Contents
Learning objectives
By the end of this section, you should be able to:
- Describe Excretion and the Excretory system.
- Describe the structure and the function of the kidney.
- Describe how Urine is formed.
What is Excretion and the Excretory system?
Every living organism's body is just like a living machine. In our previous discussions we have learnt the circulatory system and lymphatic system. If you can recall, we said that blood carry useful substances to every body tissue and take their waste away. These wastes from the tissues must not be left in the blood or in the body, hence they are removed totally from the blood and the body by a process called EXCRETION.
Several organs and tissues that work together to perform the function of removal of tissues' or metabolic waste from the blood and the body are referred to as the EXCRETORY SYSTEM.
What are the animals' body waste products that are Excreted?
Animals' body wastes that are excreted are the waste from the different tissues and organs in the body. These different tissues or organs perform metabolic activities, and in every metabolic activity, there's always a waste product which is considered as 'useless'. These useless products are the wastes that need to be excreted. They include:
- Nitrogenous wastes
- Water
- Carbon(IV)oxide
- Minerals
- Toxins
Are faeces also referred to as Excretory wastes?
Faeces which are waste products of the digestive system (or digestion) are also regarded to as useless and must be removed from the body. Faeces are however not Excreted from the body, they are Eliminated.
What is the difference between Excretion and Elimination?
Elimination is the removal of digestive waste (faeces) from the body while excretion is the removal of metabolic wastes from the body.
Hope you got that right?
Excretory mechanisms in different phyla of the Animal Kingdom
Protozoans
Protozoans are majorly unicellular organisms and they excrete their wastes using a process called exocytosis. Here, the unicellular organism simply releases the waste product into its surrounding by diffusion.
Porifera (Sponges)
Sponges do not have specialised cell structures, therefore they do not have excretory organs or system. Metabolic wastes from the mesohyl (functional cells of sponges) pass out waste products via diffusion, which is released from the sponge via osculum or holes around it. If these look new to you, read the sponges section in the classification of animals.
Cnidarians
Cnidarians lack specialized cells or organs used for excretion of their wastes. Metabolic wastes pass from the cells via diffusion which is either diffused out of the body or released out of the cnidarian via its mouth.
Flat Worms
Flatworms lack a respiratory or circulatory system; these functions occur by absorption through the body wall, they however have an excretory structure called Flame cells. Some people call this protonephridia which means the first kind of nephridium or excretory structure.
Roundworms (Nematodes)
While roundworms are a little higher than Simpler organisms, nitrogenous wastes is passed out from roundworms through diffusion, however solid wastes are excreted via tubules (in non parasitic round worms) or excreted using glands (in parasitic roundworms).
Earthworms (Annelids)
Earthworms excrete nitrogenous and solid wastes via specialized tubular structures called nephridium. At the end of each nephridium are special structures that reabsorb important components back into the worm.
Molluscs
Metabolic wastes in the coelom (blood) of molluscs collect in tubular structures called nephridia or metanephridia (can also be called 'Bojanus organs'). These tubules carry Excretory wastes outside the mollusc.
Insects and arachnids
Excretion in some arthropods (insects and arachnids) are carried out by the organs of the arthropod's gut. These organs include the midgut, the Malpighian tubules, and the structures in the hindgut, namely the ileum and rectum.
Crustaceans
Excretion in cray fish and crabs are carried out by Green glands on their antennae (also called colex glands or antennal glands).
Starfish
Echinoderms do not have a specialized excretory system. Metabolic wastes like ammonia are excreted via diffusion using the tube feet or papulae.
Fish, amphibians, reptiles, birds, mammals
All vertebrates excrete metabolic wastes from the blood using Kidney. Gaseous wastes are excreted via gills (in fish and amphibians), body (in amphibians), and lungs (in others).
Body parts that excrete wastes in higher animals
Excretory System (kidneys) – filter out excess water, metabolic waste, waste products from the blood, salt and chemicals as urine.
Respiratory System (lungs) – filter out carbon(IV)oxide, from the blood.
Integumentary System (skin) – excretes salt and organic substances as sweat, which contains trace chemical wastes, including urea.
Digestive System – indigested food leaves through large intestine
Functions of Excretory System
- Removal of metabolic wastes
- Elimination of toxins
- Elimination of excess nutrients
- Elimination of excess hormones
- Regulation of fluid volume
- Regulation of electrolytes
- Regulation of acid–base balance
- Regulation of blood volume and pressure
Nitrogenous wastes
Nitrogen wastes are by-products of protein metabolism.
Nitrogenous wastes occur in 3 main forms:
1. Ammonia - basic form of nitrogenous waste.
2. Urea - Ammonia molecule that has been concentrated by removal of water and addition of other constituents.
3. Uric acid - the most concentrated form of nitrogenous waste.
The process of excreting ammonia is Ammonotelism. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature.
Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced as a by-product of metabolism is converted into urea in the liver of these animals, and released into the blood, which is afterwards filtered and excreted out by the kidneys.
Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid, in the form of pellet, or paste, with a minimum loss of water and are called uricotelic animals.
