Question: Where does diffusion occur in the excretory system?

Disambiguated questions and answers:
context: Invertebrates lack a liver, but most terrestrial groups, like insects, possesses a number of blind guts that serve the similar functions. Marine invertebrates do not need the ammonia conversion of the liver, as they can usually expel ammonia directly by diffusion through the skin.
question: Where does diffusion occur in the marine invertebrate excretory system?
short_answers: through the skin
wikipage: Excretory system

context: In mammals, the skin excretes sweat through sweat glands throughout the body. The sweat, helped by salt, evaporates and helps to keep the body cool when it is warm. In amphibians, the lungs are very simple, and they lack the necessary means to the exhale like other tetrapods can. The moist, scale-less skin is therefore essential in helping to rid the blood of carbon dioxide, and also allows for urea to be expelled through diffusion when submerged.
question: Where does diffusion occur in the amphibian excretory system?
short_answers: skin
wikipage: Excretory system

context: One of the main functions of the lungs is to diffuse gaseous wastes, such as carbon dioxide, from the bloodstream as a normal part of respiration.
question: Where does diffusion of oxygen and carbon dioxide occur in the human excretory system?
short_answers: Lungs
wikipage: Excretory system

context: The kidneys help maintain the water and salt level of the body. Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone (ADH), resulting in water reabsorption by the kidney and an increase in urine concentration. The two factors work together to return the plasma osmolality to its normal levels.
question: Where does diffusion of salt, water, and waste occur in the excretory system?
short_answers: Kidneys
wikipage: Renal physiology

context: No context provided
question: Where does diffusion of calcium occur in the excretory system?
short_answers: Intestines

Long answers:
knowledge: content: Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) from a region of higher concentration to a region of lower concentration.
wikipage: Diffusion
long_answer: Diffusion of oxygen and carbon dioxide occur in the human excretory system in the lungs,  diffusion of salt, water, and waste occur in the kidneys and diffusion of calcium occur in the intestines. In marine invertebrates, diffusion occurs through the skin. And in the amphibian excretory system diffusion also occurs through the skin when submerged.

knowledge: content: The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. 
wikipage: Excretory system

content: The kidneys remove from the blood the nitrogenous wastes such as urea, as well as salts and excess water, and excrete them in the form of urine. 
wikipage: Excretory system

content: One of the main functions of the lungs is to diffuse gaseous wastes, such as carbon dioxide, from the bloodstream as a normal part of respiration.
wikipage: Excretory system

content: Invertebrates lack a liver, but most terrestrial groups, like insects, possesses a number of blind guts that serve the similar functions. Marine invertebrates do not need the ammonia conversion of the liver, as they can usually expel ammonia directly by diffusion through the skin.
wikipage: Excretory system

content: Most excretion of excess calcium is via the bile and feces, because the plasma calcitriol levels (which ultimately depend on the plasma calcium levels) regulate how much of the biliary calcium is reabsorbed from the intestinal contents.
wikipage: Calcium metabolism Intestine 2
long_answer: The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The kidneys remove from the blood the nitrogenous wastes such as urea, as well as salts and excess water, and excrete them in the form of urine. One of the main functions of the lungs is to diffuse gaseous wastes, such as carbon dioxide, from the bloodstream as a normal part of respiration. Invertebrates lack a liver, but most terrestrial groups, like insects, possesses a number of blind guts that serve the similar functions. Marine invertebrates do not need the ammonia conversion of the liver, as they can usually expel ammonia directly by diffusion through the skin. Most excretion of excess calcium is via the bile and feces, from the intestines, because the plasma calcitriol levels (which ultimately depend on the plasma calcium levels) regulate how much of the biliary calcium is reabsorbed from the intestinal contents.