The properties and contents of lymph

This chapter is relevant to Section I3(iv) of the 2017 CICM Primary Syllabus, which expects the exam candidates to "outline the composition and functions of lymph". It has some minimal level of exam relevance, as in Question 4 from the second paper of 2019 the college asked about "how interstitial fluid recirculates to the vascular system". Lymph is how. The question did not ask about the composition of lymph, but asked about pretty much everything else in physiology, including "anatomy of venous structures, valves and lymphatics, permeability and factors which influence permeability" and "hydrostatic forces, other pressures involved, and the role of osmotic and electric forces". In short, to answer everything on one page, one would need to restate the entire contents of several other chapters, including:

• Starling forces and fluid exchange in the microcirculation
• Movement of fluid between the intravascular and interstitial compartments
• Osmotic pressure and oncotic pressure
• Gibbs-Donnan effect

Even though it would be tempting to explain some of these concepts again (except accurately this time), this brief summary will bring them together without expanding on them, because time is finite. 

For the reader who needs a professional explanation of the lymphatic circulation, there is nothing better than this free article by Margaris & Black (2012). It would be wasteful to recommend anything else. 

Anatomy of the interstitial space

This is a massive void, containing approximately 25% of the total body water, and possessing an extremely irregular shape, as it basically wraps around cells and acellular structures, leaking through the basement of the human body. It is the whatever left behind after you take total body water and separate the intracellular fluid compartment (a quadrillion of tiny little compartments) and the intravascular compartment (one branching volume with infinite microscopic ramifications).  Nobody really thinks very much about it, but it is absolutely essential to the normal function of the human body, and by its properties and functions closely represents the primordial sea which bathed our unicellular ancestors.

Several properties of this fluid compartment can be inferred from the behaviour of molecules injected into it, for example from the experiments of Torvard Laurent (1968). In summary:

• It consists of narrow aqueous channels. The resistance to bulk flow of fluid and solute is therefore quite great.
• It is water-filled and continuous. The rate of diffusion of water through this space is roughly the same as the rate of water diffusion through itself. So, the maze of microscopic tunnels impedes bulk flow but not diffusion of water.   Guyton & Barber (1980) report that most of the time the vast majority of this fluid exists in the form of a hydrated gel, and free mobile fluid is in a minority. 

Origins and characteristics of interstitial fluid

Interstital fluid originates by transvascular fluid change, due to the action of Starling forces on the fluid inside capillaries. This transvascular fluid exchange depends on a balance between hydrostatic and oncotic pressure gradients in the capillary lumen and the interstitial fluid. This balance can be expressed as the Starling equation:

Jv = L_p S [ (P_c - P_i) - σ(Π_esl - Π_b) ]

where

• Jv is the net fluid transport, 
• L_p  is the hydraulic permeability coefficient, 
• S is the surface area,
• P_c and P_i are the capillary hydrostatic pressure and interstitial hydrostatic pressure
• σ is the reflection coefficient for protein,
• Π_esl is the oncotic pressure in the endothelial glycocalyx layer, and 
• Π_b is the oncotic pressure of the subglycocalyx.

Without revisiting the entire fascinating topic of microvascular fluid exchange, it will suffice to say that  in the capillaries, ultrafiltration takes place as the result of the capillary hydrostatic pressure being higher than the sum of the interstitial hydrostatic pressure and intravascular oncotic pressure. The balance of Starling forces wants fluid to move out of the capillaries. The ultrafiltered fluid then becomes the interstitial fluid, which has a variable (and low) hydrostatic pressure:

• Burns (skin): -20 to -30 mmHg (Lund, 1988)
• Lungs: -8 mmHg (Sven & Josipa, 2007)
• Normal subcutaneous tissue: -3 mmHg (Sven & Josipa, 2007)
• Liver, kidneys: +2 mmHg (Sven & Josipa, 2007)
• Brain: +6 mmHg (Sven & Josipa, 2007)
• Tumours: +10 to +37 mmHg (Boucher et al, 1990)

This fluid tends to be rather protein-poor, as firstly protein should not even be able to get out there (capillary endothelium barriers, etc), and secondly because the proteins that do end up out there tend to be immediately snaffled up by various membranes and reactive surfaces, i.e. by binding to bits of the interstitial space matrix they become part of the matrix and are no longer in solution. The oncotic pressure of this fluid is therefore rather low. Selen & Persson (1983) measured some rat values which ranged from  7.5 mmHg in the dehydrated animals to 2.8 mmHg in controls, giveing the average value of 5mm Hg which is usually quoted in textbooks.

