The urinary organs include the kidneys, ureters, bladder, and urethra. The Urinary system works with the other systems of the body to help maintain homeostasis. The kidneys are the main organs of homeostasis because they maintain the acid base balance and the water salt balance of the blood.
FUNCTIONS OF KIDNEYS:
- Excretion of metabolic waste products and foreign chemicals
- Regulation of water and electrolyte balances
- Regulation of body fluid osmolality and electrolyte concentrations
- Regulation of arterial pressure
- Regulation of acid-base balance
- Secretion, metabolism, and excretion of hormones
STRUCTURE OF KIDNEY
The Nephron Is the Functional Unit of the Kidney.
TYPES OF NEPHRON
Kidneys contain two types of nephrons, each located in different parts of the renal cortex:
i. Cortical nephrons and
ii. Juxtamedullary nephrons.
STRUCTURE OF NEPHRON
A nephron comprises a renal corpuscle, a renal tubule, and the associated capillary network.
Each kidney in the human contains about 1 million nephrons, each capable of forming urine.
[NOTE:The kidney cannot regenerate new nephrons.Therefore, with renal injury, disease, or normal aging, there is a gradual decrease in nephron number.]
Each nephron contains
1) a tuft of glomerular capillaries called the glomerulus, through which large amounts of fluid are filtered from the blood, and
2) a long tubule in which the filtered fluid is converted into urine on its way to the pelvis of the kidney.
NEPHRON STRUCTURES & THEIR FUNCTIONS
|NEPHRON STRUCTURE||THEIR FUNCTIONS|
• The site for blood filtration
• operates as a nonspecific filter - removes both useful and non-useful material
• the product of the glomerulus – filtrate
|2. Bowman’s Capsule|
• A sac that encloses glomerulus
• transfers filtrate from the glomerulus to the Proximal Convoluted Tubule (PCT)
|3. Proximal Convoluted Tubule (PCT)|
• A thick, constantly active segment of the nephron
• that reabsorbs most of the useful substances of the filtrate: sodium (65%), water (65%), bicarbonate (90%), chloride (50%), glucose (nearly 100%)
• The primary site for secretion (elimination) of drugs, waste and hydrogen ions.
|4. Loop of Henle|
• U-shaped tube that consists of a descending limb and an ascending limb.
• begins in the cortex, receiving filtrate from the PCT, extends into the medulla, and then returns to the cortex to empty into the distal convoluted tubule(DCT).
• Its primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop.
|5. Descending Limb of the loop of Henle|
• A part of the counter current multiplier• fully permeable to water and completely impermeable to solutes (salt particles)
• receives filtrate from the PCT, allows water to be absorbed and passes “salty” filtrate to the next segment. “Saves water and passes the salt”
|6. Ascending Limb of the loop of Henle|
• a part of the counter current multiplier
• impermeable to water and actively transports (reabsorbs) salt (NaCl) to the interstitial fluid of the pyramids in the medulla. “Saves salt and passes the water.”
• the passing filtrate becomes dilute and the interstitium becomes hyperosmotic
|7. Distal Convoluted Tubule (DCT)|
• Variably active portion of the nephron
• receives dilute fluid from the ascending limb of the loop of Henle
|8. Collecting Duct|
• variably active portion of the Nephron• receives fluid from the DCT
• The last segment to save water for the body
|9. Peritubular Capillaries|
• transport reabsorbed materials from the PCT and DCT into kidney veins and eventually back into the general circulation
• help complete the conservation process (reabsorption) that takes place in the kidney
The juxtaglomerular apparatus consists of three types of cells:
I. the Macula densa, a part of the distal convoluted tubule of the same nephron
II. Juxtaglomerular cells, (also known as granular cells) which secrete renin
III. Extraglomerular mesangial cells
CLINICAL SIGNIFICANCE OF RENIN
The Juxtaglomerular Cells secrete Renin in response to:
- Stimulation of the beta-1 adrenergic receptor
- Decrease in renal perfusion pressure (detected directly by the granular cells)
- Decrease in NaCl concentration at the macula densa, often due to an decrease in glomerular filtration rate
Urine Formation Results from:
- Glomerular Filtration,
- Tubular Reabsorption, and
- Tubular Secretion
Textbook of Medical Physiology (11th Edition)