The application relates to taking a sample of liquid contained in a flexible closed container, for example, a bag for collecting urine or blood. The problem to be resolved relates to piercing the wall of the container with a non- cutting piece, whilst obtaining a precise and clean opening for accessing the liquid and preventing any unintended flow or spill of liquid. This problem is resolved according to the present invention by a flexible bag (13, 14) for liquid, particularly a urine or blood bag which is characterised in that it contains a device (9) for taking a sample of liquid from said flexible bag. The sampling device (9) comprises a circular opening (1), arranged so that this opening is accessible from a first side of the device. The device (9) further comprises a support structure (2) on its other side, said support structure (2) comprising at least one support element (3), whose function is to distribute forces, and which is positioned at a distance from the plane of said opening (1).La solicitud se refiere a tomar una muestra de un líquido contenido en un recipiente cerrado flexible, por ejemplo, una bolsa para recolectar orina o sangre. El problema a resolverse se refiere a perforar una pared del recipiente con una pieza sin corte, mientras se obtiene una abertura limpia y precisa para acceder al líquido y evitar cualquier flujo no intencional o derrame de líquido. Este problema se resuelve de acuerdo con la presente invención por una bolsa flexible (13, 14) para líquido, particularmente una bolsa para sangre u orina que se caracteriza porque contiene un dispositivo (9) para tomar una muestra de líquido a partir de dicha bolsa flexible. El dispositivo de muestreo (9) comprende una abertura circular (1), configurada de manera que esta abertura es accesible desde una primer lado del dispositivo. El dispositivo (9) además comprende una estructura de soporte (2) en su otro lado, dicha estructura de soporte (2), que comprende al menos un elemento de soporte (
