Hormones are important messages both within the brain and between the brain and the body.

In addition to the nervous system, the endocrine system is a major communication system of the body. While the nervous system uses neurotransmitters as its chemical signals, the endocrine system uses hormones. The pancreas, kidneys, heart, adrenal glands, gonads, thyroid, parathyroid, thymus, and even fat are all sources of hormones. The endocrine system works in large part by acting on neurons in the brain, which controls the pituitary gland. The pituitary gland secretes factors into the blood that act on the endocrine glands to either increase or decrease hormone production. This is referred to as a feedback loop, and it involves communication from the brain to the pituitary to an endocrine gland and back to the brain. This system is very important for the activation and control of basic behavioral activities, such as sex; emotion; responses to stress; and eating, drinking, and the regulation of body functions, including growth, reproduction, energy use, and metabolism. The way the brain responds to hormones indicates that the brain is very malleable and capable of responding to environmental signals.

The brain contains receptors for thyroid hormones (those produced by the thyroid) and the six classes of steroid hormones, which are synthesized from cholesterol — androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, and vitamin D. The receptors are found in selected populations of neurons in the brain and relevant organs in the body. Thyroid and steroid hormones bind to receptor proteins that in turn bind to DNA and regulate the action of genes. This can result in long-lasting changes in cellular structure and function.

The brain has receptors for many hormones; for example, the metabolic hormones insulin, insulin-like growth factor, ghrelin, and leptin. These hormones are taken up from the blood and act to affect neuronal activity and certain aspects of neuronal structure.

In response to stress and changes in our biological clocks, such as day and night cycles and jet lag, hormones enter the blood and travel to the brain and other organs. In the brain, hormones alter the production of gene products that participate in synaptic neurotransmission as well as affect the structure of brain cells. As a result, the circuitry of the brain and its capacity for neurotransmission are changed over a course of hours to days. In this way, the brain adjusts its performance and control of behavior in response to a changing environment.

Hormones are important agents of protection and adaptation, but stress and stress hormones, such as the glucocorticoid cortisol, can also alter brain function, including the brain’s capacity to learn. Severe and prolonged stress can impair the ability of the brain to function normally for a period of time, but the brain is also capable of remarkable recovery.

Reproduction in females is a good example of a regular, cyclic process driven by circulating hormones and involving a feedback loop: The neurons in the hypothalamus produce gonadotropin-releasing hormone (GnRH), a peptide that acts on cells in the pituitary. In both males and females, this causes two hormones — the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) — to be released into the bloodstream. In females, these hormones act on the ovary to stimulate ovulation and promote release of the ovarian hormones estradiol and progesterone. In males, these hormones are carried to receptors on cells in the testes, where they promote spermatogenesis and release the male hormone testosterone, an androgen, into the bloodstream. Testosterone, estrogen, and progesterone are often referred to as sex hormones.

In turn, the increased levels of testosterone in males and estrogen in females act on the hypothalamus and pituitary to decrease the release of FSH and LH. The increased levels of sex hormones also induce changes in cell structure and chemistry, leading to an increased capacity to engage in sexual behavior. Sex hormones also exert widespread effects on many other functions of the brain, such as attention, motor control, pain, mood, and memory.

Sexual differentiation of the brain is caused by sex hormones acting in fetal and early postnatal life, although recent evidence suggests genes on either the X or Y chromosome may also contribute to this process. Scientists have found statistically and biologically significant differences between the brains of men and women that are similar to sex differences found in experimental animals. These include differences in the size and shape of brain structures in the hypothalamus and the arrangement of neurons in the cortex and hippocampus. Sex differences go well beyond sexual behavior and reproduction and affect many brain regions and functions, ranging from mechanisms for perceiving pain and dealing with stress to strategies for solving cognitive problems. That said, however, the brains of men and women are more similar than they are different.

Anatomical differences have also been reported between the brains of heterosexual and homosexual men. Research suggests that hormones and genes act early in life to shape the brain in terms of sex-related differences in structure and function, but scientists are still putting together all the pieces of this puzzle.

The kidneys are bean-shaped organs that serve several essential regulatory roles in vertebrate animals. They remove excess organic molecules (e.g., glucose) from the blood, and it is by this action that their best-known function is performed: the removal of waste products of metabolism (e.g., urea, though 90% of this is reabsorbed along the nephron.) They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintaining salt and water balance). They serve the body as a natural filter of the blood, and remove water soluble wastes, which are diverted to the urinary bladder. In producing urine, the kidneys excrete wastes such as urea and ammonium, and they are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol, erythropoietin, and the enzyme renin, the last of which indirectly acts on the kidney in negative feedback.

Located at the rear of the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes urine into a ureter, itself a paired structure that empties into the urinary bladder.

Renal physiology is the study of kidney function, while nephrology is the medical specialty concerned with kidney diseases. Diseases of the kidney are diverse, but individuals with kidney disease frequently display characteristic clinical features. Common clinical conditions involving the kidney include the nephritic and nephrotic syndromes, renal cysts, acute kidney injury, chronic kidney disease, urinary tract infection, nephrolithiasis, and urinary tract obstruction. Various cancers of the kidney exist; the most common adult renal cancer is renal cell carcinoma. Cancers, cysts, and some other renal conditions can be managed with removal of the kidney, or nephrectomy. When renal function, measured by glomerular filtration rate, is persistently poor, dialysis and kidney transplantation may be treatment options. Although they are not normally harmful, kidney stones can be painful, and repeated, chronic formation of stones can scar the kidneys. The removal of kidney stones involves ultrasound treatment to break up the stones into smaller pieces, which are then passed through the urinary tract. One common symptom of kidney stones is a sharp to disabling pain in the medial/lateral segments of the lower back or groin.

The lymphatic system is part of the circulatory system, comprising a network of lymphatic vessels that carry a clear fluid called lymph (from Latin lympha meaning water) directionally towards the heart. The lymphatic system was first described in the seventeenth century independently by Olaus Rudbeck and Thomas Bartholin. Unlike the cardiovascular system the lymphatic system is not a closed system. The human circulatory system processes an average of 20 litres of blood per day through capillary filtration which removes plasma while leaving the blood cells. Roughly 17 litres of the filtered plasma get reabsorbed directly into the blood vessels, while the remaining 3 litres are left behind in the interstitial fluid. One of the main functions of the lymph system is to provide an accessory route for these excess 3 litres per day to get returned to the blood.

The other main function is that of defense in the immune system. Lymph is very similar to blood plasma but contains lymphocytes and other white blood cells. It also contains waste products and debris of cells together with bacteria and protein. Associated organs composed of lymphoid tissue are the sites of lymphocyte production. Lymphocytes are concentrated in the lymph nodes. The spleen and the thymus are also lymphoid organs of the immune system. The tonsils are lymphoid organs that are also associated with the digestive system. Lymphoid tissues contain lymphocytes, and also contain other types of cells for support. The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which also includes the bone marrow, and the lymphoid tissue associated with the digestive system.

The blood does not come into direct contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles), or by lymph hearts in some animals. The organization of lymph nodes and drainage follows the organization of the body into external and internal regions; therefore, the lymphatic drainage of the head, limbs, and body cavity walls follows an external route, and the lymphatic drainage of the thorax, abdomen, and pelvic cavities follows an internal route.Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins, near their junction with the internal jugular veins.