Excretion and Urine Formation:
Excretion is the process of eliminating waste products from the body. In humans, excretion primarily occurs through the urinary system, which includes the kidneys, ureters, urinary bladder, and urethra. Urine is the waste product excreted by the kidneys.
Continuous Process of Excretion:
Excretion is a continuous process that involves the filtration of blood in the kidneys and the removal of waste products, excess water, and electrolytes from the body. The filtration occurs in the nephrons, which are the functional units of the kidneys.
Urine Formation:
Urine formation involves several steps, including filtration, reabsorption, and secretion. Here is a brief overview of the process:
1. Filtration: Blood enters the kidneys through the renal artery, and the waste products, along with water and electrolytes, are filtered out of the blood in the glomerulus. This filtered fluid is called the glomerular filtrate.
2. Reabsorption: The glomerular filtrate then passes through the renal tubules, where most of the water, electrolytes, and essential substances (glucose, amino acids, etc.) are reabsorbed back into the bloodstream. This reabsorption occurs in a selective manner, ensuring that only the necessary substances are retained.
3. Secretion: During secretion, certain substances, such as drugs, toxins, and excess ions, are actively transported from the bloodstream into the renal tubules. This further helps in the elimination of waste products.
4. Concentration: As the glomerular filtrate moves through the renal tubules, the kidneys have the ability to concentrate the urine by reabsorbing more water from the filtrate. This concentration is regulated by hormones such as antidiuretic hormone (ADH).
Storage of Urine:
After the formation of urine, it is temporarily stored in the urinary bladder. The urinary bladder is a muscular organ that can expand and contract to accommodate the storage of urine. The bladder is connected to the ureters, which are tubes that transport urine from the kidneys to the bladder.
Passage of Urine:
The passage of urine from the bladder to the outside of the body occurs through the urethra. The urethra is a tube that extends from the bladder to the external opening, allowing the controlled release of urine.
Why urine is not passed out continuously:
The urinary bladder acts as a reservoir for storing urine. It allows for the controlled release of urine, preventing constant and involuntary urination. The muscular walls of the bladder can stretch to accommodate increasing volumes of urine, and the release of urine is under voluntary control through the relaxation of the external urethral sphincter.
Therefore, urine is not passed out continuously because the urinary bladder provides a storage mechanism, allowing urine to accumulate before it is released from the body.