Injections are additions to the income in the circular flow. These are incomes which do not arise from spending by the domestic firms. These include the following:

Government expenditure in the economy (G)

Export (X)

Capital in flow

The above can be illustrated in the diagram on below

A investment expenditure (I)

Investment does not relate to the present consumer. It can come from the following: Borrowed money Contribution, profits, Money accumulated from the past.
Export (X)
Export earning constitute an injection into flows in from abroad and boosts the employment of the domestic factors of production
Government expenditure (G)
It includes goods and services provided free of charge or at subsidies prices by the state
Capital inflow (CO)
This takes place where foreigner bring capital in our economy and it contributes the increased level of production