In this article we will discuss about:- 1. Normal Values of Hepatic Circulation 2. Factors Modifying Hepatic Circulation 3. Peculiarities.
The hepatic artery is not an end artery and has got anastomotic channels with the portal vein at several levels of the interlobular regions (Fig. 7.107). The hepatic artery transfuses the portal vein as well as the sinusoid at a higher pressure head.
Normal Values of Hepatic Circulation:
i. Hepatic Blood Flow:
The portal area can hold about a quarter of the total blood volume. The average minute flow is about 100 ml per 100 gm of the liver tissue. The hepatic artery supplies about one-third to one-fourth of the total blood flow and 40% of O2 requirements. The portal vein supplies two-thirds to three- fourths of the total blood flow, 60% of O2, requirement and major part of nutrition.
ii. Hepatic Blood-Pressure:
Pressure in the portal vein is about 8-10 mm of Hg. Measured directly at operation in man; it comes to about 14-22 mm of H2O. This indicates that in the hepatic capillaries the pressure must be much lower (about 2 mm of Hg). Otherwise such a large amount of blood could not pass through it.
Factors Modifying Hepatic Circulation:
The principles are same as elsewhere. Hepatic circulation can be adjusted either by altering the lumen of the local vessels or by adjusting the rate of blood flow through them.
The following factors regulate hepatic circulation:
i. CO2 and H-Ion Concentration Changes, Metabolites, O2 Lack:
CO2 and H-ion concentration changes, metabolites, O2 lack, etc., alter the lumen of the vessels and adjust the flow as elsewhere.
Respiration helps in two ways. During Inspiration- (a) Intrathoracic pressure falls and intra-abdominal pressure rises, so that blood is sucked in by the thorax and pumped out by abdomen and thus portal flow rises, and (b) Diaphragm descends and exerts a massaging action on the liver, thus pushing out more portal blood towards the heart.
iii. Contraction of the Spleen and Movements of the Intestine:
Contraction of the spleen and movements of the intestine also help in driving blood through the liver.
iv. Systemic Blood Pressure:
Effects of systemic pressure on hepatic circulation are as follows:
(a) When pressure falls, O2 supply to the liver decreases; (i) due to less blood flow through the hepatic artery, and (ii) due to less oxygen being carried by portal blood which is more reduced owing to slow circulation,
(b) Rise of systemic pressure will cause opposite effects.
The principle is that a rise of systemic pressure, without constriction of the splanchnic vessels, will raise the hepatic flow. Opposite changes will reduce. [O2 tension in the centre of the liver lobule is much lower than in the tissues elsewhere. These cells are therefore easily injured by poisons, congestion, etc.]
v. Neurogenic Factors:
We know that the sympathetic stimulation changes the hepatic blood flow by increasing resistance of the hepatic blood vessels. But the role of vagi on the hepatic blood flow is uncertain.
vi. Vasomotor Reflex Control:
Heymans and others (1930, 1931) have shown that the volume of liver is increased following increase of pressure in the carotid sinus and the aortic arch. They have also described the importance of liver as blood depot for the maintenance of blood supply to the vital organ like the heart and brain during haemorrhage.
vii. Adrenaline and Noradrenaline:
Adrenaline has got both constrictor and dilator effects but the nature of effect is depending upon the concentration of adrenaline given. In higher concentration it has got constrictor effect whereas in lower concentration has got dilator effect. Noradrenaline has got constrictor effect.
viii. Effect of Posture:
Due to changes of posture from the supine to the upright positions, the hydrostatic factor (pgh) may decrease the blood pressure of the upper part of the body but the splanchnic bed compensates it reflexly through the redistribution of blood from its own area to the upper part of the body (mainly the heart and brain) by increasing vascular resistance. If the splanchnic nerves are sectioned then this auto-transfusion process does not take place.
ix. Effect of Exercise:
Muscular exercise decreases the splanchnic blood flow by redistributing blood to the active muscle and brain. The reduction of sympathetic blood flow is caused by increased vascular resistance.
Following haemorrhage the blood flow to the liver is decreased but the vascular resistance in the splanchnic bed remains unaltered and normal flow is resumed within an hour. Increased arteriovenous oxygen difference across the splanchnic bed actually maintains the constant oxygen supply of the liver during haemorrhage.
In severe massive haemorrhage, the liver blood flow is curtailed so greatly that the compensation becomes inadequate resulting hepatic hypoxia. This hypoxic state of the liver actually determines the irreversibility of the fatal haemorrhagic shock.
xi. Visceral Distention:
Distention of the stomach due to intake of food reduces the hepatic blood flow probably by increasing the blood volume in the spleen and mesenteric blood vessels.
xii. Clinical Conditions:
In cirrhosis of liver, the hepatic vascular resistance is increased and in order to maintain the normal blood flow the portal venous pressure is also increased (also due to decrease of blood flow). But the ultimate fate is the accumulation of fluid in the peritoneal cavity—causing dropsy. In congestive heart failure, the hepatic blood flow is also decreased greatly.
Peculiarities of Hepatic Circulation:
i. Hepatic circulation is a portal system. Blood has to pass through two capillary networks—at first, the splanchnic and then the hepatic.
ii. Although blood enters liver through two vessels (portal vein and hepatic artery), yet the outflow from the liver is only through the hepatic vein. There is no vein corresponding to the hepatic artery (contrast with pulmonary circulation).
iii. The pressure difference between portal vein and portal capillaries are not very high, yet a large amount of blood passes through the liver. Mechanical factors, mentioned above, help. Union between the radicles of the hepatic artery and portal vein possibly adds motive force to the latter.
iv. Filtration in the capillary area elsewhere takes place, because capillary pressure (32 mm of Hg) is more than colloidal osmotic pressure (25-30 mm of Hg). But in the liver, the osmotic pressure is same, while the capillary pressure is almost zero.
v. In the liver, blood comes into direct contact with the hepatic cells to some extent. This may partly explain above point.