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Introduction to human excretory system
The human excretory system consists of the kidneys, ureters, urinary bladder, and urethra. Its primary function is to filter blood, remove waste products, and excrete them as urine. The kidneys play essential roles in regulating blood composition, pH, volume, and pressure. They maintain blood osmolarity, produce hormones (calcitriol and erythropoietin), and help regulate blood glucose levels through gluconeogenesis. Nephrology is the study of kidney anatomy, physiology, and pathology, while urology deals with the urinary and male reproductive systems.
Parts of Human excretory system
Kidneys
External anatomy:
Kidneys are located on each side of the vertebral column, just below the rib cage.
The typical adult kidney measures 10–12 cm in length, 5–7 cm in width, and 3 cm in thickness. Each kidney contains an indentation called the renal hilum. This hilum serves as the entry and exit point for the ureter, blood vessels, lymphatic vessels, and nerves.
The kidneys are enveloped in three layers of tissue. The innermost layer is the renal capsule, a smooth, transparent sheet of dense irregular connective tissue that protects the kidney from trauma. Surrounding the renal capsule is the adipose capsule, which consists of fatty tissue and further shields the kidney from injury. The outermost layer, the renal fascia, is a thin layer of dense irregular connective tissue that anchors the kidney to the surrounding structures and the abdominal wall.
Internal anatomy:
Kidney has two distinct regions: the peripheral, lighter renal cortex and the deeper, darker renal medulla. The renal medulla consists of cone-shaped structures known as renal pyramids. Each pyramid has a base facing the cortex and an apex called a renal papilla pointing towards the renal hilum. The urine formed in nephron is collected in papillary ducts, then drained into minor and major calyces before reaching the renal pelvis and finally exiting through the ureter to the bladder.
Blood supply
The kidneys are incredibly well-supplied with blood vessels, receiving approximately 20-25% of the resting cardiac output. The renal artery divides into segmental arteries, interlobar arteries, and arcuate arteries, eventually branching into a series of capillaries called glomeruli. The glomeruli are unique capillaries situated between two arterioles, and they play a critical role in urine formation.
Nephron
The nephron is a microscopic structure, serving as the fundamental unit of the kidneys responsible for the formation of urine. Each kidney contains approximately one million nephrons. The nephron consists of two key components:
- The renal corpuscle comprises the glomerulus, a capillary network, and the glomerular capsule or Bowman’s capsule, which surrounds the glomerulus. The blood plasma filtered here is known as glomerular filtrate. It enters the renal tubule.
- The renal tubule is divided into three main sections:
- The PCT is tightly coiled and located in the renal cortex. PCT has simple cuboidal epithelial cells with a brush border of microvilli.
- The Henle’s loop is made of squamous epithelium and it extends into the renal medulla and returns to the cortex.
- The DCT has simple cuboidal to low columnar epithelium.
DCT leads to the collecting duct. These ducts unite and form larger papillary ducts, which ultimately drain into the minor calyces. After filtration in the nephrons, urine drains into the minor calyces, major calyces, and finally into the renal pelvis. From the renal pelvis, urine passes through the ureters, which are muscular tubes that transport urine from the kidneys to the urinary bladder. Peristaltic contractions, hydrostatic pressure, and gravity contribute to urine movement in the ureters.
Ureters
The ureters are thick-walled tubes, approximately 25-30 cm long. The ureter wall consists of three layers.
- The deepest layer, the mucosa, contains transitional epithelium and an underlying lamina propria.
- The intermediate layer, the muscularis, consists of smooth muscle fibres arranged in inner longitudinal and outer circular layers.
- The distal third layer consists of outer longitudinal muscle fibres. Peristalsis, rhythmic muscular contractions, is the primary function of the muscularis.
- The outermost layer, the adventitia, is composed of areolar connective tissue and anchors the ureters in place.
Urinary bladder
The urinary bladder is a hollow, muscular organ located in the pelvic cavity, posterior to the pubic symphysis. The bladder wall consists of three coats:
- The mucosa, composed of transitional epithelium, permits stretching, and has rugae to allow expansion.
