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Antidiuretic hormone (ADH) is produced in the hypothalamus and stored in the posterior pituitary gland. When blood osmolarity rises, ADH is released and increases water reabsorption in the distal tubule and collecting duct and reduces urine volume.

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Aldosterone is produced in the adrenal cortex to maintain a high blood volume and pressure by promoting reabsorption of NaCl and water in the distal tubules of the kidney.

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1) Secretory cells actively transport salt from the blood into the secretory tubules by countercurrent exchange.

The 1) secretory tubules make up the 1) salt glands, which empty via a duct into the nostrils.

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Salvin's albatrosses (Diomeda cauta salvini), birds that can drink seawater with no ill effects.

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Key functions of excretory systems include:

• Filtration Water and solutes are forced by blood pressure across the membranes of capillaries to produce a filtrate.
  
• Reabsorption. Valuable solutes such as glucose and amino acids are reclaimed from the filtrate by active transport.
  
• Secretion. Toxins and excess ions are extracted from body fluids to the filtrate by active transport.
  
• Excretion. The filtrate leaves the system.

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The excretory system of mammals centers on the pair of kidneys.
Urine drains from each kidney through the ureter into the urinary bladder.

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Blood enters the kidney from the outer renal cortex and filters through the inner renal medulla of functional units called nephrons . Urine is then passed from the renal pelvis to the ureter.

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Malpighian tubules of insects.

Malpighian tubules are outpocketings of the digestive tract that are immersed in hemolymph (circulatory fluid that contains the transport molecule heme).

The tubules produce nitrogenous wastes in the form of nearly dry uric acid.

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Metanephridia of an earthworm.

Each segment of the worm contains a pair of metanephridia, which collect coelomic fluid from the adjacent segment.

Fluid enters the nephrostome and passes through a tubule; the filtrate is released to the outside through the nephridiopore.

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The nephron consists of a single long tubule that ends in a swollen Bowman's capsule surrounded by a ball of capillaries called the glomerulus.

Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the Bowman's capsule.

Blood cells and large molecules such as proteins stay are too large to pass through capillary membranes and stay in the blood stream.

The filtrate consists of water and small solutes such as salts, glucose, amino acids, vitamins, and urea.

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The filtrate is passed through a series of tubules to produce urine.

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1. Proximal tubule.
   H⁺ and ammonia (NH₃) may be secreted to maintain pH. NaCl and water, along with bicarbonate (HCO3₃^-) ions. and nutrients such as glucose, amino acids, and K⁺ are reabsorbed.
   
2. Descending limb of the loop of Henle.
   Reabsorption of water continues by osmosis.
   
3. Ascending limb of the loop of Henle.
   Reabsorption of NaCl continues in this segment.
   
4. Distal tubule.
   Regulation of pH continues by the secretion of H⁺ and reabsorption of bicarbonate (HCO3₃^-).
   
5. Collecting duct.
   More NaCl, along with some urea, is reabsorbed to help maintain a high osmolarity in the inner medulla. Hormones also control reabsorption of water.

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Nitrogenous wastes are the nitrogen–containing breakdown products of proteins and nucleic acids.

The main nitrogen product of metabolism is ammonia (NH₃), which is toxic.

Some animals excrete ammonia directly, but many species first convert the ammonia to less toxic compounds that require energy to produce.

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Protonephridia: the flame–bulb system of a planarian.

A network of tubules end in a flame bulb, where beating cilia filter body fluid through the tubules and out through nephridiopores.

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