This thesis makes two main contributions to the literature. The first is to establish the existence of a capital supply channel, in particular a bank lending channel of monetary policy transmission in the UK using a clean measure of bank loan supply. In this study we exploit the revealed debt preferences of debt issuing firms by using the Becker and Ivashina (2014) fixed effects framework to isolate the impact of credit supply. By conditioning the sample on non-financial firms whose debt issuance is observed, we are able to eliminate the effects of credit demand and to isolate a clean measure for bank loan supply. In this thesis, we find that the tendency by unconstrained, non-financial firms to substitute corporate bonds for bank loans at different points of the financial cycle reflects changes in bank loan supply. We also find that the patterns of substitutability are consistent among more granular classifications of heterogeneous debt. Our results reveal that among unconstrained firms, the proportion of new bank loan issuance declines, while the proportions of corporate bonds and program debt issuance tend to increase, when faced with unfavourable credit market conditions. We then create a loan to bond substitution measure based on observed substitution behaviour of unconstrained firms. We find that this measure explains the out of sample bank loan issuance behaviour of constrained firms. As a result we conclude that the measure is able to cleanly capture changes in bank loan supply. We extend the study to examine the impact of bank loan supply on the financing, hiring and investment decisions of UK non-financial corporations. We find that bank loan supply disruptions significantly and disproportionately affect the hiring and inventory investment decisions of bank dependent firms relative to those of non-bank dependent firms. The propensity to invest or hire among bank dependent UK non-financial firms declines relative to non-bank dependent firms when bank loan supply deteriorates. Moreover, the fixed investment decisions of non-bank dependent firms tend to decline following adverse bank loan supply shocks. These results confirm the existence of a bank lending channel among UK non-financial firms, and the findings are in line with the narrow credit view of monetary policy transmission. Our second central contribution is to analyse the impact of orthogonal QE shocks, credit supply shocks, credit demand shocks, and monetary policy shocks on the aggregate debt issuance behaviour of UK non-financial firms. Using structural vector error correction models (SVECM), we show that QE shocks increase corporate bond issuance and compress term spreads, but have no effect on the policy rate. Moreover, we observe that unexpected increases in the monetary policy rate lead to a decline in corporate bonds in the short term. While credit supply shocks move aggregate bank lending and aggregate corporate bond issuance in the same direction, corporate bond issuance responds with a lag to fluctuation in credit supply. This implies that adverse credit supply shocks may produce amplified negative effects on capital supply as both corporate bonds and bank loan decline. We also establish a counterfactual for corporate bonds and bank loan issues based on our structural model. We find that the QE policies result in the Bank of England averting a decline in corporate bond issuance of between 3% and 10% during the QE period. Our findings in this thesis point towards the existence of a portfolio balance channel of QE that operates in the UK corporate bond markets during the QE period.