Recognize the cells and organs of the immune system and their functions. The key primary lymphoid organs of the immune system are the thymus and bone marrow, and secondary lymphatic tissues such as spleen, tonsils, lymph vessels, lymph nodes, adenoids, and skin and liver. Leukocytes (white blood cells) act like independent, single-celled organisms and are the second arm of the innate immune system.
Timing. Your dog can come into her first heat any time between 6 and 24 months. According to Webvet.com, small dogs come into heat sooner than larger breeds. Typically she will have two estrus cycles, or heats, a year, and each one lasts for between 18 to 24 days.
Dog in Labor Sign & Stages. The fetuses must have a normal size in relationship with the pelvis and present a normal position; their legs must be in front of the head, which comes first. The duration of labor can be from 2 to 12 hours, but it can also last up to 36 hours in first timers. The dog is anxious, agitated, she digs and sometimes vomits. The interval between two births varies between 10 and 60 minutes. This corresponds to the phase of expulsion of the placenta, which is done after every birth.