Gr 11 Kidney Structure and Ultrafiltration

What is the Role of The Kidney?

Removing Nitrogenous Waste

Nitrogenous wastes are produced from the breakdown of nitrogen-containing compounds like amino acids and nucleotides

Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body
The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat

Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats

Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms

Excretion is the removal from the body of the waste products of metabolic activity

Defecation is not considered part of excretion as faeces are undigested food remnants and not metabolic waste products

There are two key functions that an excretory system performs:

Removes nitrogenous wastes that may be toxic to the body in large concentrations
Removes excess water to maintain a suitable osmolarity within the tissues and cells

The kidney contains specialised structures called nephrons which function to filter the blood and eliminate wastes

Consequently, the composition of blood entering the kidney (via renal artery) differs to that exiting the kidney (via renal vein)

Less urea (large amounts of urea is removed via the nephrons to form urine)
Less water and solutes / ions (amount removed will depend on the hydration status of the individual)
Less glucose and oxygen (not eliminated, but used by the kidney to generate energy and fuel metabolic reactions)
More carbon dioxide (produced by the kidneys as a by-product of metabolic reactions)

The nephron is the functional unit of the kidney, with each nephron being comprised of the following components:

Bowman’s capsule – first part of the nephron where blood is initially filtered (to form filtrate)
Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs
Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient
Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs

The blood to be filtered enters the Bowman’s capsule via an afferent arteriole and leaves the capsule via an efferent arteriole

Within the Bowman’s capsule, the blood is filtered at a capillary tuft called the glomerulus
The efferent arteriole forms a blood network called the vasa recta that reabsorbs components of the filtrate from the nephron

Each nephron connects to a collecting duct (via the distal convoluted tubule), which feed into the renal pelvis

The collecting ducts are shared by nephrons and hence are not technically considered to be part of a single nephron

Click on Narrated, and text, to see and hear a very clear description of how urine formation occurs in the human Kidney.

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