20.3 Capillary Exchange

Capillary Exchange

Capillary exchange is the process of moving materials between blood and the surrounding tissues. Without it, cells wouldn't receive oxygen or nutrients, and metabolic waste would pile up. Three main mechanisms drive this exchange, and the balance of pressures across the capillary wall determines whether fluid moves out into tissues or gets pulled back into the blood.

Mechanisms of Capillary Exchange

Three distinct mechanisms move substances across the capillary wall, each handling different types of materials.

Diffusion is the most common mechanism. Solutes and gases move passively down their concentration gradients. Oxygen diffuses from blood (high concentration) into tissues (low concentration), while carbon dioxide moves the opposite direction. This happens through intercellular clefts (gaps between endothelial cells) and fenestrations (small pores in the capillary wall). The rate of diffusion is described by Fick's law, which states that diffusion increases with a larger concentration gradient, a larger surface area, and a thinner capillary wall.

Transcytosis handles larger molecules that can't slip through clefts or pores. Endothelial cells package substances like albumin and antibodies into vesicles, shuttle them across the cell, and release them on the other side. This is an active, energy-requiring process that allows selective transport of specific proteins while keeping the capillary barrier intact.

Bulk flow moves large volumes of fluid (along with dissolved solutes like water and electrolytes) between the capillary and interstitial space. Unlike diffusion, bulk flow is driven by pressure gradients rather than concentration gradients. It includes both filtration (fluid moving out) and reabsorption (fluid moving back in).

Pressures in Capillary Filtration

The direction and amount of fluid movement across the capillary wall are determined by four pressures collectively called Starling forces.

Hydrostatic pressures (physical pushing forces):

• Capillary hydrostatic pressure ($P_c$): Blood pressure inside the capillary that pushes fluid out into the interstitial space
• Interstitial fluid hydrostatic pressure ($P_i$): Pressure of fluid already in the tissue that pushes back against filtration (this value is typically very low or near zero)

Osmotic pressures (pulling forces created by proteins):

• Capillary colloid osmotic pressure ($\pi_c$): Created by plasma proteins (especially albumin) inside the capillary; pulls fluid into the capillary
• Interstitial colloid osmotic pressure ($\pi_i$): Created by proteins in the interstitial fluid; pulls fluid out of the capillary (this value is normally small)

The term oncotic pressure is used interchangeably with colloid osmotic pressure. Albumin is the primary protein responsible because of its high concentration in plasma.

These four forces combine into a single value called net filtration pressure (NFP):

$NFP = (P_c - P_i) - (\pi_c - \pi_i)$

• A positive NFP means the outward forces win, and fluid filters out of the capillary into the tissue
• A negative NFP means the inward forces win, and fluid is reabsorbed from the tissue back into the capillary

Filtration vs. Reabsorption in Capillaries

Pressures shift along the length of a capillary, so filtration and reabsorption happen at different ends.

At the arteriolar end, $P_c$ is relatively high (around 35 mmHg) because blood has just arrived from the arteriole. This high hydrostatic pressure overpowers the osmotic pull of plasma proteins, producing a positive NFP. Fluid, along with dissolved nutrients like glucose and amino acids, is pushed out into the interstitial space. This filtered fluid is called plasma ultrafiltrate, and it's how tissues receive nutrients and oxygen.

At the venular end, $P_c$ has dropped (to around 17 mmHg) because resistance along the capillary dissipates pressure. Now $\pi_c$ (about 25 mmHg) dominates, creating a negative NFP. Fluid and dissolved electrolytes are drawn back into the capillary. This reabsorption prevents excessive fluid buildup in tissues.

The balance isn't perfectly even. Filtration slightly exceeds reabsorption, resulting in a net loss of about 1–2 liters of fluid per day into the interstitial space. This excess fluid must be recovered by the lymphatic system.

Capillary Beds and Fluid Homeostasis

Capillary beds are dense networks of capillaries where exchange between blood and tissues actually takes place. Fluid homeostasis depends on keeping filtration and reabsorption in proper balance across these beds.

Three major factors influence this balance:

• Blood pressure: Higher blood pressure increases $P_c$, which increases filtration
• Plasma protein concentration: Lower protein levels reduce $\pi_c$, weakening reabsorption
• Lymphatic drainage: The lymphatic system must keep up with the net excess fluid that filtration produces

A disruption in any of these factors can tip the balance and lead to fluid accumulation in tissues.

Fate of Excess Interstitial Fluid

The lymphatic system is responsible for recovering the 1–2 liters of fluid that filtration leaves behind each day.

Here's how excess interstitial fluid returns to the bloodstream:

1. Fluid enters lymphatic capillaries (also called initial lymphatics). These have overlapping endothelial cells that act like one-way flaps, and anchoring filaments that pull them open when surrounding tissue swells with fluid.
2. Once inside, the fluid is now called lymph. Lymphatic vessels transport it through lymph nodes, where immune cells (lymphocytes and macrophages) filter it and screen for pathogens.
3. Lymph eventually drains back into the bloodstream through the thoracic duct (left side) and the right lymphatic duct (right side), both emptying into the subclavian veins.

Edema occurs when excess interstitial fluid accumulates faster than the lymphatic system can remove it, causing visible swelling. Common sites include the ankles (due to gravity) and lungs (pulmonary edema). Three main causes:

• Increased filtration: Venous obstruction raises $P_c$ (e.g., deep vein thrombosis, heart failure)
• Decreased reabsorption: Low plasma protein levels reduce $\pi_c$ (e.g., liver disease reducing albumin production, kidney disease causing protein loss)
• Impaired lymphatic drainage: Blocked or damaged lymph vessels can't remove fluid (e.g., lymphedema after lymph node removal in cancer surgery)

Severe edema disrupts normal tissue function by increasing the distance oxygen and nutrients must diffuse to reach cells, and it can compromise organ performance if fluid accumulates in critical areas like the lungs.