Chemical reactions within the cells convert nutrients into energy or convert simple substances into more complex ones.
The respiratory system consists of the esophagus, stomach, liver, gallbladder, pancreas, large intestine, small intestine, appendix, and rectum. Cellular respiration is the process carried out in the mitochondria in which oxygen is used to extract energy from nutrients. External respiration, or breathing, is the process carried out in the respiratory system in which oxygen is inhaled from the environment and carbon dioxide is exhaled. The trachea is a tube located in the abdomen with small openings called spiracles. Vertebrate lungs have different structures depending on the species:
- Amphibians: Relatively smooth wall, hollow sac-shaped organs with smooth walls.
- Reptiles: Divided into chambers by partitions.
- Birds: Connected to air sacs.
- Humans: Alveoli.
The excretory system eliminates waste products that can be toxic if levels become high. These waste products include carbon dioxide, ammonia, uric acid, mineral salts, and water. In different organisms, the excretory system can consist of structures such as Malpighian tubules, green glands, aorta, kidney, renal artery, vena cava, ureter, bladder, muscle ring, and urethra. The function of the excretory system is to filter blood to eliminate waste and keep useful substances, as well as to expel waste products as urine.
Reproduction is the process in which organisms are able to produce new living things. There are two main types of reproduction:
- Asexual reproduction: One progenitor produces a new, genetically identical living thing. The main types of asexual reproduction are budding and fragmentation.
- Sexual reproduction: Male and female produce descendants similar but not identical to them. Sexual reproduction involves the fusion of gametes. Gonads are the organs that produce the reproductive cells. Sexual dimorphism refers to the physical differences between male and female. Some animals, like earthworms and snails, are hermaphrodites, meaning they have both reproductive systems. Alternation of generations is a reproductive strategy that alternates between sexual and asexual reproduction. Fertilization is the fusion of gametes, which can occur externally or internally. Embryonic development refers to the processes that take place from the formation of the zygote to the birth of a new individual. Animals can be oviparous (embryo develops inside an egg outside the female body), viviparous (embryo develops inside the mother's body), or ovoviviparous (embryo develops inside an egg and inside the mother's body). Post-embryonic development is the process that takes place from birth until an individual is capable of reproduction. There are two types of post-embryonic development: direct development, where the offspring are similar in appearance to the adult, and indirect development, where the young, called larvae, go through profound changes until they become adults.