Urine Production In Kidney

Picture Source:http://www.ivy-rose.co.uk/Topics/Urinary_System_Nephron_Diagram.php

Urine Production in kidney occurs in 3 stages:

Ultrafiltration, Reabsorption and Secretion.

Ultrafiltration
-Afferent Arteriole transport blood into glomerulus while Efferent Arteriole transport blood away from glomerulus.
-Afferent Arteriole has bigger diameter than Efferent Arteriole.
-Rate of blood flow into glomerulus through Afferent Arteriole is higher compared to rate of blood flow out of glomerulus through Efferent Arteriole.
-The difference in rate of blood flow facilitates creation of high hydrostatic pressure in glomurulus.
-The pressure created causes blood from glomerulus to be filtered into Bowman’s capsule across endothelium and Bowman’s capsule’s wall.
-Ultrafiltration Occurs.
-Fluid formed in Bowman’s Capsule is Glomerular Filtrate.
-Filtrate has same composition with blood except it lacks Red Blood Cells, Plasma Protein and Platelets which are too large to be filtered across endothelium.
-Ultrafiltration is facilitated by these adaptation:
.Blood that flows through afferent arteriole has high pressure as it is directly connected to aorta though renal artery.
.Presence of podocytes that wraps around capillaries in glomerulus to increase surface area for ultrafiltration.
.Both endothelium and Bowman’s Capsule wall are one-cell thick each, the wall is made of squamous epithelial cell.

Reabsorption:
-From Bowman’s Capsule, filtrate flows into proximal convoluted tubule in which reabsorption process occurs.
-Selected substances from filtrate in tubule are reabsorb into blood in peritubular capillary.
-Examples of substances reabsorb are:
.100% of nutrients such as glucose and amino acids through active transport and facilitated diffusion.
.Minerals such as Na+ ion through active transport.
.Water through osmosis.
-Adaptation:
.Highly convoluted to increase surface area.
.Microvilli on surface of epithelial cells that is facing into lumen of proximal convoluted tubule.
.The epithelial cells have high density of mitochondrion to generate energy for active transport.

Counter current multiplier process:
-Descending arm is permeable to water while ascending arm is NOT permeable to water.
-Na+ and Cl- diffuses out of filtrate into medulla region in the thinner segment of ascending arm.
-These ion are actively transported into medulla region from thicker segment of ascending arm
-These processes lower water potential of medulla region.
-Water in filtrate in descending arm diffuses into medulla region and then into blood in vasa recta (capillaries in medulla) through osmosis.
-Reabsorption of water in Loop of Henle occurs through counter-current multiplier process.
-Adaptation- the longer rhe Loop of Henle , the more water reabsorb into capillary.

Secretion:
-Fom Loop of Henle, filtrate flows into distal convoluted tubule in which active secretion process occurs.
-Excretory substances such as urea, uric acid, ammonia, excess H+ ions, drugs and other substances are actively secreted from blood in peritubular capillaries into filtrate in the tubule(distal).
-This process enables kidney to function as an excretory organ.
-From distal convoluted tubule, filtrate flows into collecting duct in which more reabosorption of water occurs before the filtrate flows into pelvis of urine.