What is Excretion?

The expulsion of nitrogenous wastes (non-gaseous) such as urea, ammonia, uric acid with water, some salts, and pigments out of the body is termed excretion. It helps to maintain the homeostatic conditions (constant internal composition) in the body. Different organisms excrete different types of waste products. Based on the chemical nature of the nitrogenous compounds produced as the waste products, the organisms are divided into the following three types:

1. Ammonotelic: In these organisms, ammonia and ammonium salts are the main waste products.
2. Ureotelic: These animals excrete urea.
3. Uricotelic: The main waste product in these animals is uric acid.

The organs involved in the excretory system also vary in different animal groups. For example, in cockroaches, Malpighian bodies are involved in excretion, whereas the major excretory organ in humans is the kidneys.

Human Excretory System

The excretory system includes two kidneys, a urinary bladder, one pair of ureters, and a urethra.

Kidneys

The shape of a kidney is like a bean. The kidneys are paired structures that vary between 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness. These are dark red in color and present between the last thoracic and third lumbar vertebra levels in the upper part of the abdominal cavity. One kidney lies on both sides of the vertebral column. It has an inner concave surface; the concavity is known as hilus renal or hilum. The position of both the kidneys is asymmetrical. The left kidney is present slightly towards the upper side, and the right kidney is comparatively lower in position, which might be due to the presence of the liver on the right side.

The internal structure of the kidney

Each kidney contains millions of nephrons. The outer region of the kidney is darker, granular, and dotted; this part is called the renal cortex. In contrast, the inner central region of the kidney is lighter and striated, which is known as the medullary pyramid. The medullary pyramids further project into minor and major calyces. The extension of the renal cortex in between the medullary pyramids is termed as columns of Bertini.                                                                                            

Functions of the kidney

The primary role of the kidney is associated with the regulation of body fluids. It also regulates the blood pH levels, electrolytes concentration, and chemical composition of the body fluids.

Ureters

These are paired, narrow, muscular, tubular structures in the excretory system of humans. They vary from about 25-30 cm in length. The lining of the ureters is composed of transitional epithelium. They arise from the hilum of the kidney known as the renal pelvis (upper broader part of the hilum). They run along the dorsal body wall (backward) and open up in the urinary bladder.                                                  

Functions of ureters

They transport urine from the kidneys to the urinary bladder through peristaltic movements.

Urinary bladder

It is a large, expandable, thin-walled sac-like organ in the excretory system of humans. It is present in the abdominal cavity within the pelvis region. Its inner walls are lined by detrusor muscles (smooth muscles). The bladder temporarily stores urine. The distensible wall of the bladder is maintained by certain transitional muscles known as the urothelium. These muscles increase the urine storage capacity from 250 to 750 ml. The upper part of the bladder is broader and is called the body of the bladder, whereas the lower part is narrow and referred to as the neck of the bladder. An internal urethral orifice is present that marks the origin of the urethra. Two sphincters guard the origin of the urethra: the outer sphincter, made up of striated muscle fibers and the inner sphincter, made up of smooth muscle fibers.

Urethra

It is a tubular and muscular structure that extends from the neck of the urinary bladder to the outside. It is involved in the elimination of urine out of the body. The male urethra and female urethra are different. The male urethra is about 20 cm long and opens at the tip of the penis as a urinogenital aperture. It is differentiated into three parts: prostatic, membranous, and penile. The ejaculatory duct, prostate gland, and Cowper's glands are associated with the male urethra. In males, the urethra serves as a common passage for urine and seminal fluid. The female urethra is about 4 cm in size. It opens in the vestibule as a urethral orifice in front of the vaginal aperture. It is undifferentiated and lacks the associated glands.  

Structure of a nephron

Nephrons are the functional as well as structural units of the kidneys. These are the smallest units of the excretory system that remove or extract wastes from the blood. Cortical and juxta-medullary nephrons are the two types of nephrons.

Cortical nephrons 

These constitute about 85% of the total nephrons. These are present in the upper part of the renal cortex. In these types of nephrons, the afferent arteriole is broader than the efferent arteriole. In the loop of Henle, the descending limb is thin-walled, and the ascending limb is thick-walled. These types of nephrons are involved in the formation of urine.

Parts of a cortical nephron:

1. Bowman's capsule 

It is present in the cortex region of the kidney. It is a double-walled cup-like structure lined by semipermeable and thin squamous epithelium made up of fat cells. Its inner layer is the visceral layer, and the outer layer is called the parietal layer. Bowman's capsule's inner (visceral) layer is formed of a special type of epithelial cells known as foot cells or podocytes. These podocytes have feet-like structures termed pedicels and minute pores known as slit pores (thousands to millions in number). The Bowman's capsule possesses a cavity in which a group of 50 capillaries is present. This group of capillaries is known as the glomerulus. The endothelium of the glomerular capillaries has several minute pores called fenestrae. A membrane known as the basement membrane is present between the endothelium of glomerular capillaries and the visceral epithelium. The glomerular epithelium, basement membrane, and visceral membrane together form a dialyzing membrane or filtering membrane. This membrane acts as an ultrafilter and performs ultrafiltration.

2. Renal tubule 

It is a long, coiled structure composed of the following parts:

• Proximal convoluted tubule (PCT): It is located in the cortex of the kidney and is a complex structure. The lining of PCT is brush bordered and possesses many microvilli. The cells of PCT bear several mitochondria involved in the active transportation and selective reabsorption of ions and molecules such as calcium, sodium, glucose, amino acids from the filtrate that passes through the nephron.
• Loop of Henle: It is a U-shaped structure that extends in the medulla region. It has two limbs: ascending limb (thick), lined by cuboidal epithelium, and descending limb (thin), lined by squamous epithelium. The descending limb is permeable to water and impermeable to sodium chloride. In contrast, the ascending limb is permeable to electrolytes and impermeable to water.
• Distal convoluted tubule (DCT): It is present in the cortex region and convoluted. It is permeable to sodium and potassium ions. It ends up in the collecting tubule.

Each collecting duct receives approximately eight uriniferous tubules. Collecting ducts join and form the larger ducts of Bellini. These ducts pass through the conical ends of the papillae called renal pyramids (15-16). The pyramids further open into minor calyces and then into larger major calyces. Major calyces open up into the funnel-shaped renal pelvis.

Juxta-medullary nephrons 

These constitute approximately 15% of the total nephrons. They are present in the deeper part of the cortex region of the kidney. The diameters of afferent and efferent arterioles are the same. In the loop of Henle, both ascending limb and descending limb are thin-walled and long sized. The efferent arterioles form the vasa recta (a loop-like structure). The juxta-medullary nephrons make the urine concentrated and aid in sodium reabsorption. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

• Bachelors of Science in Biology
• Master of Science in Zoology 

Related Concepts 

1. Excretory system of different invertebrates
2. Formation of urine
3. The organ associated with the excretory system
4. Excretion of urine
5. Excretory system in mammals
6. Physiology of excretory system in humans