.

4.  CO₂ transport

• About 85% of the CO₂ produced by respiration diffuses into the red blood cells and forms H₂CO_3 (carbonic acid) under the control of carbonic anhydrase - enzyme found in red blood cells.

• The HCO₃⁻ diffuses out of the red blood cell into the plasma. This leaves a shortage of negatively charged ions inside the red blood cells. (To compensate for this, chloride ions move from the plasma into the red blood cells. This restoration of the electrical charge inside the red blood cells is called the chloride shift.)

• About 5% of the CO₂ produced simply dissolves in the blood plasma.

• Some CO₂ diffuses into the red blood cells but instead of forming H₂CO₃, attaches directly onto the Hb molecules to form carbaminohaemoglobin. Since the CO₂ doesn't bind to the haem groups the Hb is still able to pick up O₂ or H⁺.

Adaptation to high altitude

• At high altitudes, the air is less dense and the pO₂ is lower than at sea level -->Hb is less saturated with O₂ in the lungs and delivers less O₂ to body tissues.
• After some time at high altitude, the number of red blood cells in the blood ↑--> there are more Hb molecules in a given volume of blood.
• -->Even though each Hb molecule carries less O₂ on average than at sea level, the fact that there are more of them helps to supply the same amount of O₂ to respiring tissues.

Athletes may make use of this by training at high altitude before an important competition. When they return to low altitude, their extra red blood cells can supply O₂to their muscles at a greater rate than in an athlete who has not been to high altitude, giving them a competitive advantage.

 

Syllabus 2015 (o) [PA] describe the structure of red blood cells, phagocytes (macrophages and neutrophils) and lymphocytes; (p) state and explain the differences between blood, tissue fluid and lymph; (q) describe the role of haemoglobin in carrying oxygen and carbon dioxide (including the role of carbonic anhydrase, the formation of haemoglobinic acid and carbaminohaemoglobin); (r) describe and explain the significance of the oxygen dissociation curves of adult oxyhaemoglobin at different carbon dioxide concentrations (the Bohr effect); (s) describe and explain the significance of the increase in the red blood cell count of humans at high altitude;

Syllabus 2016  - 2018 8.1    The circulatory system d)   observe and draw the structure of red blood cells, monocytes, neutrophils and lymphocytes using prepared slides  and photomicrographs e)   state and explain the differences between blood, tissue fluid and lymph f) describe the role of haemoglobin in carrying oxygen  and carbon dioxide with reference to the role of carbonic  anhydrase, the formation of haemoglobinic acid and carbaminohaemoglobin (details of the chloride shift are not required) g)   describe and explain the significance of the oxygen dissociation curves of adult oxyhaemoglobin at different carbon dioxide concentrations (the Bohr effect) h)   describe and explain the significance of the increase in the red blood cell count  of humans at high altitude