The Function of the Mammalian Kidney: Ultrafiltration and Selective Reabsorption

What is ultrafiltration in the kidney?

-Small molecules filtered out of the blood and into the Bowman's capsule, forming glomerular filtrate

What is the differences in lumen size between both arterioles and is caused by that?(3)

-The afferent arteriole is wider than the efferent arteriole

-This then results in a hydrostatic pressure gradient in the glomerulus

-High hydrostatic pressure in the glomerulus forces small molecules (water, glucose, ions, urea) out of the blood into the Bowman's capsule.

What substances may be found in the Glomerular filtrate?

-Amino acids

-Water

-Glucose

-Urea

-Inorganic ions

Why do blood cells and large proteins stay inside the Glomerulus despite the hydrostatic pressure gradient?

-Too large to pass through the perforations in the capillaries

State the process of ultrafiltration in the kidney.

-In the Bowman's capsule, a high hydrostatic pressure gradient is created due to the afferent arteriole having a wider diameter than the efferent ateriole

-This causes smaller molecules to be forced out of the glomerulus and into the Bowman's capsue

-(such as: amino acids, water, glucose, urea, inorganic ions)

-Blood cells and large proteins remain as they're too large to pass through the capillary perforations

State the properties of the basement membrane involved in ultrafiltration and how it aids the process.(2)

-The basement membrane is made of a mesh of collagen fibres and glycoproteins

-It further acts as a filter for ultrafiltration.

What are podocytes and what is their function?

-Modified endothelial cells which have gaps that allow small molecule

-Makes up the epitelium of the Bowman's Capsule

What structure does Selective reabsorption take place in?

-Proximal Convoluted Tubule

What is the purpose of Selective Reabsorption

-The reabsorption of useful substances into the blood

-examples being: water; salts; glucose; amino acids

What are the adaptations of the Proximal Convoluted Tubule which aid in reabsorption?(4)

-Microvilli

-Co-transporter proteins

-Many mitochondria

-Tightly packed cells

How does microvilli serve as an adaptation for the PCT?

-Has many microvilli, which greatly increases the surface area for substance reabsorption

How does mitochondria serve as an adaptation for the PCT?

-Has many mitochondria to provide ATP for the sodium-potassium pump proteins as well as other active transport processes involved

How do the cotransporter proteins serve as an adaptation for the PCT?

-Allows for charged solutes to be transported from the filtrate

How are glucose and amino acids reabsorbed in the PCT?

-Via active transport and facilitated diffusion. They are co-transported with sodium ions (Na⁺) across the membrane of the microvilli.

How do the cells being packed tightly serve as an adaptation for the PCT?

-Means that no fluid can pass between the cells

What is the process of reabsorption?(Part 1- Na-K Pumps)

-Sodium-Potassium pumps in the cells lining the PC actively transport sodium ions out of epithelial cells and into the blood

What is the process of reabsorption?(Part 2- Concentration of Na)

-This lowers the concentration of sodium ions in the epithelial cells, causing the sodium ions in filtrate to diffuse down the concentration gradient into the epithelial cells

What is the process of reabsorption?(Part 3- Co-transporter proteins)

-The sodium ions move via co-transporter proteins, and as they move in, the proteins transport another solute(e.g. glucose) in at the same time

What is the process of reabsorption?(Part 4- Diffusion into the blood)

-When in the epithelial cells, the solutes diffuse down their concentration gradients into the blood

What is the role of the Loop of Henle?

-To create a low water potential (high salt concentration) in the tissue of the medulla. This allows water to be reabsorbed by osmosis from the collecting duct later on. (It is a counter-current multiplier).

What are the features of the ascending loop of Henle?

-Impermeable to water, and Na+ and Cl- are pumped out, decreases the water potential of interstitial fluid

-So Na+ and Cl- diffuse out of it

-So Water potential increases due to solute removal

What are the features of the descending loop of Henle?

-It is permeable to water, so water leaves via osmosis due to the low water potential of the medulla

-Low permeability to ions as few transport proteins

How does filtrate become more concentrated when it moves down the loop?(5)(counter current system)

-The medulla has a low water water due to the action of the ascending limb

-The descending limb is permeable to water

-Water moves out of the tubule into the medulla by osmosis

-The descending limb is impermeable to salts, so they remain inside

-This causes the filtrate volume to decrease and its concentration to increase

What is the role of the Distal Convoluted Tubule (DCT) and Collecting Duct (CD)?

-Osmoregulation. They fine-tune the water potential of the blood by varying the amount of water reabsorbed from the filtrate, under the control of the hormone ADH.