An aerosol-generating system, comprising: an aerosol-generating device comprising a power source; and a cartridge removably couplable to the aerosol-generating device, the cartridge comprising: a liquid storage portion configured to hold a liquid aerosol-forming substrate; and a heater assembly comprising: an electrical heating element configured to heat the liquid aerosol-forming substrate to form an aerosol; and a capillary body having a porous outer surface and being configured to convey the liquid aerosol-forming substrate to the electrical heating element, wherein the electrical heating element is fixed to and embedded in a porous end surface of the capillary body; the aerosol-generating system further comprising: a mouthpiece portion having an air inlet and an air outlet; wherein a first side of the heater assembly is arranged to receive the liquid aerosol-forming substrate and an airflow channel is disposed at an opposite side of the heater assembly to the first side and adjacent to the electrical heating element, the airflow channel defining a first airflow path that extends past the electrical heating element and is configured to convey the aerosol, wherein a second airflow path extends from the air inlet towards the heater assembly in a first direction, and a third airflow path extends from the heater assembly to the air outlet, in a second direction, wherein the first direction is opposite to the second direction, wherein the first airflow path provides a fluid connection between the second airflow path and the third airflow path, and wherein the power source of the aerosol-generating device is configured to supply power to the heater assembly. an aerosol-generating device comprising a power source; and a cartridge removably couplable to the aerosol-generating device, the cartridge comprising: a liquid storage portion configured to hold a liquid aerosol-forming substrate; and a heater assembly comprising: an electrical heating element configured to heat the liquid aerosol-forming substrate to form an aerosol; and a capillary body having a porous outer surface and being configured to convey the liquid aerosol-forming substrate to the electrical heating element, wherein the electrical heating element is fixed to and embedded in a porous end surface of the capillary body; a liquid storage portion configured to hold a liquid aerosol-forming substrate; and a heater assembly comprising: an electrical heating element configured to heat the liquid aerosol-forming substrate to form an aerosol; and a capillary body having a porous outer surface and being configured to convey the liquid aerosol-forming substrate to the electrical heating element, wherein the electrical heating element is fixed to and embedded in a porous end surface of the capillary body; an electrical heating element configured to heat the liquid aerosol-forming substrate to form an aerosol; and a capillary body having a porous outer surface and being configured to convey the liquid aerosol-forming substrate to the electrical heating element, wherein the electrical heating element is fixed to and embedded in a porous end surface of the capillary body; the aerosol-generating system further comprising: a mouthpiece portion having an air inlet and an air outlet; wherein a first side of the heater assembly is arranged to receive the liquid aerosol-forming substrate and an airflow channel is disposed at an opposite side of the heater assembly to the first side and adjacent to the electrical heating element, the airflow channel defining a first airflow path that extends past the electrical heating element and is configured to convey the aerosol, wherein a second airflow path extends from the air inlet towards the heater assembly in a first direction, and a third airflow path extends from the heater assembly to the air outlet, in a second direction, wherein the first direction is opposite to the second direction, wherein the first airflow path provides a fluid connection between the second airflow path and the third airflow path, and wherein the power source of the aerosol-generating device is configured to supply power to the heater assembly. a mouthpiece portion having an air inlet and an air outlet; wherein a first side of the heater assembly is arranged to receive the liquid aerosol-forming substrate and an airflow channel is disposed at an opposite side of the heater assembly to the first side and adjacent to the electrical heating element, the airflow channel defining a first airflow path that extends past the electrical heating element and is configured to convey the aerosol, wherein a second airflow path extends from the air inlet towards the heater assembly in a first direction, and a third airflow path extends from the heater assembly to the air outlet, in a second direction, wherein the first direction is opposite to the second direction, wherein the first airflow path provides a fluid connection between the second airflow path and the third airflow path, and wherein the power source of the aerosol-generating device is configured to supply power to the heater assembly.