Filtration and Reabsorption in Capillary Beds

Pressures

There’s a hydrostatic pressureThe force exerted by a fluid, such as the pressure of blood pushing against the walls of capillaries, which is the pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. of waterThe universal solvent essential for life.. It’s kind of like if you had a water balloon. That pressure is the water exerting force from the inside out of the balloon. And there’s also something called colloidA mixture where small particles are dispersed but not dissolved in a liquid. osmotic. A colloid is just a substance in water that attracts water. Basically, these are solutes. That’s what a colloid is. Colloids attract water.  We measure colloids in your blood by the blood colloid osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments.. So, let’s say that you have one of these house plants. 

Movements

In the capillary below in the picture, the hydrostatic pressure is present. This pressure of water pushes water and solutes out of the capillary to the surrounding tissues. The colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries attracts that water back. There are two kinds of exchange. There’s filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre, which is the blue arrow pushing solutes and water from the capillary to the surrounding tissues. And then there is reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p, which is mostly just water. That’s the Green Arrow, representing the attraction back into the capillary of fluids. The diagram also shows a capillary with surrounding tissue cellsThe basic structural and functional units of life.. There is open space between the capillary and the tissue cells. This open space would be the interstitial fluids.

Filtration is depicted on the left side of this diagram. The arrows show substances moving from the capillary to the interstitial fluidThe fluid surrounding cells within tissues.. They also indicate the movementA fundamental property of life involving motion of the body or its parts. into the intracellular fluidThe fluid inside a cell, primarily composed of cytosol. of the tissue cells. Filtration is the movement of plasmaThe liquid component of blood. components into the interstitial fluids. It pushes water out, but most of the larger solutes stay in. Some solutes, like glucoseA simple sugar that is the main source of energy for cells. and amino acids, can escape. This process is driven by the hydrostatic pressure of water.

Arrows on the right side of the diagram show reabsorption. They represent substances that move from the interstitial fluids back into the plasma. This is when mainly fluid moves back into the capillary. Water is attracted to the solutes inside the capillary, which is driven by the colloid osmotic pressureThe force exerted by water moving across a membrane due to osmosis..

Net Filtration Pressure

The diagram in your book illustrates blood entering a capillary from an arteriole. As blood enters the capillary bed, filtration occurs, pushing substances out of the capillary. Later, reabsorption takes place as substances move back into the capillary. This creates a net filtration pressure—the difference between filtration and reabsorption pressures. Since filtration exceeds reabsorption, the excess fluids are managed by the lymphatics. The lymphatic system is a drainage system. It takes up the difference, including larger substances like white blood cells and proteinsLarge molecules made of amino acids with various functions in the body.. It returns them to circulation at the subclavian vein.

Tissue Perfusion

Filtration and reabsorption are crucial for tissue perfusion—the exchange of oxygen, glucose, and water into tissues. Good tissue perfusionThe delivery of oxygen and nutrients to tissues via the blood supply. ensures proper distribution of oxygen, carbon dioxide, and hormones. It removes toxins and pathogens via the lymphatic system because these cannot re-enter capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues.. A common issue arises from an imbalance in filtration and reabsorption. This issue is edemaExcess fluid in interstitial spaces., caused by excess fluid in the tissues. This can result from increased vessel permeability, a lack of colloids or proteins in the capillaries, or elevated hydrostatic pressure. When colloids are insufficient, water cannot be reclaimed through reabsorption, leading to swelling. Bruising, starvation, or high blood pressure can exacerbate this condition. Prolonged issues may result in peripheral congestion, often associated with problems in the right side of the heart.