Capillary Circulation and Cardiac Output Control

Vocabulary and physiological concepts covering the interaction between cardiac and peripheral factors, Starling forces, and capillary-interstitial fluid exchange.

Steady State Intercept Point

The point where cardiac function and venous return (VR) curves intersect, representing a state where $VR = CO = 5\,L/min$ and RAP is maintained at $0\,mmHg$.

Cardiac Output (CO) Control Factors

Controlled by preload, afterload, inotropy, and heart rate (HR).

Venous Return (VR) Control Factors

Controlled by mean systemic filling pressure (MSFP), resistance to venous return (RVR), and right atrial pressure (RAP).

Cardiac Output Curve (Muscular Exercise)

Shifts up and to the left due to increased sympathetic activity (increasing heart rate and inotropy) and decreased total peripheral resistance via vasodilation.

Venous Return Curve (Muscular Exercise)

Shifts up and to the right due to increased sympathetic-mediated vein contraction (increasing MSFP), muscle pumping, and arteriolar dilation.

Diffusion

Quantitatively the most important mechanism for the exchange of materials between capillaries and interstitial fluid.

Vesicular Transport

The mechanism utilized in capillary circulation for the exchange of macromolecules.

Capillary Filtration Coefficient (k)

A value proportionate to the permeability of the capillary and the area available for filtration.

Outward Starling Forces

Forces that move fluid from capillaries into the interstitium, consisting of Capillary hydrostatic pressure ($Pc$) and Interstitial colloid osmotic pressure ($\pi_i$).

Inward Starling Forces

Forces that move fluid from the interstitium into the capillaries, consisting of Interstitial hydrostatic pressure ($Pi$) and Capillary colloid osmotic pressure ($\pi_c$).

Trans-Capillary Filtration Equation

The formula for bulk flow fluid movement defined as: $\text{Fluid movement} = k [(Pc + \pi_i) - (Pi + \pi_c)]$.

Arteriolar End Net Force (Muscle Capillary)

A net force of $11\,mmHg$ calculated as $(37 + 0) - (1 + 25)$, which moves fluid out into the interstitial space.

Venular End Net Force (Muscle Capillary)

A net force of $-9\,mmHg$ calculated as $(17 + 0) - (1 + 25)$, which moves fluid into the capillary from the interstitial space.

Intestines and Lungs Capillaries

Examples of specific tissues where fluid moves into the capillaries (absorption) along their entire length.

Renal Glomeruli Capillaries

Examples of specific capillaries where fluid moves out (filtration) along their entire length.

Daily Capillary Filtration Volume

Approximately $24\,L$ of fluid are filtered per day, with $85\%$ being reabsorbed at the venous end and the remainder returning via lymphatics.