Week 9 - Gaseous exchange and transport

Question 1

What are the 3 phases of respiration?

Answer 1

Pulmonary ventilation
External respiration
Internal respiration

Question 2

What is pulmonary ventilation?

Answer 2

Breathing
the intake of 02 and exhalation of C02

Question 3

What is external respiration?

Answer 3

→ Diffusion of O2 from air in the alveoli of the lungs to blood in pulmonary capillaries and the diffusion of CO2 in the opposite direction

Question 4

External respiration process?

Answer 4

• deoxygenated blood (depleted of some O2) coming from the right side of the heart is converted into oxygenated blood (saturated with O2) that returns to the left side of the heart.
• blood flows through the pulmonary capillaries, picks up O2 form alveolar air and unloads CO2 into alveolar air.
• each gas diffuses independently from the area where it’s partial pressure is higher to the area where partial pressure is lower (across a concentration gradient) until equilibrium reached.

Question 5

What is the partial pressure of alveolar air and pulmonary capillaries?

Answer 5

In alveolus = 105mmHg
In blood - 40mmHg (even lower when exercising)

goes from high conc. to low conc.

Question 6

Why is the PO2 of blood in the pulmonary veins slightly less than the PO2 in the pulmonary capillaries?

Answer 6

As blood leaving pulmonary capillaries near alveolar spaces mixes with a small volume of blood that has flowed through conducting portions of the respiratory system.

100mmHg

Question 7

What is internal respiration?

Answer 7

→ The left ventricle pumps oxygenated blood into the aorta and through the systemic arteries to systemic capillaries.

• the exchange of oO2 and CO2 between systemic capillaries and tissue cells is called internal respiration or systemic gas exchange
• as O2 leaves the bloodstream, oxygenated blood is converted into deoxygenated blood.
• occurs in tissues throughout the body

Question 8

What is the partial pressure of oxygen in the systemic capillaries and the tissue cells?

Answer 8

Systemic capillaries = 100mmHg
Tissue cells = 40mmHg

Question 9

Process of internal respiration?

Answer 9

• The pO2 of blood pumped into systemic capillaries is higher than the PO2 in tissue cells because the cells constantly use O2 to produce ATP.
• Due to this pressure difference, oxygen diffuses out of the capillaries into tissue cells and blood pO2 drops to 40mmHg by the time the blood exists systemic capillaries.
• CO2 diffuses in the opposite direction → tissue cells constantly producing CO2, the pCO2 of cells (45mmHg at rest) is higher than that of systemic capillary blood (40mmHg).
• Leads to CO2 diffusing from tissue cells through interstitial fluid into systemic capillaries until pCO2 in blood increases to 45mmHg.
• Deoxygenated blood then returns to heart + is pumped to lungs for another cycle of external respiration.

Question 10

When a person is at rest, how much oxygen do the tissue cells need?

Answer 10

25% of available O2 in oxygenated blood

Question 11

How do oxygen demands for tissue cells change when exercising?

Answer 11

During exercise there is more O2 diffusing from blood into the metabolically active cells such as the contracting skeletal muscle fibres.

These active cells use more O2 for ATP production so O2 content of deoxygenated blood drops below 75%.

Question 12

The rate of pulmonary and systemic gas exchange depends on what factors?

Answer 12

→ Partial pressure difference of gases
→ Surface area available for gas exchange
→ Diffusion distance
→ Molecular weight and solubility of the gases

Question 13

What impact does partial pressure have?

Answer 13

• Alveolar pO2 must be higher than blood for oxygen to diffuse from alveolar air into the blood.
• The rate of diffusion is faster when the difference between pO2 in alveolar air and pulmonary capillary blood is larger; diffusion is slower when the difference is smaller.
• The differences between pO2 and pCO2 in alveolar air versus pulmonary blood increase during exercise.
• The partial pressures of O2 and CO2 in alveolar air also depend on rate of air flow in/out of lungs.
• Certain drugs (morphine) slow ventilation, thereby decreasing the amount of O2 and CO2 that can be exchanged between alveolar air and blood.

Question 14

Impact of surface area available on gas exchange?