Osmoregulators VS Osmoconformers
Osmoregulators are those animals that maintain a constant internal osmotic environment in spite of changes in their external environment, while osmoconformers are those animals whose body fluids are in balance with their environment.
In other words, osmoconformers are animals that have their body fluid concentration same as their environment. For example, a marine animal that its body fluids match the same concentration as the seawater around it is an osmoconformer. On the other hand, osmoregulators are animals that have their body fluid concentration as different compared to their environment, therefore they have to balance the differences in osmotic pressure by a process called osmoregulation.
Excretory organs in higher animals
The main organs of the excretory system in higher animals are:
- A pair of Kidney
- A pair of Ureter
- Urinary Bladder
- Urethra
These components, together, carry out the excretory system’s function of regulating the volume and composition of animals' body fluids, removing waste products from the blood, and expelling the waste and excess water from the body in the form of urine, hence, they can also be called Urinary system.
Kidney
Kidney is the principal Excretory organ in higher animals.
It occurs in form of various shapes in different animals. It is bean shaped in goats, heart shaped in horse, oval shaped in birds, flattened in pigs and lobulated in cattle. It generally occur in pair, one on the left and the other on the right.
The kidneys are located behind the peritoneum, hence they are called retroperitoneal organs.
Kidneys perform the major function of separating wastes from the blood, reabsorbing important nutrients, then passing final wastes, down to the other organs, for disposal.
The sizes of the kidney in different animals vary, some appear as big as the size of an adult human's fist, while some others are even bigger.
A kidney is divided into 3 parts:
- Cortex
- Medulla
- Calyx or Pelvis
Kidneys are usually surrounded by an adipose tissue called Capsula adiposa.
The cortex of kidneys is where blood is filtered.
The medulla is the middle structure of the kidney, and it contains the collecting ducts which carry urine to the pelvis.
The pelvis is a hollow cavity where urine accumulate and drain into the ureter.
PRO TIP: Anywhere you see the word Renal, you should know that what is being discussed is kidney related.
How does kidney produce urine
Unclean blood passes through the kidney where it is filtered and cleansed, while the waste products form urine.
This unclean blood is brought into the kidney via the Renal Artery.
Blood passes through tiny microscopic filtering factories that remove wastes from blood in the kidney. These microscopic filtering factories are the functional units of the kidney, and they are called Nephron. Nephrons make up the cortex of the kidney.
After Nephron has separated wastes from blood, cleansed blood is sent to the Renal vein, while waste is sent to pelvis.
This is the summary of urine production, now let us discuss it extensively.
Urine production in the Kidney is divided into 3 major processes
- Ultrafiltration
- Selective reabsorption
- Hormone secretion
Funny enough these 3 major processes take place in the nephron.
Ultrafiltration
As you can see the word filtration, what is basically happening here is filtration of wastes from blood.
How is waste filtered from blood?
Waste is filtered from blood with the aid of nephron components called glomerular corpuscle which makes up the glomerulus and the bowman's capsule. Hence, ultrafiltration can also be called glomerular filtration.
Ultrafiltration involves the filtration of blood through tiny network of capillaries called glomerulus.
What happens in the glomerulus?
Blood enter the capillaries of the glomerulus from the Renal artery through what is called afferent arteriole (or juxtaglomerular arteriole, JGA).
With blood now in the capillaries of the glomerulus, the walls of these capillaries contain numerous, very tiny pores called fenestrae. These tiny pores (fenestra) allow water and solutes to pass through, but don't allow cellular components of blood and large proteins.
Now you can see why it is called filtration? it just follows the same process of like you filtering water from a mixture of water and other components. Do you get what happens in the glomerulus now?
To understand glomerular filtration better, watch this video:
When ultrafiltration has taken place in the glomerulus, water and solutes that pushed through the walls of the capillaries are passed down to the glomerular capsule or bowman's capsule which is a cup-like structure that collects the water + solute (glomerular filtrate). Cleansed blood (that did not pass through the fenestra) from the glomerulus pass through another vessel or pathway known as the efferent arterioles, these arterioles will later collect to form the venules that will enter the renal vein.
Selective reabsorption
Water + solutes or glomerular filtrate from the bowman's capsule, pass into nephron tubules or nephric tubules.
These glomerular filtrate are not of the same composition as urine because glomerular filtrate still contain useful nutrients and ions.
Reabsorption is very selective, and it occurs all along the nephric tubules. At the end of the day, 99% of glomerular filtrate get reabsorbed, while about 1% actually leaves the body as urine.
How does selective reabsorption occur?
Reabsorption starts with the proximal convoluted tubule. Glomerular filtrate that is passed down from bowman's capsule, move along the proximal convoluted tubule for reabsorption.
In the proximal convoluted tubule, is where majority (about 80%) of useful materials are absorbed from the glomerular filtrate. The cell lining of this tubule have microvilli (finger-like structures used for absorption) and more mitochondria. All small proteins, glucose and amino acids are reabsorbed. Most water and salts are also reabsorbed in the proximal convoluted tubule.
