Excretory System | Lucent for GK - UPSC PDF Download

Introduction

The human excretory system is composed of several organs, each playing a vital role in waste elimination. Let us explore these organs in detail:

Kidneys
Positioned on either side of the backbone, nestled within the protective embrace of the ribs and back muscles, the kidneys are the bean-shaped powerhouses of the excretory system. Each adult kidney measures approximately 10-12 cm in length, 5-7 cm in width, and weighs around 120-170g. Characterized by an inner concave structure, the kidneys serve as the gateway for blood vessels, ureters, and nerves, all of which enter through a notch known as the hilum. The renal pelvis, a spacious funnel-shaped area within the kidney, contains multiple projections called calyces.

Ureters
Emerging from each kidney's renal pelvis, a pair of slender, muscular tubes known as the ureters transport urine to the urinary bladder.

Urinary Bladder
The urinary bladder, resembling a sac-like muscular structure, functions as a storage facility for urine. The process of expelling urine from the bladder is known as micturition or urination.

Urethra
The urethra, originating from the urinary bladder, acts as a conduit for the expulsion of urine from the body. In males, it also serves as a common pathway for both urine and sperm. Sphincter muscles guard the opening of the urethra.

Structure of the Kidney

Understanding the structure of the kidneys is crucial to comprehend their excretory functions. Let us examine the key components of a kidney:

Capsule
The outer layer of the kidney is called the capsule. Inside the kidney, two distinct zones exist: the cortex (outer zone) and the medulla (inner zone). The cortex extends between medullary pyramids, forming renal columns known as columns of Bertin.

Nephrons
Nephrons, the functional units of the kidney, play a pivotal role in the excretory process. Each nephron comprises two main parts: the glomerulus and the renal tubule. The glomerulus consists of a cluster of capillaries formed by afferent arterioles, while blood from the glomerulus is carried away by efferent arterioles. The renal tubule commences with a cup-shaped structure called Bowman's capsule, which encloses the glomerulus. Together, the glomerulus and Bowman's capsule form the malpighian body. Adjacent to Bowman's capsule, we find the highly coiled proximal convoluted tubule.

Henle's Loop
Proceeding from the proximal convoluted tubule, we encounter Henle's loop, which consists of an ascending and descending limb. The ascending loop then continues as a distal convoluted tubule. The distal convoluted tubules of numerous nephrons empty into the collecting duct. The kidney's cortical region encompasses the malpighian corpuscle, proximal convoluted tubule, and distal convoluted tubule, while the medullary region houses the loop of Henle. Two types of nephrons exist: cortical and juxtamedullary. In cortical nephrons, the loop of Henle is relatively short, extending only slightly into the medulla. Conversely, juxtamedullary nephrons boast a lengthy loop of Henle, deeply penetrating the medulla.

Excretion in Humans

Excretion, the process by which metabolic waste is eliminated from the body, occurs through various bodily organs and internal processes. While lower organisms primarily rely on simple diffusion for excretion, the human body necessitates more complex and specific mechanisms to remove waste effectively. Humans produce a range of waste products, including carbon dioxide, water, urea, ammonia, uric acid, and various chemicals and toxins derived from medications and hormones. In order to rid the body of these waste materials, more intricate processes come into play.

Kidney's Role in Excretion
Blood contains both beneficial and harmful substances, requiring the presence of kidneys to separate the two. The kidneys achieve this through a process of filtration and reabsorption, ultimately producing urine. The structural filtration unit within the kidney, known as the nephron, serves as the site where blood is filtered. Each kidney harbors millions of nephrons. The capillaries of the kidneys filter the blood, reabsorbing essential substances such as glucose, amino acids, salts, and the required amount of water, before returning the cleansed blood to circulation. Excess water and nitrogenous waste in humans are converted into urine, which then passes through the ureters to the urinary bladder.

Mechanism of Excretion in Humans

The excretory process in humans unfolds through the following steps:

Urine Formation
Urine formation occurs within the nephrons and involves the following stages:

• Glomerular Filtration: The initial step in urine formation, where excess fluid and waste products are filtered from the blood into the kidney's urine collection tubules and subsequently expelled from the body. The Glomerular Filtration Rate (GFR) represents the amount of filtrate produced by the kidneys per minute.
• Tubular Reabsorption: This process entails the absorption of ions and molecules such as sodium ions, glucose, amino acids, and water. While water is passively absorbed, glucose and sodium ions undergo active processes of absorption.
• Secretion: Potassium ions, hydrogen ions, and ammonia are secreted to maintain equilibrium within the body fluids.

Various tubules within the kidney contribute to the different stages of urine formation. For instance:

• Glomerulus: Filters the blood.
• Proximal Convoluted Tubules (PCT): Reabsorb water, ions, and nutrients. They also remove toxins and aid in maintaining ionic balance and pH levels by secreting potassium, hydrogen, and ammonia into the filtrate and reabsorbing bicarbonate ions.
• Descending Loop of Henle: Permeable to water, resulting in the concentration of the filtrate as it becomes impermeable to electrolytes.
• Ascending Loop of Henle: Impermeable to water but permeable to electrolytes, leading to the dilution of the filtrate as electrolytes move from the filtrate to the medullary fluid.
• Distal Convoluted Tubule (DCT): Facilitates the reabsorption of water and sodium ions, while aiding in maintaining pH and ionic balance through ion secretion and reabsorption, similar to the PCT.
• Collecting Duct: Reabsorbs a significant amount of water from the filtrate.

Micturition
Once the urinary bladder becomes stretched and filled with urine, receptors on its walls send signals to the Central Nervous System, initiating the relaxation of sphincter muscles to facilitate the release of urine. This process is known as micturition.

Dialysis
Under certain circumstances, such as poor blood flow to the kidneys, infections, or injuries, the kidneys may fail to perform their functions adequately. In such cases, artificial kidneys, known as dialysis machines, are employed to filter and purify the blood. Dialysis plays a crucial role in maintaining fluid and electrolyte balance when kidney function is impaired.

Other Modes of Excretion

Apart from the urinary system, other organs also participate in waste elimination. The skin, lungs, and even eyes contribute to the excretion of waste products in various forms. Sweating, for example, serves as a mode of excretion, expelling water, urea, and salts through the skin. The lungs assist in the exhalation of gaseous wastes like carbon dioxide and nitrogen.

Conclusion

The human excretory system, a remarkable orchestration of organs and processes, ensures the efficient elimination of waste products from our bodies. By understanding the intricate workings of this system, we gain insights into the vital role it plays in maintaining homeostasis. The kidneys, with their nephrons and elaborate filtration processes, stand as the cornerstone of waste removal. However, let us not overlook the contributions of the skin, lungs, and other organs that aid in the excretory journey. Embracing a holistic perspective on the excretory system empowers us to appreciate the intricacies of our own bodies and cultivate a deeper understanding of human biology.