Vascular awall metabolism: starling balance

2021-05-29 01:59 PM

The amount of filtrate that exits from the arteriolar ends of the capillaries is approximately equal to the amount of filtrate returned to circulation by absorption. The difference of that very small amount of fluid to the heart by the lymphatic route.

The main function of microcirculation is to transport nutrients to tissues and remove cellular waste products. Small arterioles control blood flow to individual tissues and their local condition, by controlling the diameter of the arterioles. Thus, in most cases, each tissue's regulation of flow is related to its own needs.

The walls of the capillaries are very thin and are composed of a layer of highly permeable endothelial cells. Thus, water, cellular nutrients and cellular secretion products can be exchanged quickly and easily between tissues and circulating blood.

The human body's peripheral circulatory system has about 10 billion capillaries with an estimated total surface area of ​​500 to 700 square meters (about one-eighth of the surface area of ​​a football field). Thus, any functioning cell has a capillary that feeds it no more than 20-30 micrometres away.

More than a century ago, Ernest Starling showed that under normal conditions, the capillary membrane exists in a state of near equilibrium. That is, the amount of filtrate that exits from the arteriolar ends of the capillaries is approximately equal to the amount of filtrate returned to circulation by absorption. The difference of that very small amount of fluid to the heart by the lymphatic route.

The following table shows the principles of Starling equilibrium. For this table, the pressures in the arterial and venous capillaries are averaged to calculate the mean functional capillary pressure for the entire length of the capillaries. The calculated mean functional capillary pressure was 17.3 mmHg.

Mean force that moves fluid out (mmHg)

Average capillary pressure: 17.3

The negative pressure of free fluid in interstitial fluid: 3.0

The colloidal osmotic pressure of interstitial fluid: 8.0

Total thrust out: 28.3

Mean force that moves fluid inward (mmHg)

Plasma colloid osmotic pressure: 28.0

The total force put in: 28.0

The average sum of forces

Out: 28.3

Inside: 28.0

True outward thrust: 0.3

Thus, for total capillary circulation, we find a near-equilibrium between the sum of the extrinsic forces, 28.3 mmHg, and the total inward forces, 28.0 mmHg. This slight imbalance of forces, 0.3 mmHg, causes fluid to be filtered into the interstitial space rather than reabsorbed. This excess filtrate is called net filtration, and it is the amount of filtrate that must be returned to circulation through the lymphatic system. The normal rate of net filtration in the whole body, excluding the kidneys, is only about 2 ml/min.