Excretory system: Introduction
The excretory system is a bodily system in a living organism that disposes waste from the body. The excretory system involves numerous body organs like sweat glands, lungs, and the renal system to excrete different kinds of waste products like sweat, carbon dioxide, and urea.
Let us understand how the human excretory system works and enables us to dispose of our body’s waste.
Human Excretory System (Excretory System Organs)
The human excretory system organs include:
- A pair of kidneys
- A pair of ureters
- A urinary bladder
- A urethra
A pair of Kidneys
Every human being is born with two kidneys. The kidneys are bean-shaped organs located on either side of the backbone. The ribs and back muscles protect the kidneys. Each adult human kidney is 10-12 cm long, 5-7 cm wide, and weighs 120-170g.
The structure of the kidney in detail is as follows:
The renal capsule refers to the fibrous tissue that surrounds the kidney. The inside of the kidney has two zones – the renal cortex and the renal medulla. The renal cortex is the outer layer of the kidney, while the renal medulla is the inner layer. The renal cortex extends between the medullary region as renal columns. These renal columns are also known as columns of Bertin or Bertin columns.
Nephrons are the functioning units of the kidney that produce urine while removing waste and excess substances from the blood. Each human kidney has around 1,000,000 nephrons.
A nephron consists of two parts – the renal tubule and the renal corpuscle.
- Renal corpuscle – The cup-shaped end at the top of a nephron is called the bowman’s capsule. The bowman’s capsule consists of a collection of capillaries created by called the glomerulus. These capillaries get formed by arterioles that transport blood to the glomerulus in the kidney. The bowman’s capsule and the glomerulus together make up the renal corpuscle.
- Renal Tubule – The renal tubule further extends from the bowman’s capsule. The renal tubule has five different parts – the proximal tubule, the loop of Henle, the distal convoluted tubule, the connecting tubule, and the collecting ducts. The highly coiled structure right next to the bowman’s capsule is the proximal tubule.
Loop of Henle
The next part of the tubule is the loop of Henle that consists of an ascending limb and a descending limb. The ascending loop continues as a distal convoluted tubule. The distal convoluted tubule then connects with the collecting ducts via the connecting tubule. The fluid reaching the collecting ducts is called urine.
A pair of Ureters
The collecting ducts coming out of the kidneys through the renal pelvis send out urine via a pair of thin muscular tubes called the ureters. In an adult human being, the length of the ureters is usually 20–30 cm.
A Urinary Bladder
The urinary bladder is a hollow, muscular sac-like structure that stores urine. The urine gets stored there until it gets secreted by humans through urination.
This tube emerges from the urinary bladder and aids in the removal of urine from the body. It is the most common pathway for sperms and urine in males. The muscles at the end opening of the urethra are called the Sphincter muscles that allow voluntary control over urination.
Mechanism of Excretion in Humans
Our bodies have a mechanism in which many functions like respiration, circulation, digestion occur simultaneously. As such, many waste products created in our body take diverse forms that include carbon dioxide, water, and nitrogenous compounds such as urea, ammonia, and uric acid. Therefore, the entire excretory system works to remove waste products from our bodies.
Now that we know the components of the excretory system, let us understand how it works!
Since blood includes both beneficial substances and toxins, it enters the kidneys via the capillaries in the glomerulus for purification. The bowman’s capsule filters out the waste substances from the blood as the walls of glomerulus capillaries are thin and selectively permeable. The capillaries reabsorb beneficial components such as glucose, amino acids, salts, and the needed quantity of water and send out the purified blood from the bowman’s capsule to the heart via the posterior vena cava.
In humans, the excretion process takes place in the following steps:
In the nephrons, the formation of urine involves the following steps:
- Glomerular Filtration : It is the first stage in the production of urine. The excess fluid and waste products from the kidney get filtered out into the collecting ducts to remove from the body. Such waste is called the glomerular filtrate. Glomerular Filtration Rate (GFR) is the quantity of filtrate generated by the kidneys per minute.
- Tubular Reabsorption: It is the process of absorbing ions and substances such as sodium ions, glucose, amino acids, water, etc.
- Secretion: To keep the bodily fluids in balance, potassium ions, hydrogen ions, and ammonia get secreted.
Micturition or Urination
Urine produced by the nephrons gets transported to the urinary bladder. It gets stored there until the central nervous system (CNS) sends voluntary signals. The straining of the urinary bladder sends signals to CNS as it fills with urine. Stretch receptors on the bladder walls transmit signals to the CNS in response.
The CNS sends motor messages to the urinary bladder, prompting the bladder muscles to contract and the urethral sphincter to relax, causing urine to be released. The process of releasing urine is known as micturition or urination. The neurological mechanism causing urination is called the micturition reflex.
An adult person excretes 1 to 1.5 litres of urine every day on average. The urine produced is a watery fluid that is light yellow, slightly acidic and has a pH equal to 6. Each day, 25-30 grams of urea gets excreted in the human body. A variety of factors can influence the properties of urine. Urine analysis aids in the clinical diagnosis of various metabolic disorders and renal dysfunction. The presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine, for instance, indicate diabetes mellitus.
Sometimes, the kidneys fail to purify blood under specific conditions, such as inadequate blood supply to the kidneys, infections, trauma, and so on. In such cases, artificial kidneys get leveraged to filter blood, and this process is called dialysis.
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