Analysis of the factors influencing peritubular PCO2 in the rat

We have developed a mathematical model to assess the relative contributions of several factors to the high CO2 partial pressures observed in rat peritubular capillaries. This model is based on a single nephron and focuses specifically on the CO2 partial pressure differences (delta PCO2) between peritubular capillaries and the afferent arteriole. The model is formulated by writing steady-state mass balances for the glomerulus, proximal tubule, and peritubular capillaries in addition to equilibrium relationships for CO2, HCO3-, blood protein buffers, and hemoglobin carbamino compounds. Principal input parameters include glomerular blood flow rate, rates of HCO3- and water reabsorption, and the rate of metabolic CO2 production. Under conditions representative of normal Munich-Wistar rats, the model predicts delta PCO2 to be 4.1 mmHg, in approximate agreement with experimental observations reported elsewhere. Metabolic CO2 production is responsible for roughly half of this predicted delta PCO2, the remainder being attributable to reabsorption processes. In examining the sensitivity of delta PCO2 to changes in physiological conditions, we consistently found it to be inversely related to glomerular blood flow rate. The influence of changes in HCO3- reabsorption on delta PCO2 is variable and highly dependent on the arterial acid-base status and the ratio of HCO3- reabsorption to water reabsorption.