At the end of the proximal convoluted tubule, the resulting waste is then passed down into the Loop of Henle. The Loop of Henle is that U-like structure that you can see in the nephron. What is majorly reabsorbed in the Loop of Henle are sodium and potassium ions, and it makes use of a particular process called counter current mechanism.
Resulting waste from the Loop of Henle, is then led up to the distal convoluted tubule and collecting tubule. In this region, there's presence of highly concentrated salts which reabsorb water from the tubules, however, reabsorption level is partially controlled by hormones.
To understand selective reabsorption better, watch this video:
Hormone Secretion
Two hormones play a major role in the formation of urine. They are:
- Aldosterone
- Antidiuretic hormone (ADH)
Aldosterone
Aldosterone is a steroid hormone that is made in the cortex (outer layer) of the adrenal gland.
Aldosterone have a plasma half life of about 20 minutes. Its production is stimulated by Renin (or angiotensin II) released from the kidney.
The major function of aldosterone is to stimulate the reabsorption of sodium ions by addition of potassium ions into the urinary waste.
Antidiuretic Hormone (ADH)
Antidiuretic hormone also called arginine vasopressin is a hormone that is produced in the hypothalamus of the brain and then stored in the pituitary gland (posterior pituitary gland) from where it is released when needed. What ADH basically does is to direct kidneys to absorb more water. ADH release by the pituitary gland is dependent on the water level of the blood. More ADH means more water absorption and less urine, less ADH means less water absorption and more urine. When blood has too much water, small amounts of ADH will be released by the pituitary gland. This makes the kidney to reabsorb small amount of water, therefore releasing more urine.
How Urine leaves the kidney and the body
After urine has been formed and concentrated by several nephrons, it is collected in Collecting ducts which extend out to the medulla of the kidney. Several collecting ducts from millions of nephrons converge into pelvis, where urine is poured into. The pelvis leads out of the kidney into another tube called ureter.
The ureter then goes straight down into the bladder, where urine is temporarily stored.
Bladder is a sac-like muscular organ that stores urine. The bladder expands like an elastic sac to hold more urine. As it reaches full capacity, the process of micturition, or urination, begins. Involuntary muscle movements (from the internal urethral sphincter) send signals to the nervous system, putting the decision to urinate under conscious control (opening or closing of the external urethral sphincter). Urethra, which is another tube, leads out of the bladder to the outside of animals, through which urine is finally released to the environment.
Other functions of the Kidney
Control rate of erythropoiesis
Kidneys produce hormone (erythropoietin) that regulates erythropoiesis (formation of Red Blood Cells).
Affects the absorption of Calcium from intestine
Help to regulate blood pressure & volume
Kidneys release an enzyme called renin which triggers production of angiotensin II.
Angiotensin causes:
- Vasoconstriction (tightening of blood vessels) which raises Blood Pressure.
- Release of ADH conserves water to raise blood pressure.
- Maintenance of high filtration pressure in Renal corpuscles.
The one thing is that, blood pressure is directly affected by the volume of fluids retained or removed from body:
Kidneys directly affect blood–water/salt volume by altering salt and water reabsorption under influence of Aldosterone and ADH.
While, Aldosterone promotes salt retention and therefore water retention by kidneys, ADH promotes water retention by kidneys
Regulate pH of body fluids
Disorder of the kidney and excretory system
NEPHRITIS: Inflammation of the nephron or kidney
POLYCYSTIC KIDNEY DISEASE (PKD): causes many fluid filled cysts to grow in the kidneys. PKD cysts can make the kidneys much larger and cause them not to function at all.
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BACTERIAL INFECTIONS: can get into the bladder and ureters from the urethra. Infections in the kidney can permanently damage nephrons, and cause the kidney to not work properly.
KIDNEY STONES: minerals can build up in the kidney tubules and act as a blockage to the free flow of fluids. This leads to fluid build up in certain organs, or Oedema.
Many kidney problems can be treated by DIALYSIS, where a machine acts as a kidney. Kidney transplant is an alternative to dialysis.
Summary
🎯Excretion is the removal of metabolic wastes from the body.
🎯Several organs that work together to remove metabolic wastes in the body make up the excretory system.
🎯Removal of faeces from the body is termed elimination.
🎯 Metabolic wastes include
- Nitrogenous wastes
- Water
- Carbon(IV)oxide
- Minerals
- Toxins
🎯Simple organisms excrete using diffusion. Worms excrete using flame cells or nephridium. Arthropods excrete using Malpighian tubules. Vertebrates excrete using kidney.
🎯Osmoregulators are those animals that maintain a constant internal osmotic environment in spite of changes in its external environment, while osmoconformers are those animals whose body fluids are in balance with its environment.
🎯 Excretory waste in vertebrates occur majorly in the form of urine.
🎯 Excretory organs in vertebrates are
- A pair of Kidney
- A pair of Ureter
- Urinary Bladder
- Urethra
🎯The functional unit of a kidney is nephron.
🎯Urine formation take 3 major processes
- Ultrafiltration
- Selective reabsorption
- Hormone Secretion
🎯 Urine leaves the kidney into the Ureter. Into the bladder. Into the urethra. Out of the body.
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