Medical apparatus for supplying a medication fluid into an organism and for detecting a substance of the organism, the medical apparatus comprising: a casing (101, 201, 301, 401, 501, 601) having a chamber (143, 243, 343, 443) for accommodating the medication fluid (133, 233, 333, 433) and having an opening (145, 245, 246, 345, 445) in fluid communication with the chamber; a cannula (103, 203, 303, 403) having a lumen (161, 261, 361, 461) being in fluid communication with the chamber; an insertion needle (105, 205) having a receptacle, wherein at least a portion of the insertion needle is removably arrangeable within the lumen of the cannula; a sensor system (107, 207, 307) for detecting the substance of the organism, wherein the receptacle of the insertion needle is configured to removably at least partially receive the sensor system; characterised by a seal (153, 253, 353, 453), wherein the seal is adapted to annularly sealingly surround the insertion needle, when the insertion needle is arranged in the opening (145, 245, 246, 345, 445), wherein the seal is adapted to annularly sealingly surround a portion (139, 239, 339, 439) of the sensor system, when the insertion needle is removed from the opening and the portion (139, 239, 339, 439) of the sensor system is arranged in the opening (145, 245, 246, 345, 445), wherein the seal is adapted to seal the chamber from fluid communication via the opening (145, 245, 246, 345, 445) when the insertion needle is arranged in the opening, wherein the seal is adapted to seal the chamber from fluid communication via the opening (145, 245, 246, 345, 445) when the insertion needle is removed from the opening and a portion (139, 239, 339, 439) of the sensor system is arranged in the opening. a casing (101, 201, 301, 401, 501, 601) having a chamber (143, 243, 343, 443) for accommodating the medication fluid (133, 233, 333, 433) and having an opening (145, 245, 246, 345, 445) in fluid communication with the chamber; a cannula (103, 203, 303, 403) having a lumen (161, 261, 361, 461) being in fluid communication with the chamber; an insertion needle (105, 205) having a receptacle, wherein at least a portion of the insertion needle is removably arrangeable within the lumen of the cannula; a sensor system (107, 207, 307) for detecting the substance of the organism, wherein the receptacle of the insertion needle is configured to removably at least partially receive the sensor system; a seal (153, 253, 353, 453), wherein the seal is adapted to annularly sealingly surround the insertion needle, when the insertion needle is arranged in the opening (145, 245, 246, 345, 445), wherein the seal is adapted to annularly sealingly surround a portion (139, 239, 339, 439) of the sensor system, when the insertion needle is removed from the opening and the portion (139, 239, 339, 439) of the sensor system is arranged in the opening (145, 245, 246, 345, 445), wherein the seal is adapted to seal the chamber from fluid communication via the opening (145, 245, 246, 345, 445) when the insertion needle is arranged in the opening, wherein the seal is adapted to seal the chamber from fluid communication via the opening (145, 245, 246, 345, 445) when the insertion needle is removed from the opening and a portion (139, 239, 339, 439) of the sensor system is arranged in the opening.