The reproductive systems differ markedly between the sexes but share the purpose of producing gametes, supporting fertilization, and in females, nurturing developing offspring. In the male, paired testes lie in the scrotum, which regulates their temperature for spermatogenesis. Within each testis, seminiferous tubules contain the developing sperm cells. The epididymis is where sperm mature and are stored, and the vas deferens carries sperm during ejaculation, mixing with secretions from the seminal vesicles and prostate gland to form seminal fluid that is expelled through the urethra within the penis. In the female, the ovaries house oocytes within follicles and release them cyclically. The uterine tubes capture the ovulated oocyte and are typically the site of fertilization; the resulting embryo then travels to the uterus, where the endometrium provides a vascular lining for implantation and pregnancy. The vagina serves as the birth canal and receptacle for semen, while the external vulva protects the vaginal opening. The menstrual cycle prepares the uterine lining each month for possible pregnancy through coordinated hormonal changes.
The skin, the largest organ in the body, forms the integumentary system and serves as the body's primary interface with the environment. It consists of three layers with distinct functions. The outermost epidermis is keratinized stratified squamous epithelium that continuously sheds and renews itself, providing a waterproof barrier. Beneath it lies the dermis, a thicker layer of connective tissue containing blood vessels, nerves, sensory receptors, hair follicles, and the glands that produce sweat and sebum. The deepest layer, the hypodermis, is subcutaneous tissue dominated by adipose cells that cushion underlying structures, store energy, and insulate the body. Together these layers protect against pathogens and dehydration, regulate temperature through sweating and vasodilation, provide sensation, and contribute to vitamin D synthesis.
Two additional regulatory systems complement the organs already described. The lymphatic system returns interstitial fluid to the bloodstream as lymph, absorbs dietary fats from the digestive tract, and supports immunity through lymphoid organs and tissues, including lymph nodes that filter lymph, the spleen that filters blood, and the thymus where T lymphocytes mature; lymphatic vessels generally parallel the venous system. The endocrine system consists of ductless glands that release hormones directly into the bloodstream to regulate distant targets and maintain homeostasis. Examples include the pituitary gland, often called the master gland for its broad hormonal influence, the thyroid gland regulating metabolism, the adrenal glands mediating stress responses, the pancreas controlling blood glucose through insulin and glucagon, and the gonads producing sex steroids. Hormonal output is governed by feedback loops that keep internal conditions stable.