The Excretory System


Core Concepts

Different habitats present different challenges to the water and electrolyte balance of the animals that inhabit them.  

The Sodium-Potassium pump represents a common mechanism in maintenance of water and electrolyte balance

The ability to form a concentrated urine is critical to the success of animals in terrestrial habitats

The functions of the Excretory System (with emphasis on terrestrial animals, especially mammals)

Water Conservation

Reduction of Osmotic Stress & Osmoregulation

Removal of Nitrogenous Wastes from the Body

Regulation of Blood Volume and Concentration


The Mammalian Kidney Is an Important Adaptation to Life on Land; Its Functional Unit is the Nephron

The mammalian kidney consists of an outer cortex and an inner medulla. It is composed of units called nephrons. Each nephron consists of a glomerulus, situated in the cortex of the kidney, and a long U-shaped tubule (the loop of Henle) that extends into the medulla of the kidney and connects to collecting tubules which eventually merge to join the ureter. 

The kidney has an outer region called the cortex and an inner region called the medulla.  As one progresses inward from the cortex to the inner medulla, the concentration of solutes increases from about 300 mosm/L to 1200 mosm/L.  The effects of this on water and solute balance will be discussed below.



Phase I - Filtration in Bowman's Capsule

The glomerulus consists of a cuplike structure, Bowman’s capsule, within which lies a cluster of capillaries, and a hairpin-shaped tubule that runs from Bowman’s capsule into the medulla of the kidney and to collecting ducts in the medulla. The capillaries are extremely thin-walled, and significantly more permeable to plasma than ordinary capillaries. Moreover, the diameter of the arteriole as it leaves Bowman’s capsule is less than its entering diameter. Surrounding the capillaries are cells known as podocytes - they create a network of cytoplasmic extensions which aid in filtration.  All of these factors combine to increase blood pressure and force large quantities of plasma out of the capillaries into the Bowman’s capsule and down the tube of the nephron. Small solutes (especially salts and urea, but also other water soluble molecules) are also forced out of the bloodstream with the plasma. Larger proteins, and cells remain in the capillaries, creating a very hypertonic solution. 

The general strategy here is this: the blood plasma is full of nutrients, proteins, ions, water, and other dissolved particles, some of which the body needs, some of which the body must remove.  To remove the wastes, a large portion of the plasma is filtered from the blood and then the substances the body needs are put back into the blood.  The substances that the body doesn't need are left behind and removed during urination.

Phase II - Reabsorption in the Proximal Tubule

The lumen wall of the epithelial cells of the proximal tubule is like the lumen wall of the small intestine - both are bordered with millions of microvilli to increase surface area.  The role of these epithelial cells is to reabsorb ions, nutrients, and water and transport them to the blood vessels nearby.  

1.  A Na+/K+ ATPase located on the basolateral membrane of the epithelial cell (the side of the cell opposite the lumen) actively pump Na+ out of the cell into the blood.  This sets up a strong concentration gradient in the cell

2.  The gradient created by the Na+/K+ ATPase provides a potential to allow Na+  contransporters in the apical membrane to reabsorb nutrients and electrolytes.  Water also flows in via osmosis

3.  Solutes exit the epithelial cells and enter the blood through channels

4.  Water flows from the epithelial cells into the blood via osmosis.

Note that because osmosis occurs, the osmolarity of the filtrate remains isotonic.  The volume decreases 

 

 

 

 

Phase III - Creation of an Osmotic Gradient in the Loop of Henle

The loop of Henle has three distinct regions, the thin-walled descending limb, the thin-walled lower portion of the ascending limb and the thick-walled upper portion of the ascending limb.  The descending limb of the loop is highly permeable to water but almost completely impermeable to solutes.  As the filtrate travels down the descending loop, water will flow from the loop into the surrounding medium via osmosis.  When the filtrate reaches the hairpin turn, it is isotonic with the surroundig medium (about 1200 mosm/L)

The lower portion of the ascending branch of the loop of Henle, however, is highly permeable to Na+ and Cl-, moderately permeable to urea, and almost completely impermeable to water.  As it travels up into the  less-concentrated regions of the medulla, Na+ and Cl- will passively diffuse across the membrane. As the filtrate continues up the thick portion of the loop of Henle, Na+ and Cl- are actively pumped out of the filtrate into the surrounding medium.  This requires energy, but helps to maintain the osmotic concentration gradient in the medulla.

The water and solutes that flowed into the medulla can be reabsorbed by the vasa recta - a network of capillaries that surround the loop of Henle and reabsorb water and solutes filtered from the blood.

 

No ADH Present - Collecting Duct is NOT permeable to water and large volume of urine is produced

ADH Present - Collecting Duct is permeable to water and a small volume of urine is produced

Phase IV - Regulating Water and Electrolyte Balance in the Distal Tubule and the Collecting Duct

The first three steps in urine formation, filtration, reabsorption, and establishment of an osmotic gradient, result in a fluid that is slightly hypotonic to blood.  The major solutes still present in this fluid are urea and other wastes.  Electrolytes and water are always absorbed by the distal tubule, the amount of Na+, Cl-, and water absorbed is variable.  Regulation of these processes are under hormonal control.

If the Na+ levels in the blood are low, the hormone aldosterone is released. which leads to reabsorption of Na+ and Cl- in the distal tubule.  Water will also flow into the tubule via osmosis.  However, if a person is dehydrated, the hormone ADH (antidiuretic hormone) is released.  This causes aquaporin channels to be inserted in the membrane of the collecting duct so that large quantities of water can be reabsorbed.