Physiology: Splanchnic and Hepatic Circulation

Question 1

How much of the CO does the splanchnic circulation receive?

Answer 1

25% of CO

25% of oxygen consumption

Question 2

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

Answer 2

Celiac trunk
SMA
IMA

Question 3

What does the celiac artery supply?

Answer 3

Stomach
Spleen
Pancreas

Question 4

What does the SMA supply?

Answer 4

Intestine

Pancreas

Question 5

What does the IMA supply?

Answer 5

Intestine

Question 6

Go over diagram of distribution of splanchnic circulation

Answer 6

xx

Question 7

What happens in the high splanchnic circulation?

Answer 7

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

Question 8

Where does most of the GI blood flow go to?

Answer 8

Mucosa

Question 9

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

Answer 9

Energy for forming secretions

Absorption for digested food

Question 10

Describe the regulation of splanchnic blood flow.

Answer 10

Autoregulation of blood flow in the GI tract

Question 11

Describe how autoregulation in the GI tract occurs.

Answer 11

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)

Question 12

What system regulates splanchnic flow?

Answer 12

Sympathetic nervous system

Question 13

What is heavily innervated by the SNS in splanchnic flow?

Answer 13

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

Question 14

Describe how the SNS controls blood flow.

Answer 14

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.

Question 15

Describe what happens to blood flow during digestion and absorption.

Answer 15

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

Question 16

What mechanisms are involved in gastric mucosal blood flow?

Answer 16

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

Question 17

Describe the arterial and venous flow in the villus.

Answer 17

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.

Question 18

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

Answer 18

Countercurrent blood flow

Question 19

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

Answer 19

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.

Question 20

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

Answer 20

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.

Question 21

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

Answer 21

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

Question 22

Describe how much CO the liver receives.

Answer 22

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

Question 23

Describe portal venous blood flow.

Answer 23

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

Question 24

Describe hepatic arterial blood flow.

Answer 24

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

Question 25

Go over image of blood entering vs leaving liver

Answer 25

xxx

Question 26

Describe the vascular arrangement in the liver.

Answer 26

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

Question 27

Describe the arrangement of lobules and sinusoids.

Answer 27

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

Question 28

What do the sinusoids carry and where do they drain?

Answer 28

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

Question 29

Do bile ducts and sinusoids flow in the same direction?

Answer 29

No opposite directions

Question 30

What are sinusoids lined by?

Answer 30

Layer of endothelial cells with clusters of pores/fenestrae | Very leaky and allows plasma proteins to pass through it

Question 31

What regulates portal venous blood flow?

Answer 31

Dependent on the vascular resistance of the GI tract

Question 32

What regulates hepatic arterial blood flow?

Answer 32

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

Question 33

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

Answer 33

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)

Question 34

How much can the liver expand by?

Answer 34

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

Question 35

What happens to the liver in times of circulatory stress?

Answer 35

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)

Question 36

What causes cirrhosis?

Answer 36

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

Question 37

What is characteristic of cirrhosis?

Answer 37

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

Question 38

Describe what happens in liver cirrhosis/consequences.

Answer 38

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.

Question 39

What is normal portal pressure vs portal HTN?

Answer 39

Normal portal pressure = 5-10mmHg | Portal HTN = 12mmHg or higher

Question 40

List haemodynamic consequences of portal HTN.

Answer 40

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

Question 41

What are portal systemic shunts?

Answer 41

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

Question 42

What does portal systemic shunts lead to?

Answer 42

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