| 1. | Get an overview of the excretory system |
| 2. | Know the different parts of the excretory system |
| 3. | Understand the excretory organs and their functions |
| 4. | Know the mechanism of excretion |
| 5. | Basic steps in the urine formation process |
| 6. | Understand how the kidney function is controlled |
The excretory system consists of organs that remove metabolic wastes and toxins from the body. In humans, this includes the removal of urea from the bloodstream and other wastes produced by the body. The removal of urea happens in the kidneys, while solid wastes are expelled from the large intestine.
The human excretory system organs include:
Kidneys are bean-shaped structures located on either side of the backbone and are protected by the ribs and muscles of the back. Each human adult kidney has a length of 10-12 cm, a width of 5-7 cm and weighs around 120-170 g.
The kidneys have an inner concave structure. At the center, there is a notch called hilum through which the blood vessels and nerves enter the organ. Towards the inner surface of the hilum, there is a large funnel-shaped space called renal pelvis with projections called calyces.
Kidneys are the primary excretory organ in humans and are located one on each side of the spine at the level of the liver. They are divided into three regions
The structural and functional unit of the kidney is the nephron. Each kidney consists of millions of nephrons that are all functioning together to filter urine and expel the waste products.
Each nephron consists of the following parts:
Bowman’s capsule – This is the first part of the nephron which is a cup-shaped structure and receives the blood vessels. Glomerular filtration occurs here. The blood cells and proteins remain in the blood.
Proximal Convoluted Tubule – The Bowman’s capsule extends downwards to form the proximal tubule. Water and reusable materials from the blood are now reabsorbed back into it.
The Loop of Henle – The Proximal Convoluted Tubule leads to the formation of a U-shaped loop called the Loop of Henle. It has three parts – the descending limb, the u-shaped bend, and the ascending limb. It is in the area that the urine becomes concentrated as water is reabsorbed. The descending limb is freely permeable to water whereas the ascending limb is impermeable to it.
Distal Convoluted Tubule – The Loop of Henle leads into the distal convoluted tubule which is where the kidney hormones cause their effect. And the distal convoluted tubule leads to the collecting ducts.
Collecting duct – The distal convoluted tubule of each nephron leads to the collecting ducts. The collecting ducts together form the renal pelvis through which the urine passes into the ureter and then into the urinary bladder.
A thin muscular tube called the ureter comes out of each kidney extending from the renal pelvis. It carries urine from the kidney to the urinary bladder.
It is a sac-like structure that stores urine until micturition. Micturition is the expulsion of urine from the body. The urine is carried to the bladder through the ureters.
This is a tube that arises from the urinary bladder and helps to expel urine out of the body. The urethra is shorter in females and longer in males. In males, it acts as the common path for sperms and urine. Its opening is guarded by a sphincter that is automatically controlled.
The urine is formed in the nephrons and involves the following steps:
Glomerular Filtration – It is the primary step in urine formation. In this process, the excess fluid and waste products from the kidney are filtered out of the blood into the urine collection tubules of the kidney and eliminated out of the body. Small ions such as sodium and potassium pass freely, but large molecules such as proteins, hemoglobin and albumin do not permeate. The amount of filtrate produced by the kidneys every minute is known as Glomerular Filtration Rate.
Tubular reabsorption – It is the absorption of ions and molecules such as sodium ions, glucose, amino acids, water, etc. Water involves passive absorption, while glucose and sodium ions are absorbed by an active process.
Secretion – Potassium ions, hydrogen ions, and ammonia are secreted out to maintain the equilibrium between the body fluids.
The functions of the various tubules involved in the process are:
The urinary bladder is stretched and gets filled with urine formed in the nephrons. The receptors present on the walls of the urinary bladder send signals to the Central Nervous System, thereby, allowing the relaxation of sphincter muscles to release urine. This is known as micturition.
The activity of the nephron in the kidney is controlled by a person’s choices, environment, and hormones. For example, if a person consumes a large amount of protein, much urea will be in the blood from the digestion of the protein. Also, on a hot day, a body will retain water for sweating and cooling, so the amount of urine is reduced.
Humans produce a hormone called antidiuretic hormone (ADH), also known as vasopressin, which is secreted by the posterior lobe of the pituitary gland. It regulates the amount of urine by controlling the rate of water absorption in the nephron tubules.
Hormones from the cortex of the adrenal glands also control the content of urine. These hormones promote reabsorption of sodium and chloride ions in the tubules. Thus, they affect the water balance in the body because the water flows in the direction of high sodium and chloride content.
Apart from the above, there are other organs that also perform some form of excretion.
Skin – The skin is a secondary excretory organ since sweat glands in the dermis can remove salts and some excess water. The skin also has sebaceous glands that can secrete waxy lipids.
Lungs – They are the primary respiratory organs and help expel carbon dioxide.
Liver – The liver is the main detoxifying organ of the body, especially for nitrogenous wastes. It is the first line of defense when it comes to hormones, fats, alcohol, and drugs. The liver aids in the elimination of excess fats and cholesterol from the body.
Large intestine - The liver is also necessary for the removal of the decomposed hemoglobin, some drugs, excess vitamins, sterols, and other lipophilic substances. These are secreted along with bile and finally removed from the body through feces via the large intestine. The large intestine, therefore, plays a role in excretion, especially for hydrophobic particles.
The excretory system performs many functions such as
