All of these are functions of the kidney except for excretion of proteins. In fact, the kidney should never secrete protein or glucose (sugars) unless there is a disease state present. For example, in diabetics who have damaged kidneys (diabetic nephropathy), there is protein in the urine. To a degree, the amount of protein in the urine determines how bad their kidney disease is.

Glucose is initially filtered at the glomerulus but then all reabsorbed in the proximal convoluted tubule. If it appears in the urine, it is termed glucosuria and indicates that the patient has diabetes (impaired insulin secretion or sensitivity that causes elevated levels of glucose in the blood).

Antidiuretic hormone (ADH) is useful if an individual becomes dehydrated. When dehydrated, ADH will make the collecting ducts permeable to water. This allows water to be reabsorbed into the body from the filtrate in the kidneys. By moving water from the filtrate to the body, the urine becomes more concentrated and the body retains water. This process helps to prevent dehydration.

Oxytocin and antidiuretic hormone (ADH) are both produced by the hypothalamus and released from the posterior pituitary. In women, oxytocin is important during childbirth because it regulates uterine contractions. It also acts as a hormone in the brain and is important to sexual arousal, recognition, and trust. Antidiuretic hormone, also known as vasopressin, is an important hormone that induces increased water reabsorption in the kidney to increase blood volume.

Thyroid hormone (thyroxine or T4) and calcitonin are both produced in the thyroid. Thyroid hormone is extremely important in the regulation of metabolism. Calcitonin in the bones inhibits the activity of osteoclasts, which break down bone matrix. This decreases the amount of calcium in the blood. In the kidney, calcitonin inhibits the reabsorption of calcium. Thyroid-stimulating hormone is used to stimulate the thyroid to release thyroid hormone, and is produced by the anterior pituitary.

Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland and plays an important role in reproduction, growth, and development, particularly during puberty. 

The nephrons of the kidney serve to balance and concentrate the filtrate in order to generate urine and eliminate nitrogenous wastes, while conserving valuable nutrients and ions. Filtration separates fluids from the cells of the blood in Bowman's capsule. Reabsorption refers to the process by which particulates and ions are removed from the filtrate and returned to the blood, while secretion refers to the addition of compounds from the blood into the filtrate. These processes are critical to establishing the proper urine composition.

While much of reabsorption and secretion occurs passively due to the manipulation of ion gradients, it is important to note that the generation of ion gradients usually requires active transport and the expenditure of energy. As such, the excretory system does utilize energy and involves active transport.

Filtrate that enters the renal corpuscle in Bowman’s capsule is forced in due to hydrostatic blood pressure; this is a passive mechanism. However, as the filtrate is reabsorbed in the proximal convoluted tubule (PCT), some molecules exit the tubule via active transport mechanisms. Later, as filtrate travels through the descending limb of the loop of Henle, water moves out passively with its concentration gradient, as do sodium ions in the thin ascending limb. In the thick ascending limb, the concentration of salt has increased in the medulla, causing salt to exit the tubule via active transport mechanisms. Finally, in the distal convoluted tubule and collecting duct, water reabsorption occurs passively through aquaporins, but some reabsorption and secretion may occur through active transport.

Active transport occurs in the proximal convoluted tubule, ascending loop of Henle, distal convoluted tubule, and collecting duct.

Renin is released from these cells in response to decreased arteriole blood pressure. The renin-angiotensin-aldosterone system is then activated to maintain blood pressure by various ways including stimulating thirst, reabsorbing sodium in the kidney, and increasing vasopressin release.

Aldosterone is released by the adrenal glands and works to increase sodium retention which increases water retention and thereby increases blood volume to increase blood pressure. If sodium flows back into the cells from the kidney tubules (so that it is not secreted as urine), water also flows back as an osmotic effect. Additionally, aldosterone also stimulates thirst and decreases vasopressin secretion, all to increase blood volume to increase blood pressure.

The spleen is like a giant lymph node, and it is organized in a somewhat similar manner. Although it can be surgically removed if it is damaged, such patients are at life-long risk of death from fairly ordinary infectious processes. The spleen is a reservoir of immune competence. Blood passes through the spleen for exposure to white blood cells. When the white blood cells detect antigens or foreign particles in the blood, they initiate the immune response. The spleen is essentially a screening center to check the blood for contaminants.

Adaptive immunity occurs when antibodies are produced as a result of exposure to a pathogen or immunization. These antibodies are specific for the particular microorganism and memory cells are produced. Cell-mediated immunity is a direct form of defense based on the action of lymphocytes to attack foreign cells and destroy them. Congenital immunity is immunity one is born with. This may result from antibodies received from the mother's blood. Innate immunity is not pathogen-specific and includes the secretion of proteins and the activities of natural killer cells. Passive immunity involves the introduction of preformed antibodies into an unprotected individual. This may occur through infusion of immune globulin or antibodies that pass from the mother to the fetus through the placenta.