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GI Block Module > Physiology: Splanchnic and Hepatic Circulation > Flashcards

Physiology: Splanchnic and Hepatic Circulation Flashcards

1
Q

How much of the CO does the splanchnic circulation receive?

A

25% of CO

25% of oxygen consumption

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2
Q

Name the branches of the aorta going to the splanchnic circulation.

A

Celiac trunk
SMA
IMA

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3
Q

What does the celiac artery supply?

A

Stomach
Spleen
Pancreas

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4
Q

What does the SMA supply?

A

Intestine

Pancreas

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5
Q

What does the IMA supply?

A

Intestine

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6
Q

Go over diagram of distribution of splanchnic circulation

A

xx

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7
Q

What happens in the high splanchnic circulation?

A

Nutrients absorbed from the intestinal lumen for transport to the liver for storage or transformation or for direct supply to general circulation

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8
Q

Where does most of the GI blood flow go to?

A

Mucosa

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9
Q

What is the function of the 65% blood flow to the mucosa?

A

Energy for forming secretions

Absorption for digested food

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10
Q

Describe the regulation of splanchnic blood flow.

A

Autoregulation of blood flow in the GI tract

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11
Q

Describe how autoregulation in the GI tract occurs.

A

In the stomach, small intestine and colon.
More prominent in the fed state than in the fasting state
More prominent in the mucosa (more metabolically active)

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12
Q

What system regulates splanchnic flow?

A

Sympathetic nervous system

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13
Q

What is heavily innervated by the SNS in splanchnic flow?

A

Blood vessels in the GI tract are heavily innervated by the SNS

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14
Q

Describe how the SNS controls blood flow.

A

Activation of the SNS causes alpha adrenoreceptor mediated vasoconstriction.
Diversion of blood (200-300mL) from the GIT to the vital organs when MAP reduced.
During haemorrhage, GI blood flow can be reduced to 25% of resting flow following SNS activation.

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15
Q

Describe what happens to blood flow during digestion and absorption.

A

Within 20min of the ingestion of a meal, GI blood flow can increase significantly
Increase in flow is almost exclusively confined to the MUCOSA - up to 6 fold increase in flow

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16
Q

What mechanisms are involved in gastric mucosal blood flow?

A 
Metabolic vasodilation (adenosine) 
Dilator action of GI hormones (CCK, vasoactive intestinal peptide, gastrin, secretin) 
Bradykinin
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17
Q

Describe the arterial and venous flow in the villus.

A

Arterial flow into the villus and venous flow out of the villus are in opposite directions to each other and are close to each other.

18
Q

What do we call the opposite flow of the arterial blood and venous blood in villi?

A

Countercurrent blood flow

19
Q

Describe the flow of blood to the tip of the villus.

A

Much of the blood oxygen is transported from the arterioles into adjacent venules via capillaries without being carried to the tip of the villus.
As much as 80% of the oxygen may take this short circuit.

20
Q

When may the short circuiting of blood to the tip of the villus be harmful?

A

Under normal circumstances this is not harmful
In circulatory shock, cells in the top of the villus may become ischaemic and die.
This occurs because flow is reduced.

21
Q

Describe what will occur if severe hypoperfusion occurs in the GI tract.

A

Not tolerated for long and disruption of the mucosa will reduce its barrier function.
Endotoxins can enter the systemic circulation causing sepsis/septic shock.

22
Q

Describe how much CO the liver receives.

A

25% in order to allow it to attend to its metabolic functions
A total liver blood flow of 1.5L/min is normal

23
Q

Describe portal venous blood flow.

A

Venous, partly de-oxygenated, but nutrient rich
70-80% of total liver blood flow
Usually low pressure: 5-10mmHg
Site of portal vascular resistance is ill-defined under physiological conditions

24
Q

Describe hepatic arterial blood flow.

A

Well oxygenated
20-30% of total liver blood flow
Blood pressure equivalent to MABP
Hepatic arterial resistance resides in the hepatic arterioles which protects the fragile sinusoids from high pressures

25
Q

Go over image of blood entering vs leaving liver

A

xxx

26
Q

Describe the vascular arrangement in the liver.

A

4 lobes - left, right, quadrate, caudate
Each lobe is made up of hexagonal lobules
Portal triad - branch of portal vein, hepatic artery and bile duct.
At the centre of the lobule is the central vein

27
Q

Describe the arrangement of lobules and sinusoids.

A

Lobules are made up of hepatocytes arranged in pairs of columns radiating from the central vein.
Between 2 pairs of hepatocyte columns are the sinusoids.
A bile canaliculus runs between 2 columns of hepatocytes and drains into a bile duct

28
Q

What do the sinusoids carry and where do they drain?

A

Sinusoids carry portal venous blood mixed with hepatic arterial blood.
Drain into central vein and then the hepatic vein.

29
Q

Do bile ducts and sinusoids flow in the same direction?

A

No opposite directions

30
Q

What are sinusoids lined by?

A

Layer of endothelial cells with clusters of pores/fenestrae

Very leaky and allows plasma proteins to pass through it

31
Q

What regulates portal venous blood flow?

A

Dependent on the vascular resistance of the GI tract

32
Q

What regulates hepatic arterial blood flow?

A

Under sympathetic tone (alpha adrenoreceptor mediated vasoconstriction - arterioles)
Decrease pressure in arterioles where high pressure is present

33
Q

What does the liver act as a reservoir for and why/how?

A

Blood is expandable/compressible organ
Large amounts of blood can be stored in its blood vessels (sinusoids and hepatic veins)
Acts as a blood reservoir in times of excess blood volume, capable of supplying extra blood in times of reduced flow (hypovolemia)

34
Q

How much can the liver expand by?

A

Expandable venous organ
Normal liver blood volume is 500mL or 10% of total blood volume
Cardiac failure - volume of blood can rise to 1L

35
Q

What happens to the liver in times of circulatory stress?

A

The SNS causes vasoconstriction of the hepatic veins & a large volume of blood is discharged in the systemic circulation within 1-4min
Single most important source of extra blood in times of need (heavy exercise, severe haemorrhage)

36
Q

What causes cirrhosis?

A

Liver injury (chronic hep B or C infection, iron overload, copper overload, recurrent bile duct injury, alcohol)

37
Q

What is characteristic of cirrhosis?

A

Accumulation of extra-cellular matrix proteins, tissue contraction and derangement of blood flow

38
Q

Describe what happens in liver cirrhosis/consequences.

A

Blood flow within liver is disrupted.
Portal and hepatic venules are found within the fibrous septa.
Constriction or distortion of portal venules and hepatic venules lead to increased vascular resistance.
Portal vascular resistance increases.
The increases portal vascular resistance leads to a rise in portal pressure = portal HTN.

39
Q

What is normal portal pressure vs portal HTN?

A

Normal portal pressure = 5-10mmHg

Portal HTN = 12mmHg or higher

40
Q

List haemodynamic consequences of portal HTN.

A

Primary consequence - development of portal-systemic shunt (PSS)

41
Q

What are portal systemic shunts?

A

Collateral blood vessels, which allow the blood which normally passes through the liver to bypass it

42
Q

What does portal systemic shunts lead to?

A

Reduction in portal venous (PV) blood flow: >80% of the portal venous blood flow can bypass hepatic sinusoids in liver cirrhosis

GI Block Module (8 decks)

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