MOVEMENT OF SUBSTANCES BETWEEN BODY FLUID COMPARTMENTS

DR. P. HAMAMBULU

Transport across capillaries

• The capillary wall separates plasma from interstitial fluid.

• Substance exchange between blood plasma and interstitial fluid is through endothelial wall of the capillaries

• Various substances diffuse into interstitial fluid from blood plasma while others diffuse from interstitial fluid into plasma

Transport across capillaries

• Examples substances transported from plasma into interstitial fluid include nutrients(glucose, amino acids, fatty acids), electrolytes(Na⁺,K⁺,Cl⁻,Ca²⁺ e.t.c),hormones and gases mainly Oxygen

• Examples of substances transported from interstitial fluid into blood plasma are various metabolic products e.g. water, urea, CO₂ and various electrolytes

Transport across capillaries

• Capillary wall is a thin membrane made up of endothelial cells

• Substances pass through the junctions between endothelial cells and through fenestrations when they are present.

• Some also pass through the cells by vesicular transport

• The factors other than vesicular transport that are responsible for transport across the capillary wall are diffusion and filtration

Transport across capillaries

• Diffusion is the movement of substances from higher concentration to lower concentration i.e. down their concentration gradient.

• Filtration is the process by which fluid is forced through a membrane or other barrier because of a difference in pressure on the two sides.

Forces producing movements of substances between compartments

• There are four forces that influence movement of fluid between plasma and interstitial fluid

(a) Capillary (hydrostatic) pressure (Pc)(b)Interstitial fluid (hydrostatic) pressure(Pi)(c)Plasma oncotic pressure(∏c)(d)Interstitial fluid oncotic pressure(∏i) Oncotic pressure is also called colloid osmotic

pressure

Forces producing movement of substances between compartments

• Capillary (hydrostatic) pressure (Pc) is the pressure of blood fluid against capillary walls

• Capillary pressure tend to force fluid from capillaries into the interstitial spaces

• Interstitial fluid (hydrostatic) pressure(Pi) is of interstitial fluid against capillary wall

• Interstitial fluid pressure tend to force fluid from interstitial spaces into the capillaries

Forces producing movement of substances between compartments

• Plasma oncotic pressure(∏c) is that force which tends to force water into the plasma by osmosis due to plasma proteins in the plasma

• Interstitial fluid oncotic pressure(∏i) which is due to presence of proteins in interstitium tends to cause osmosis outward through the capillary membrane into interstitium

Forces producing movement of substances between compartments

• The rate of filtration at any point along a capillary depends on a balance the above forces sometimes called the Starling forces i.e. hydrostatic pressure gradient and osmotic pressure gradient

• The difference between the hydrostatic pressure in the capillary and the hydrostatic pressure of the interstitial fluid is called hydrostatic pressure gradient

• The difference between the colloid osmotic pressure of plasma and colloid osmotic pressure of interstitial fluid is called osmotic pressure gradient

Forces producing movement of substances between compartments

Fluid movement = k[(Pc– Pi) – (πc– πi)]Where, k= capillary filtration coefficient Pc = capillary hydrostatic pressure Pi = interstitial hydrostatic pressure πc = capillary colloid osmotic pressure πi = interstitial colloid osmotic pressureThe capillary filtration coefficient (k) takes into

account, and is proportional to, the permeability of the capillary wall and the area available for filtration

Forces producing movement of substances between compartments

• The lymphatic system provides a route for the transport of fluids and protein away from the interstitium

• System of fine lymphatic channels throughout the body passing via lymph nodes to thoracic duct

• Valves ensure one-way flow

Oedema

• Defined as increased volume of interstitial fluid in a tissue or organ

• May be localised or generalised (systemic)• Causes of edema include;(i)Raised capillary pressure(ii)Reduced oncotic pressure(iii)Endothelial damage (inflammation)(iv)Impaired lymphatic drainage

(i)Raised Capillary Pressure

Cardiac failure right ventricular failure - systemic oedema left ventricular failure - pulmonary

oedema congestive cardiac failure - both

Local venous obstruction deep vein thrombosis external compression

(ii)Reduced oncotic pressure

Renal disease loss of albumin across glomerulus

Hepatic disease inadequate albumin synthesis

Malnutrition inadequate albumin synthesis

Lymphatic obstraction

Renal disease loss of albumin across glomerulus

Hepatic disease inadequate albumin synthesis

Malnutrition inadequate albumin synthesis

Ascites and generalized edema

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