Rebecca is at home sick. This involves the lymphatic system which is currently fighting the virus in her body. Lymph, a clear fluid that contains infection-fighting white blood cells, circulates the lymphatic system being filtered at the many lmyph node sites in the body. Here, lymph enters and white blood cells immediately attack any bacteria or viruses present.
Rebecca tries to breathe through her nose but quickly realizes that she is congested. The congestion is an immune response to natural killer cells doing their job to fight the infection. There is extra mucus, a substance that lines all cavities of the body, in her septum, and her nasal cavities, specifically her sinus, have inflamed inner linings.
Rebecca tries to reduce her congestion by taking medicine. The medicine is going through her digestive system which is distributing the nutrients into her bloodstream. The medicine reaches her stomach where it is broken down into chyme, a mixture of broken down food and gastric juices. It then moves to the small intestine where peristalsis occurs, and the food is mixed up into a thick fluid. The lining of the small intestine absorbs nutrients and transfers them into the bloodstream.
Rebecca then goes to the bathroom and finds out she's gotten her period. This directly involves the reproductive system. Each month, her egg begins dissolving if it is not fertilized. Currently, her uterine lining is shedding and the blood flows from the opening of her cervix and out of her vagina. Menstruation usually lasts from 3-7 days, and Rebecca may experience PMS and cramps.
Rebecca goes to the kitchen because she's suddenly craving potato chips, but finds the bag empty. This craving is due to the endocrine system. Hormones and glands of the endocrine system regulate metabolism and digestion. By continuously snacking all day, she is spiking her insulin levels repeatedly which causes cravings due to the lack of insulin when she doesn't eat. The pancreas secreting both insulin and glucagon secretes the different substance at different times. When the levels of insulin are low, the pancreas secretes glucagon, which metabolizes fat for energy. However the pancreas will never release the fat burning enzyme if she is always spiking her insulin levels.
Rebecca goes to the grocery and gets chips, medicine, and tampons. She is currently using her muscular system to carry the bag. When a muscle contracts, the filaments inside muscle fibers slide. The myosin filaments' (thick) head attach and detach to the actin filaments' (thin) binding sites. When attaching, the filaments slide towards the center of the sarcomere, tiny contractile unit. This whole sliding process is triggered by calcium ions initiating the bonding of myosin and actin.