Kidneys

Overview

What are the kidneys role?

The kidney is a vital organ and has a key role in:

• Filtering the blood, removing waste products from it
• Maintaining and regulating water potential and electrolyte balance
• Excreting waste products as urine

Structure of kidneys

Mammals have 2 kidneys, each made of 3 main regions:

• Cortex
  • Outer region
  • This includes: Glomerulus, Bowman’s capsule, the PCT & DCT
• Medulla (m for middle region)
  • Middle region
  • Loop of Henle
• Renal Pelvis
  • Collecting urine from collecting ducts, passes to ureter

A nephron is the functional unit of the kidney (image below)

Osmoregulation stages

The kidneys have to maintain the water potential of the blood plasma and the tissue fluid, this is done through the following stages:

1. Ultrafiltration: Formation of glomerular filtrate
2. Selective Reabsorption of glucose & water at Proximal Convoluted Tubule (PCT)
3. Maintenance of a sodium ion gradient in medulla by Loop of Henle
4. Reabsorption of water by Distal Convoluted Tubule (DCT) and collecting ducts

Nephron with key structures and stages of osmoregulation

Ultrafiltration

How is a high hydrostatic pressure maintained?

• The diameter of afferent arteriole (the arteriole that takes blood into the glomerulus) is greater than the efferent arteriole (takes filtered blood towards the capillaries) this results in the blood in the glomerulus being under high hydrostatic pressure.

Glomerular Capillary Walls

The glomerular capillary wall has layers the blood has to pass through in the ultrafiltration process:

• Gaps in the Endothelium - this has pores which prevent red blood cells passing through
• Basement membrane - the main filter which prevents proteins from being filtered out the blood
• Epithelium - Gaps between the podocytes
  • These podocytes are specialised cells attached to the basement membrane, they allow filtrate to pass between them

These 3 layers prevent larger molecules, such as red blood cells and proteins passing through, so these remain in the blood. The substances (smaller molecules) that pass through enter the Bowman’s capsule and are known as the glomerular filtrate.

High levels of protein in the urine, known as proteinuria or albuminuria, can occur when the glomeruli are damaged, allowing proteins to pass through basement membrane from the blood

Selective Reabsorption

As this glomerular filtrate passes into the proximal convoluted tubule (PCT), selective reabsorption occurs.

This is where the body reabsorbs ‘useful’ substances back into the bloodstream, glucose and water.

Adaptations of PCT

To ensure the useful substances (glucose) can be reabsorbed, the PCT has some specific features

• Microvilli on epithelium of PCT to provide a large surface area for reabsorption of useful materials from glomerular filtrate into capillaries
• Lots of mitochondria to produce ATP for active transport
• Many carrier proteins/channel proteins for facilitated diffusion
• Many carrier proteins for active transport
• Cells packed together (no fluid can pass between cells)

Selective Reabsorption Process

1. Active Transport: Sodium ions (Na+) are actively transported out of the PCT epithelial cells and into blood capillaries. This reduces the Na+ concentration in epithelial cells lining the PCT

2. Co-transport: Na+ moves from the PCT lumen into the epithelial cells, down its concentration gradient. Na+ is co-transported by carrier proteins with substances like glucose and amino acids into the epithelial cells.

3. Facilitated diffusion & osmosis: Glucose and amino acids move into the capillaries from the PCT via facilitated diffusion. Due to the movement of solutes, water also moves via osmosis into epithelial cells & capillaries

Loop of Henle

The Loop of Henle is a long curved tubule (see image) which extends into the medulla. It’s responsible for maintaining the gradient of sodium ions in the medulla so water can be reabsorbed into the blood. We will look at how the kidney does this below.

Sodium ions are actively transported out of the ascending limb.

This creates a lower water potential in the medulla, due to the concentration of these sodium ions.

Water moves out of descending limb

As there is a low water potential in the medulla, water moves out of the descending limb into the medulla. It moves into interstitial space, the space between kidney tubules and blood vessels and can then be reabsorbed into the blood through the capillary network

Filtrate becomes more concentrated

As the filtrate travels down the descending limb it becomes progressively more concentrated (most concentrated is at the bottom of the hairpin turn)

Sodium ions diffuse out of bottom of ascending limb

Sodium ions also diffuse out of the bottom of the ascending limb, this causes the water potential of the medulla to decrease even further

Reabsorption of water

With a low water potential in the medulla, water can move out the distal convoluted tubule (DCT) and collecting duct by osmosis into the medulla (interstitial space). This water can then be reabsorbed into the blood through the capillary network.

The volume of water which is reabsorbed into the blood via the capillary network is controlled by changing the permeability of the DCT and collecting duct. This is done by antidiuretic hormone (ADH).

ADH

Osmoreceptors detect low water potential of blood

• Osmoreceptors in the hypothalamus detect low water content of the blood (decrease in water potential)
• Hypothalamus produces ADH
• This is passed into the posterior pituitary gland and from here it releases ADH into the bloodstream

ADH binds to receptors on DCT & collecting duct

• ADH passes to the kidneys and binds to receptors on cell membrane of the DCT & collecting duct
• This increases the permeability of DCT and collecting duct walls

How ADH increase the permeability of DCT & Collecting Ducts?

• When ADH binds to receptors on cell membrane of DCT & Collecting Duct
• This leads to activation of enzyme (phosphorylase)
• This causes vesicles with aquaporins (channel proteins act as water channels) to move and fuse with cell membrane
• This makes them more permeable to water
• More water leaves the collecting duct/DCT by osmosis down concentration gradient and returns to the blood

Summary diagram

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