VO2 and Specific Circulations

Question 1

DO2

Answer 1

O2 delivery - the total amount (volume) of O2 delivered by the left ventricle to the tissues.

DO₂ = O₂ concentration of arterial blood x CO

O2 concentration of arterial blood = Hb bound O₂ + dissolved O₂

Hb bound O₂ = 1.34 x Hb concentration x O₂ saturation

DO₂ = 1.34 x Hb x cardiac output

Question 2

VO₂

Answer 2

Tissue O₂ consuption.

O₂ consumption is the total amount (volume) of O₂ consumed by teh left ventricle to the tissues. O₂ deliver is also called **VO₂. **

Higher workload (eg exercise) requires greater metabolic demand from tissue and therefore greater amounts of O₂ are consumed by the tissue in aerobic metabolism.

The maximal O₂ consumption of the body is called the VO₂ max.

VO₂ = O₂ content of arterial blood - O₂ content of venous blood x CO

In other words: *the volume (ml blood) of blood delivered to the tissues by the heart per minute, multiplied by how much of the oxygen is removed by the tissues per ml of blood. *

Fick principle

VO₂ = a-v O₂ difference x cardiac output

Question 3

Balance of oxygen supply and demand

Answer 3

Supply and demand are normally balanced so that what the tissue needs, the tissue gets. If this doesn’t happen, oxidative phosphorylation is limited and lactate will accumulate (anaerobic metabolism).

• Oxygen delivery (DO₂) is normally in excess of oxygen demand/consumption*. What this means is that more oxygen is delivered to the tissue than is actually needed. This gives the tissues a reserve of oxygen that it can draw on if demand suddenly increases or if blood flow falls.
• Some tissues have more reserve than others*.

Question 4

Skeletal muscle

Answer 4

Myocardial O₂ consumption = SMVO₂

SMVO₂ = a-v O₂ difference x muscle blood flow

Control of skeletal muscle blood flow

At rest

• SM plays an important role in regulating arterial pressure at rest. This is because SM vasculature represents a large proportion of the ‘total vascular bed’ of the body.
• Baroreflex control via SNS and PNS predominates.

Exercise

• Neural control is (mostly) overridden by local metabolism.
• Signals include bradykinin, prostaglandings, adenosine, CO₂ and lactate.
• NO released in response to increased shear stress.

Question 5

The heart

Answer 5

Resting a-v O₂ difference in the heart is much higher than in skeletal muscle.

• In the heart at rest: 120-140mL O2/L blood
• Skeletal muscle at rest: 50ml O2/L blood

This means that at rest the heart is already nearly maximally extracting oxygen.

Any increase in myocardial work (and therefore an increase in MVO₂) must come about by increasing coronary blood flow.

At rest, the coronary blood flow averaged about 5% of the CO. In heavy exercise the flow rate increases by x3-4

Question 6

Perfusing pressure

Answer 6

The perfusing pressure is the driving pressure between teh arterial and venous ends of an organ.

However, we need to remember that the blood must flow through that organ, and tehrefore external pressure or tension in that organ may also limit perfusion.

Question 7

Phasic characteristic of coronary blood flow

Answer 7

During systole, ventricular contraction compresses the vessels and occludes blood flow.

During diastole relaxation of the myocardium allow blood to be re-established (reactive hyperemia).

These changes are not evident in the right ventricle as right ventricular systolic pressures are much lower.

Question 8

Effect of heart rate on coronary blood flow

Answer 8

• A LV flow principally diastolic it can be greatly altered by HR.
• Systole requiring both active contraction and relaxation occupies a great and greater % of each cardiac cycle as HR quickens.
• At very fast rates eg >200/min because diastolic time is so short marked LV ischaemia can occur even with normal coronaries.

Question 9

Regulation of coronary blood flow

Answer 9

At rest, the heart extracts 70-80% of the O₂ from each unit of blood delivered. Any increase in O₂ delivery must come about through increased blood flow.

The most important determinant of coronary blood flow if perfusing pressure is adequate is the O₂ demand of the myocardium.

Coronary vessels are particularly sensitive to increases in metabolic by-products (particularly adenosine).

Neural effects via sympathetic alpha receptors (constrictor) and non-innvervated beta receptors (dilator) are of much lesser importance.

Thus coronary flow is principally controlled locally.

Question 10

Cerebral circulation

Answer 10

Although representing only 2% of the body mass by weight, the brain receives 13-14% of the cardiac output and consumes 16-20% of total oxygen at rest.

Because the cerebral vessels are located within a rigid cranium, intracranial pressure (ICP) is more significant than tissue hydrostatic pressure measured elsewhere in the body. It is normally 0-10 mmHg. Very high ICP reduces CBF and also affects venous outflow by causing some collapse of cerebral veins.

Cerebral blood flow is 50-60 mL/min per 100g tissue and remains remains remarkably constant.

Intrinsic mechanisms maintaining blood flow are particularly weel developed in cerebral arteries and arterioles.

Extrinsic mechanisms largely play a secondary role.

Question 11

Renal circulation

Answer 11

Although the kidneys represent only 0.4% of the body mass, they receive 20% of the cardiac output.

The circulatory anatomy is unique in that the glomerular capillaries are located between two sets of arterioles

This means that hydrostatic pressures in glomerular capillaries is especially high (55-60 mmHg), favouring filtration along their whole length

Regulation of renal blood flow is complex:

• The smaller arteries and arterioles show powerful myogenic autoregulation
• In addition, the blood flow to the kidney is very sensitive to sympathetic vasoconstrictor influences and to a variety of regulatory hormones.

Question 12

Cutaneous circulation

Answer 12

• The skin gets much more blood flow than it needs to sustain its own metabolic demands
• The skin blood flow is almost solely determined by the thermo-regulatory requirements of the body
• Under thermoneutral conditions, flow amounts to 250ml/ m2 BSA (7-8% of cardiac output)
• This value can be reduced sevenfold in the cold, or increased sevenfold during exercise in the heat, making the cutaneous circulation the most variable of all
• Control is via the sympathetic nervous system (directly and via adrenaline)
  *

Question 13

Re-distribution of cardiac output during dynamic exercise

Answer 13

• During heavy exercise, CO increases.
• A very large increase in blood flow occurs in skeletal muscle
• Substantial increases also occur in the coronary and cutaneous circualtions.
• On the other hand, renal and splanchnic blood flow is reduced.
• The cerebral circulation maintains a constant blood flow.