Answer 14

• The surface area of the alveoli is huge (75m2)
• In addition, many capillaries surround each alveolus, so many that as much as 900ml of blood is able to participate in gas exchange at any instant.
• Any pulmonary disorder that decreases the functional surface area of the respiratory membranes decreases the rate of external respiration e.g. in emphysema alveolar walls disintegrate, so the surface area is smaller than normal and pulmonary gas exchange is slowed.

Question 15

Impact of diffusion distance?

Answer 15

• The respiratory membrane is very thin, so diffusion occurs quickly.
• Also, the capillaries are so narrow that the red blood cells must pass through them in single file, which minimises the diffusion distance from an alveolar air space to haemoglobin inside red blood cells (they are squashed against wall).
• Build up of interstitial fluid between alveoli, as occurs in pulmonary edema slows the rate of gas exchange as it increases diffusion distance.

Question 16

Impact of molecular weight and solubility of the gases?

Answer 16

• As O2 has lower molecular weight than CO2, it could be expected to diffuse faster.
• HOWEVER the solubility of CO2 in the fluid portions of the respiratory membrane is about 24 greater than that of O2.
• Net outward CO2 diffusion occurs 20 times more rapidly than net inward O2 diffusion.
• Consequently, when diffusion is slower than normal O2 insufficiency (hypoxia) typically occurs before there is significant retention of CO2 (hypercapnia).

Question 17

What are proteins?

Answer 17

→ proteins represent the structural composition of all living organisms
→ proteins contribute to biochemical processes that preserve life
→ proteins are complex macromolecules (polymers) and have high molecular weight + are made up of structural units (monomers) called amino acids.
→ amino acids are the protein’s building units.

Question 18

What are amino acids?

Answer 18

They are organic compounds made up of hydrogen, oxygen, carbon and nitrogen atoms.

Amino acids are made up of a basic group (amino group NH2), an acidic group (carboxyl group COOH), a hydrogen atom, and a terminal group R which differs one amino acid to another.

Question 19

What is a dipeptide compound?

Answer 19

The combination of two amino acids

Question 20

What is a polypeptide?

Answer 20

The protein chain formed of several amino acids

Question 21

What is the quaternary structure of protein?

Answer 21

Proteins that consist of 2 or more chains of polypeptide.

Different poly peptide chains form a structure due to linkage of polypeptide chains with each other - referred to as protein sub units.

These sub units are all the same in haemoglobin. - 4 polypeptide chains.

Question 22

Key points about oxygen transport?

Answer 22

Oxygen does not dissolve easily in water, so only about 1.5% of inhaled O2 is dissolved in blood plasma, which is mostly water.

About 98.5% of blood O2 is bound to haemoglobin in red blood cells.

Question 23

What do each of the 4 polypeptide chains in haemoglobin have?

Answer 23

iron ion
each of these ions can bind with oxygen

Question 24

What is the structure of haemoglobin?

Answer 24

→ Haemoglobin is a globular protein which is an oxygen-carrying pigment found in vast quantities in RBC’s

→ It has a quaternary structure - 4 p.p. chains.

→ These chains or subunits are globin proteins (two a- globins and two b-globins) and each subunit has a prosthetic haem group.

→ The four globin subunits are held together by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards (helping preserve 3D spherical shape) and the hydrophilic R groups are facing outwards (helps to maintain its solubility).

→ The prosthetic haem group contains an iron II ion (Fe2+) which is able to reversibly combine with an oxygen molecule forming oxyhaemoglobin and results in the haemoglobin appearing bright red.

→ Each haemoglobin with the four haem groups can therefore carry 4 oxygen molecules (8 oxygen atoms).

Question 25

What is the function of haemoglobin? (simple)

Answer 25

Responsible for binding oxygen in the lung and transporting the oxygen tissue to be used in aerobic metabolic pathways.

Question 26

What are the 3 functional properties of haemoglobin?

Answer 26

→ Oxygen is not very soluble in water and haemoglobin is, meaning oxygen can be carried more efficiently around the body when bound to haemoglobin.

→ The presence of the haem group (and Fe2+) enables small molecules like oxygen to be bound more easily as each oxygen molecule binds it alters quaternary structure.

→ This causes haemoglobin to have higher affinity for subsequent O2 molecules and they bind more easily.

