The outlet of urine from the body is through urethra. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is released through the urethra. Ureters are the long tubes, which connect the kidneys with the urinary bladder. Urinary bladder is muscular sac where urine is stored until the pressure of the expanded bladder leads to urge to pass it out through the urethra. Anus is the excretory opening at the end of the digestive tract, it controls the excretion of faeces produced during digestion.

Along with the removal of waste, kidneys also maintain constant blood pressure. One of the major roles of the kidneys is maintaining the proper levels of electrolytes (such as sodium and potassium) in the body. The amount of electrolytes in the body influences the amount of fluid in the body. When electrolyte levels are high, the body retains more water, which in turn increases the volume of the blood. More blood volume results in higher blood pressure. Thus, the kidneys maintain blood pressure by indirectly controlling the amount of blood in the body. 

Urine produced in the kidneys pass through ureters into urinary bladder. Kidneys filter out waste products from the blood. Just as CO₂ is removed from the blood in the lungs, Nitrogenous wastes like urea or uric acid are removed from blood in the kidneys. This process is called excretion. Excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is released through the urethra. Ureters are the long tubes, which connect the kidneys with the urinary bladder. Urinary bladder is a muscular sac where urine is stored until the pressure of the expanded bladder leads to urge to pass it out through the urethra. Vena cava is a large vein, which brings blood from the various parts of body to the heart. Aorta is the largest artery and it arises from the left ventricle and supplies oxygenated blood (not in case of pulmonary artery) to various parts of body.

Kidneys filter out nitrogenous waste from blood. Also, they help in removal of excess of water from the body. Kidneys are the vital organs for survival. Several factors like infections, injury or restricted blood flow to kidneys, reduce the activity of kidneys. This leads to accumulation of poisonous wastes in the body. The composition of blood is disrupted. It may even lead to death of an organism.

Apart from kidneys, liver and skin also help in elimination of nitrogenous excretion. Liver, the largest gland in our body secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Formation of urea also takes place in liver. Most of these substances eliminate through digestive wastes. The sweat and oil glands in the skin also eliminate substances through their secretions, e.g. sweat produced by the sweat glands comprises water, NaCl, small amounts of urea, lactic acid etc. Similarly, oil glands in the skin eliminate certain sterols, hydrocarbons and waxes. Lungs, however, do not excrete nitrogenous wastes, but remove large amounts of carbon dioxide and significant amounts of water every day.

In unicellular organisms, e.g. amoeba, paramecium, there are no separate excretory organs. They excrete out waste through general body surface through simple diffusion. The biological process involved in the removal of harmful metabolic wastes from the body is called excretion. These metabolic wastes include the nitrogenous materials. Different organisms use varied strategies for this. Many unicellular organisms remove these wastes by simple diffusion. Diffusion is the movement of particles from the region of high concentration to the region of low concentration. Filtration – It is a physical process used to separate solids from liquid or gases. Reabsorption occurs in kidney, which selectively reabsorbs substances like glucose, protein, sodium, which had already been secreted in the renal tubule. These reabsorbed substances return to blood. Translocation is the transport of soluble products of photosynthesis by the food conducting tissues, called phloem. Therefore, simple diffusion is the correct answer.

 Excess water is excreted from plants through the stomata in leaves.  Plants can get rid of excess water by transpiration and guttation.

Shedding of body parts for getting rid of waste material is a characteristic of plants. Plants store the wastes in some organs like leaves or stalk, which ultimately die and are shed. In various other creatures like birds, mammals and amphibians, there is shedding of feathers or outer skin or even wings. This is different from the shedding of leaves or stalks in plants, which occur to get rid of wastes which are stored in them.

Vacuoles are the membrane-bound organelles of plant cells. These store food or waste products.

Water droplets are formed by the process of transpiration.  The liquid that comes out of tree bark is actually the excretory wastes like resins, gums, alkaloids (wastes obtained from nitrogenous substances), rubber, etc. Plants do not have special excretory system. They excrete out these waste materials through the bark. Cell sap is the liquid that is stored in the cell vacuole and composed of water and sugars.

Filtration of blood occurs in the kidneys in a cup shaped part of nephron, which receives the glomerular filtrate is called Bowman’s capsule.  

In crustaceans like prawns, paired green glands are the excretory organs. In insects like cockroaches, malpighian tubules perform the task of excretion.

Molluscs have one or two pairs of renal organs such as sponge-like kidneys.

Proboscis glands are the excretory organs of hemichordates.

Mesonephros kidney is the functional kidney of petromyzon, fishes and amphibians.