- The intermediate muscularis, or detrusor muscle, has three layers of smooth muscle fibers facilitating bladder contraction.
- The outermost coat, the adventitia, a layer of areolar connective tissue.
The trigone, a triangular area in the floor of the bladder, contains the openings of the two ureters and the internal urethral orifice.
Urethra
The urethra is a small tube that carries urine from the internal urethral orifice to the exterior of the body.
- In males, urethra is subdivided into the prostatic urethra, membranous urethra, and penile urethra. It helps in carrying both urine and semen in males.
- In females, the urethra is shorter and lies posterior to the pubic symphysis. So, females are more susceptible to urinary tract infections (UTIs) compared to males.
Summary for human excretory system
The human excretory system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys play a vital role in filtering blood, regulating blood composition, pH, volume, and pressure, and producing hormones. Each kidney has a complex internal structure called the nephron, which is responsible for urine formation. Urine is collected in the renal pelvis, drained into the ureters, stored in the urinary bladder, and excreted through the urethra. Nephrology focuses on the study of kidney anatomy and function, while urology deals with the urinary and male reproductive systems.
Frequently Asked Questions on human excretory system
What is the function of the kidneys?
kidneys are responsible for filtering blood to remove waste products and excess substances, such as electrolytes and water, to form urine. They also play essential roles in regulating blood composition, pH, volume, and pressure, as well as producing hormones and maintaining blood osmolarity.
What are nephrons?
Nephrons are the functional units of the kidneys, responsible for the formation of urine. Each kidney contains approximately one million nephrons, which consist of a renal corpuscle (glomerulus and Bowman's capsule) and a renal tubule (proximal convoluted tubule, nephron loop, and distal convoluted tubule).
How does urine formation occur?
Urine formation occurs through a process called filtration, reabsorption, and secretion. Blood is filtered in the glomerulus of the renal corpuscle, and the filtrate (glomerular filtrate) passes through the renal tubule. Along the tubule, substances are reabsorbed back into the bloodstream, and some substances are actively secreted into the tubule. The final product, urine, is then collected in the renal pelvis and drained through the ureters into the urinary bladder.
What is the role of the urinary bladder?
The urinary bladder stores urine until it is excreted from the body through the urethra. When the bladder fills with urine, stretch receptors send signals to the brain, leading to the sensation of needing to urinate. The micturition reflex, a combination of involuntary and voluntary muscle contractions, allows urine to be expelled from the bladder through the urethra.
How does the urethra differ between males and females?
In males, the urethra is longer and extends through the prostate, deep muscles of the perineum, and penis. It serves as a passageway for both urine and semen. In females, the urethra is shorter and located posterior to the pubic symphysis. It is solely responsible for carrying urine from the bladder to the exterior of the body.
What is the role of the Juxtaglomerular Apparatus (JGA) in the kidneys?
The Juxtaglomerular Apparatus (JGA) is a specialised structure located near the glomerulus of the nephron. It consists of the glomerular afferent and efferent arterioles and the macula densa, which are specialized tubular epithelial cells. The JGA plays a crucial role in regulating glomerular hemodynamics and controlling the release of renin, an enzyme that helps regulate blood pressure and fluid balance.
What is the significance of renal pyramids in the kidney?
Renal pyramids are cone-shaped tissues found in the renal medulla of the kidney. These structures contain the nephrons' loops of Henle and collecting ducts, which are essential for urine formation and concentration. The arrangement of renal pyramids helps in the efficient filtering and concentration of urine before it is drained into the minor calyces and eventually into the renal pelvis.
What do the Columns of Bertini do in the kidney?
Columns of Bertini are extensions of the renal cortex that project into the renal medulla between the renal pyramids. These columns provide structural support to the kidney and help maintain its shape. They also serve as pathways for blood vessels and urinary tubules, facilitating the transport of filtrate and urine through the kidney.
