In a quantitative model, heterogeneous agents trade securities of different maturity and riskiness. Risk-tolerant investors issue collateralized bonds to obtain leverage and to insure the risk-averse. Despite the presence of higher return assets, the most risk-tolerant hold long-maturity safe assets, which they value as good collateral. The value of collateralizability is high when safe bond quantities are low. Given measured variations in safe bond quantities, the model replicates the dynamics of lending volumes and generates large, volatile credit spreads and return predictability. I use the model to study the effects of central bank asset purchases.