Excretory System

Excretory system definition

By consuming food and water, the body of a living organism is constantly taking in water and nutrients from its environment. Without a proper mechanism for getting rid of substances, the body may accumulate toxic waste and water, damaging the body's internal balance.

The excretory system helps maintain the body's Homeostasis by disposing of metabolic wastes and excess water.

In many animal groups ranging from insects to humans, the excretory system also plays a role in Osmoregulation, the process of maintaining a balance between salt and water across membranes in the body's fluids.

Excretory system diagram

Before we go through the specific components and functions of the excretory system, take a moment to look at the parts of the human excretory system in the diagram below (Fig. 1). This will give you an idea of how different excretory organs work together.

Excretory system parts

Recall that Animal Body Systems vary among animal groups.

While the structure and functions of excretory organs vary among different animal groups, one characteristic they have in common is that they typically consist of a network of tubules with enough surface area for water and solutes–including nitrogenous wastes–to pass through.

In many Animals, excess water and waste are removed from the bloodstream by producing liquid waste called urine. Urine is made through the following basic steps:

1. Filtration: Bodily fluid (such as Blood) comes into contact with epithelium, a layer of Cells that lines organs and glands. Blood pressure drives filtration through the selectively permeable membrane of the epithelium.

   1. Large molecules, including Cells and Proteins, cannot pass through this membrane, so they remain in the fluid, while water and small molecules like sugars and amino acids pass through, forming a solution called filtrate.

2. Reabsorption: Valuable molecules like vitamins and hormones are selectively recovered and returned to the bodily fluid, leaving only a fraction of the filtrate to be transported to the bladder.

3. Excretion: The processed filtrate containing metabolic wastes is released from the body in the form of urine.

Now that we have a general idea of how urine is formed, let's look at how this process varies among animal groups. First, we will look at excretory organs typically found in vertebrates, including humans. Then we will go through several examples of animal groups where an entirely different set of organs performs excretion.

Vertebrate excretory system organs

The kidneys are usually considered the primary excretory organ in vertebrates. The kidneys are part of the urinary system, which also includes the ureters, urinary bladder, and urethra, responsible for transporting, storing, and disposing of urine, respectively.

The kidneys are made up of highly structured tubules and tightly linked with a network of capillaries.

They are enclosed in three layers of tissue: the renal fascia, the perirenal fat capsule, and the renal capsule. The kidneys also have three inner regions: the cortex, medulla, and pelvis, located in the hilum. The hilum serves as the passageway for Blood vessels and nerves to enter and exit the Kidney. It is also the exit point for the ureters.

Nephrons– tiny structures that serve as the building blocks of the kidneys—filter out elements from the Blood, restore what is required to the bloodstream, and eliminate the excess as urine. Each Kidney consists of over a million nephrons.

Similar to the steps of urine production that we discussed earlier, the kidneys filter blood in three basic steps (Fig. 2):

1. Glomerular filtration: the nephrons filter blood that runs through the glomerulus, a network of capillaries near the end of a kidney tubule. Almost all solutes, except for Proteins, are filtered out.

2. Tubular reabsorption: the filtrate is collected, and most of the solutes get reabsorbed in the renal tubules, a long tubule that emerges from the glomerulus.

3. Tubular secretion: more solutes and wastes are released into the distal tubules. The collecting ducts gather filtrate from the nephrons and fuse it in the medullary papillae, from which the filtrate–now called urine–eventually flows to the ureters.

By filtering blood and regulating salt and water balance in bodily fluids, the kidneys play an integral role in Osmoregulation and excretion in vertebrates.

Due to differences in their external environments, there are adaptive variations in the structure and function of kidneys across vertebrate groups.

For example, most Mammals have the ability to dispose of salts and nitrogenous wastes while conserving water; they can adjust the volume and solute concentration of their urine based on their water and salt balance as well as their rate of urea production:

• When a mammal takes in a lot of salt but very little water, it can excrete urea and salt in small amounts of hyperosmotic urine (meaning, solute concentration is higher in urine than in blood), minimizing Water Loss.

• When a mammal takes in minimal salt but plenty of water, it can excrete urea and salt in large volumes of hypoosmotic urine (meaning, solute concentration is lower in urine than in blood), minimizing salt loss.

On the other hand, freshwater fishes and amphibians tend to produce large volumes of dilute urine because they are hyperosmotic to their surroundings. Therefore, to conserve salt, their tubules reabsorb ions from the filtrate.

In marine bony fishes, kidneys help in disposing of divalent ions (those with a charge of 2+ or 2-), such as calcium (Ca²⁺), magnesium (Mg²⁺), and sulfate (SO₄^2-), through urine production and excretion. Marine fishes take in large amounts of these ions because of constant seawater uptake.

Invertebrate excretory system facts

While the kidneys and their ducts are primarily responsible for nitrogenous excretion and osmoregulation, these functions are not always performed by the same set of organs in other animal groups. In the following section, we will discuss the excretory systems called protonephridia, metanephridia, and Malpighian tubules.

Protonephridia

Flatworms do not have a body cavity. Instead of kidneys, they have unique excretory systems called protonephridia (Fig. 3).

Protonephridia are a network of highly-branched tubules. Each protonephridium's branches are capped with cellular units known as flame bulbs. Cilia cover the tubule of each flame bulb.

The beating of the cilia takes water and solutes from the interstitial fluid through the flame bulb, releasing filtrate into the tubule network. The filtrate flows outward through the tubules and exits as urine through excretory pores on the body's surface. Because freshwater flatworm urine is low in solutes, its secretion also helps to maintain a balance in water concentration inside and outside its body.

Other Animals that have protonephridia include tapeworms, mollusk larvae, and lancelets.

Metanephridia

Earthworms and other annelids have special excretory organs called metanephridia, consisting of tubules with cilia. Each segment of the earthworm has a pair of metanephridia (Fig. 4). As the cilia move, fluid is pulled into a tubule with a storage bladder that opens to the outside.

Earthworm metanephridia control water inflow by creating diluted urine. The epithelium recovers most solutes and returns them to the blood in the capillaries. Nitrogenous waste remains in the tubule and is expelled into the environment.

Malpighian tubules

Insects and other terrestrial arthropods possess Malpighian tubules. The Malpighian tubules of an ant are shown in Figure 5 below.

Malpighian tubules are lined with microvilli that reabsorb water and nutrients and maintain osmotic balance. These tubules work together with special glands in the rectum.

These excretory systems lack filtration found in the majority of the other excretory systems. Exchange pumps lining the tubules pump H+ ions into the cell and K+ or Na+ ions out. The movement of ions alters the osmotic pressure, allowing water, electrolytes, and nitrogenous waste to enter the tubules.

The nitrogenous wastes, mostly insoluble uric acid, are released as near-dry materials along with the feces, which helps them conserve water. This crucial adaptation contributes to their survival in dry environments.

Excretory system diseases

Diseases affecting the excretory system include:

1. Kidney stones, which are solid, pebble-like materials that form in one or both kidneys from substances found in urine.

2. Urinary Tract Infection, which is when Bacteria enters the urethra and infect the urinary tract.

3. Uremia, which is characterized by imbalances in the body's fluids, electrolytes, and hormones, as well as metabolic abnormalities.

4. Nephritis, where tissues in the kidney are inflamed, hindering them from filtering waste from blood.

5. Incontinence, where there is a loss of control in urination.