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Chapter 4: Urinary System -NBF Short Notes
Nitrogenous Waste Products and Habitats
Animals excrete nitrogenous waste products like ammonia, urea, and uric acid. The type of waste an animal excretes depends on the availability of water in its habitat.
- Ammonia: Excreted by ammonotelic animals, primarily aquatic animals like fish. It's highly toxic and requires a large amount of water for dilution.
- Urea: Excreted by ureotelic animals, including marine and most terrestrial animals. It's less toxic than ammonia and requires less water for excretion.
- Uric Acid: Excreted by uricotelic animals, such as reptiles, insects, land snails, and birds, who need to conserve water. It is the final product of purine breakdown, is the least toxic, and is excreted as a semi-solid paste, requiring minimal water.
The Urinary System of Man
The human urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The kidneys are bean-shaped organs located in the back that filter blood and produce urine. Ureters transport urine from the kidneys to the bladder for storage, and the urethra expels it from the body.
Structure of the Kidney
External Structure
The kidney is enclosed by three layers: the renal fascia (outermost), the perirenal fat capsule (middle), and the renal capsule (innermost). The concave part where vessels and nerves enter is the hilum.
Internal Structure
The kidney has three main regions:
- Cortex: The outer, granular layer containing the functional units called nephrons.
- Medulla: The middle region composed of cone-shaped renal pyramids. The tips of these pyramids, called renal papillae, point towards the renal pelvis. The spaces between the pyramids are the renal columns.
- Renal Pelvis: The inner, funnel-shaped region that collects urine. It branches into major calyces and minor calyces.
The Nephron: The Functional Unit
The nephron is the basic functional unit of the kidney, responsible for filtering blood and forming urine.
Main Parts of a Nephron
- Renal Corpuscle: The filtering component, made up of the glomerulus (a network of capillaries) surrounded by the Bowman's capsule.
- Renal Tubule: A long, coiled tube with distinct sections:
- Proximal Convoluted Tubule: The first highly coiled part.
- Loop of Henle: A U-shaped loop with a descending and ascending limb.
- Distal Convoluted Tubule: A coiled part that connects to the collecting duct.
- Collecting Duct: Receives filtrate from multiple nephrons and empties into the renal pelvis.
Types of Nephrons
There are two main types:
- Cortical Nephrons: The most common (85%), located primarily in the cortex with a short Loop of Henle. They are involved in maintaining fluid balance.
- Juxtamedullary Nephrons: The remaining 15%, located closer to the medulla-cortex junction with a long Loop of Henle, crucial for producing concentrated urine.
Kidney Blood Vessels
Blood flows into the kidney through the renal artery, which branches into afferent arterioles leading to the glomerulus. Blood leaves the glomerulus via efferent arterioles and enters the peritubular capillaries and vasa recta. The filtered blood eventually exits the kidney through the renal vein.
Urine Formation
Urine is formed in three stages:
- Glomerular Filtration: Blood is filtered in the glomerulus. Water, ions, and small molecules are forced out to form glomerular filtrate.
- Tubular Reabsorption: Essential substances like 99% of water, glucose, and amino acids are reabsorbed from the filtrate back into the blood.
- Tubular Secretion: Additional waste products, excess ions, and toxins are actively secreted from the blood into the renal tubule, fine-tuning urine composition.
Countercurrent Multiplier Mechanism
The countercurrent multiplier system, involving the Loop of Henle and the vasa recta, creates an osmotic gradient in the kidney medulla. The ascending limb actively pumps out solutes, while the descending limb is permeable to water, allowing it to move out into the increasingly concentrated interstitial fluid. The countercurrent flow in the vasa recta maintains this gradient, enabling the production of concentrated urine.
Hormonal Regulation
The hormone aldosterone, secreted by the adrenal cortex, acts on the distal tubules and collecting ducts. It increases the reabsorption of sodium (Na+) and water, thereby increasing blood volume and pressure, and further concentrating the urine.
Functions of the Kidney
- Excretory Function: Filters nitrogenous waste from the blood.
- Osmoregulatory Function: Maintains the body's water balance by producing diluted or concentrated urine as needed.
Disorders of the Urinary Tract
Urinary Tract Infections (UTIs)
UTIs are common, with a higher risk for women due to a shorter urethra. Infections can affect the urethra (urethritis), bladder (cystitis), or kidneys (pyelonephritis). They are treated with antibiotics and can be prevented by good hygiene and hydration.
Kidney Stones
Hard mineral deposits that form in the urinary tract. Causes include high levels of calcium, oxalate, or uric acid. Types include calcium oxalate (most common), calcium phosphate, and uric acid stones. Symptoms include severe back pain and blood in the urine. Treatment options include Extracorporeal Shock Wave Lithotripsy (ESWL) and Percutaneous Nephro Lithotripsy (PCNL).
Kidney Failure
A decline in kidney function.
- Chronic Renal Failure: A gradual, irreversible decline caused by infections, nephritis, high blood pressure, or diabetes. It can lead to end-stage renal disease (ESRD) and uremia.
- Acute Renal Failure: A sudden decline caused by trauma, dehydration, obstruction (e.g., kidney stones), or severe nephritis.
Dialysis and Kidney Transplant
Dialysis
An artificial method to remove toxins from the blood.
- Haemodialysis: Uses an external filter (dialyzer) to cleanse the blood outside the body.
- Peritoneal Dialysis: Uses the body's own peritoneal membrane as a natural filter inside the abdomen.
Kidney Transplant
A surgical procedure to place a healthy kidney from a donor into a patient with kidney failure. The process requires compatibility of blood group and Human Leukocytes Antigen (HLA). The main problem is rejection by the recipient's immune system, which is managed with immunosuppressant drugs.
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