→ Iron II ion in the prosthetic haem group also allows O2 to reversibly bind as none of the amino acids that make up the p.p. chains in haemoglobin are well suited to binding w/ O2.

Question 27

What is the formula for the reversible reaction where oxygen and haemoglobin form oxyhemoglobin?

Answer 27

Hb + O2 ⇋ Hb-O2

Reduced haemoglobin + Oxygen ⇋ Oxyhemoglobin

The 98.5% of the O2 that is bound to haemoglobin is trapped inside RBC’s, so only the dissolved O2 (1.5%) can diffuse out of tissue capillaries into tissue cells.

Question 28

What are the factors that promote O2 binding and dissociating from haemoglobin?

Answer 28

→ The affinity of haemoglobin for carbon monoxide is 245-fold higher than that for oxygen. Therefore haemoglobin will preferentially bind carbon monoxide over oxygen following an exposure.

→ This decreases the oxygen carrying capacity of the blood and alters the dissociation curve of oxyhaemoglobin. As a result, the rate at which oxygen is delivered to cells is reduced, interfering with cellular respiration and causing tissue hypoxia.

Question 29

What is the relationship between haemoglobin and oxygen partial pressure?

Answer 29

The higher the pO2, the more O2 combines with Hb.

→ When reduced haemoglobin (Hb) is completely converted to oxyhaemoglobin (Hb-O2), the haemoglobin is said to be fully saturated; when haemoglobin consists of a mixture of Hb and Hb-O2, it is partially saturated.

→ The perfect saturation of haemoglobin expresses the average saturation of haemoglobin with oxygen. For instance, if each haemoglobin molecule has bound two O2 molecules, then the haemoglobin is 50% saturated as each Hb can bind a maximum of four O2.

Question 30

What are the 3 ways that carbon dioxide is transported?

Answer 30

Under normal resting conditions, each 100ml of deoxygenated blood contains the equivalent of 53mL of gaseous CO2, which is transported in 3 main forms:

1. Dissolved CO2 - &%
2. Carbamino compounds - 23%
3. Bicarbonate ions - 70%

Question 31

What is dissolved CO2?

Answer 31

The smallest percentage - about 7% - is dissolved in blood plasma.
On reaching the lungs, it diffuses into alveolar air and is exhaled.

Question 32

How do carbamino compounds work ?

Answer 32

→23% CO2 combines with the amino groups of amino acids and proteins in blood to form carbamino compounds.
→Because the most prevalent protein in blood is haemoglobin (inside RBC’s), most of the CO2 transported in this manner is bound to haemoglobin.
→The main CO2 binding sites are the terminal amino acids in the two alpha and two beta goblin chains.

Question 33

What is CO2 that has bound to haemoglobin called?

Answer 33

Carbaminohaemoglobin (Hb-CO2)

Question 34

What is the formation of carbominohaemoglobin influenced by?

Answer 34

• Formation is greatly influenced by pCO2.
  → For example, in tissue capillaries pCO2 is relatively high, which promoted formation of Hb-CO2.
  → But in pulmonary capillaries, pCO2 is relatively low, and the CO2 readily splits apart from globin and enters the alveoli by diffusion.

Question 35

What are bicarbonate ions?

Answer 35

70% of CO2 is transported in blood plasma as bicarbonate ions (HCO3-).

Question 36

How is CO2 transported through bicarbonate ions?

Answer 36

→ As CO2 diffuses into systemic capillaries and enters red blood cells, it reacts with water in the presence of the enzyme carbonic anhydrase (CA) to form carbonic acid - which dissociates into H+ and HCO3-.
→ Thus, as blood picks up CO2, HCO3- accumulates inside RBCs.
→ Some HCO3- moves out into the blood plasma, down it’s concentration gradient.
→ In exchange, chloride ions (Cl-) move from plasma into RBCs. This exchange of negative ions, which maintains the electrical balance between blood plasma and RBC cytosol, is known as the chloride shift.
→ The net effect of these reactions = CO2 removed from tissue cells + transported in blood plasma as HCO3-.
→ As blood passes through pulmonary capillaries in the lungs, all of these reactions reverse and CO2 is exhaled.

Question 37

How does the oxygen dissociation curve work?

Answer 37

When pO2 high, haemoglobin binds with large amounts of O2 and is almost 100% saturated.