Electrolyte composition of interstitial fluid

The electrolyte composition of interstitial fluid is largely the same as that of circulating plasma. The main difference is due to the Gibbs-Donnan effect which develops as the consequence of albumin, a large negatively charged molecule, being excluded from the interstitium by a semipermeable membrane. The result of this is a distribution of cations out of the interstitial fluid and into the plasma. This contributes about 7-10mmHg to the colloid osmotic pressure exerted by albumin. The difference in distribution between th interstitial and intravascular compartments is generally referred to as the Gibbs-Donnan Factor. For example, the value of this factor for monovalent cations is 0.95 (i.e. the sodium concentration in the interstitial fluid is 0.95 × the concentration in plasma). 

Functions of the interstitial fluid

Katz (1980) does an excellent job of discussing this "forgotten organ" in case anybody is interested in that sort of thing. He calls it the "policeman for every nutrient or waste molecule trafficking between the cell and the blood capillary". The main functions of the interstitial fluid can be summarised as:

• Conduit for the diffusion of small molecule nutrients and gases out of capillaries and to the cells
• Medium for the diffusion of medium molecules acting as paracrine signalling mediators
• Carrier for products of metabolism and definctioned tissue components which need to be cleared from the intercellular spaces 

When does interstitial fluid become lymph?

An excellent question which nobody can answer. Given the vast range of compositions seen in regionally varied interstitial fluid, it would be impossible to base one's definition on the content of the fluid itself. Most authors shrug their shoulders and say that you call it "lymph" when it's in the lymphatic vessel (Margaris & Black, 2012).

Anatomy of lymphatic vessels

Terminal lymphatic capillaries have generally been thought of as blind-ended tubes (Casley-Smith, 1980), but forty years later authors have generally been describing them as open-ended (Hansen et al, 2015), i.e. at the end of every terminal lymphatic capillary there is an open portal leading back into the circulatory system.

Like capillaries of the blood, these vessels are made up of a single layer of endothelial cells. When seen under the microscope, these vessels are usually collapsed, with irregular walls and discontinuous basement membranes (remember, the point is not to keep things out). These vessels are so floppy and shapeless that special anchoring filaments are required to keep them stretched open and patent, so they don't completely flatten and collapse when the interstitial pressure increases (Leak & Burke, 1968).

Even at this small scale (10 –60 μm diameter), these vessels have valve-like functions, if not actual valves. When pressure inside them increases,  they tend to impede back flow.  They start having actual physical valves (bicuspid ones) when they get to around 0.2mm in diameter (Margaris & Black, 2012). The larger of these vessels (pre-collecting and collecting lymphatics) can sometimes even have smooth muscle in their walls, with the promise of contractility and propulsion.

The lymphatic circulation

The lymphatic circulation network can be describes as follows, in order of what drains into what:

• Lymphatic capillaries (initial lymphatics)
• Pre-collecting lymphatics (primary valves)
• Collecting lymphatics (secondary bicuspid valves)
• Lymphangions is the term given to length of lymphatic vessels between two valves
• Lymph nodes
• Thoracic duct
• Back to the venous circulation

The lymphatic network is asymmetrical: most of the body (83% of total flow) drains into the thorac duct which empties into the junction of the left IJ and subclavian veins, whereas the right side of the head, right chest and right arm all empty into the right subclavian vein. 

The liver contributes about 50% of the total body lymph. Hepatic lymph is highly concentrated, up to 60g/L of protein is present, and it is rich in chylomicrons, which makes it easy to identify when a thoracic duct has been injured during surgery. 

• About 3 litres of lymph are produced every day
• That makes about 120ml per hour (100mls of which returns via the thoracic duct
• This volume increases massively with exercise

The flow of lymph is propelled by:

• Contraction of contractile smooth muscle of the lymphatic vessels, which is peristalsis-like
• Contraction of skeletal muscle surrounding the lymphatics
• Transmitted pulsation of neighbouring arterial structures
• Presence of one-way valves
• Decreased intrathoracic pressure associated with normal breathing
• Postural changes (i.e. with sleep)

Contents and properties of lymph

To quote Casley-Smith (1980), "In brief, the tissue lymphatic system is a leaky swamp through which material flows due to the vagaries of adjacent pressure changes". At the most basic level, it's just interstitial fluid with 20g/L of extra protein. Much of the extra protein is albumin. Not much fibrinogen makes it out into the lymphatics, because of how large its molecule is. In spite of this, even without any platelets and with minimal fibrinogen, lymph can still clot if given half a chance.

Additonally,  lymph accumulates other proteins in the intercellular space. These are not anything like the plasma proteins which are intentionally secreted into the bloodstream for some sort of purpose. These are products of lytic extracellular matrix processing, tissue remodeling, cellular metabolic activities and the products of cell death. The vast majority of these proteins will undergo phagocytosis by tissue macrophages and antigen presenting cells long before they reach the lymph nodes, let alone the venous circulation, which is why you never see them in the blood. A s you might expect, this means all of these proteins are going to be pretty  tissue-specific, i.e. the proteome of lymph has a lot of regional variation.