When pO2 is low, haemoglobin is only partially saturated.

In pulmonary capillaries, where pO2 is high, a lot of O2 binds to haemoglobin.

In tissue capillaries, where pO2 = lower, haemoglobin does not hold as much O2, and the dissolved O2 is unloaded via diffusion into tissue cells.

Question 38

How does the oxygen dissociation curve work (2)?

Answer 38

• When pO2 is between 60-100mmHg, haemoglobin is 90% or more saturated with O2, thus, blood picks up a nearly full load of O2 from the lungs even when the pO2 of alveolar air is as low as 60mmHg.
• The Hb-pO2 curve explains why people can still perform well when they have certain cardiac and pulmonary diseases, even though pO2 may drop as low as 60mmHg.
• At pO2 of 40%, haemoglobin is still 75% saturated with oxygen. However, oxygen saturation of Hb drops to 35% at 20mmHg.

Question 39

What occurs to the Hb-pO2 curve between 40-20mmHg?

Answer 39

Between 40 and 20 mmHg, large amounts of O2 are released from haemoglobin in response to only small decreases in pO2.

In active tissues such as contracting muscles, pO2 may drop well below 40mmHg.

Then, a large percentage of the O2 is released from haemoglobin, providing more O2 to metabolically active tissues.

Question 40

What are the other factors affecting the affinity of haemoglobin for oxygen?

Answer 40

• Acidity
• Partial pressure of CO2
• Temperature
• BPG

Question 41

How does acidity impact oxygen dissociation curve (Bohr effect)

Answer 41

• When pH decreases - entire curve shifts to the right; at any given pO2, Hb is less saturated with O2, a change termed the Bohr effect.
• The Bohr effect works both ways: An increase in H+ in blood causes O2 to unload from haemoglobin, and the binding of O2 to haemoglobin causes offloading of H+ from haemoglobin.

Question 42

What is the explanation for the Bohr effect?

Answer 42

→ Haemoglobin can act as a buffer for hydrogen ions.

→ But, when H+ ions bind to amino acids in haemoglobin, they alter it’s structure slightly, decreasing it’s oxygen carrying capacity.

→ Thus, lowered pH drives O2 off haemoglobin, making more O2 available for tissue cells.

→ By contrast, elevated pH increases the affinity of haemoglobin for O2 and shifts the oxygen - haemoglobin dissociation curve to the left.

Question 43

How does the partial pressure of carbon dioxide influence the Hb-pO2 dissociation curve?

Answer 43

→ CO2 can also bind to haemoglobin, and the effect is similar to that of H+ (shifting the curve to the right). As pCO2 rises, haemoglobin releases O2 more readily.
→ pCO2 and pH = related as low blood pH results from high pCO2.
→ As CO2 enters blood, much of it is temporarily converted to carbonic acid (H2CO3), a reaction catalysed by an enzyme in red blood cells called carbonic anhydrase.
→ The carbonic acid thus formed in RBC’s dissociated into hydrogen ions and bicarbonate ions. As the H+ increases, pH decreases.
→ Thus - increased pCO2 produces more acidic environment, which helps release O2 from haemoglobin.
→ Decreased pCO2 (and elevated pH) shifts the saturation curve to the left.

Question 44

What is BPG?

Answer 44

biphosphoglycerate

Question 45

What is VO2?

Answer 45

Oxygen uptake = the amount of oxygen the body takes in and utilises.

Can be measured through gas analysis of oxygen content in inspired air vs oxygen content of expired air.

Question 46

What is VO2 max?

Answer 46

Maximal amount of oxygen that body can uptake and utilise.

The point at which oxygen uptake plateaus and shows no further increase in response to additional workload.

Question 47

What is VO2 max dependant on?

Answer 47

gender
height
weight
lung function
fitness level
the activity they are performing

It is exercise specific - higher for activities involving large muscle groups → VO2 max increases w/aerobic training.

Question 48

What is global oxygen delivery?

Answer 48

Total amount of oxygen delivered to the tissues in entire body per minute, regardless of the distribution of blood flow.

The product of total blood flow or cardiac output (CO) and the oxygen content of arterial blood - usually expressed in ml O2/min

Oxygen delivery (mL O2/min) = CaO2 x CO