Renal System 1: Introduction

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What is the Renal System?

The renal/urinary system is made up of the kidneys, ureters, bladder and urethra.

The five basic functions of the renal system are:
  1. Water and ion balance
  2. Excretion of metabolites and foreign chemicals
  3. Regulation of blood pressure
    • Sodium balance
    • Renin-angiotensin system
  4. Regulation of red blood cell production (erythropoietin)
  5. Regulation of vitamin D activity
However, the two basic functions of the kidney are:
  1. Concentrate the filtrate in the nephrons by altering tubular secretion and tubular reabsorption of water and ions
  2. Absorb and retain valuable materials for use by other tissues (sugar and amino acids)
The nephron is the functional unit of the kidney, and there are two types of nephron with the same function; cortical and juxtamedullary nephrons. Both these nephrons function to maintain a water and ion balance.

The tubule of a nephron runs across the axis of the cortex to the medulla, and is surrounded by a capillary network. It is this relationship which allows for filtration of blood and reabsorption and secretion of substances overall.

The renal corpuscle is the site of blood filtration in the nephron. It contains the glomerulus, a knot of capillaries, and the glomerular capsule. The glomerular capsule is connected to the initial segment of the renal tubule and forms the outer wall of the renal corpuscle, encapsulating the glomerular capillaries.

Filtration at the renal corpuscle is passive, where a high blood pressure forces water and small solutes across the membrane into the capsular space. These solutes include metabolic wastes, excess ions, glucose, free fatty acids, amino acids and vitamins. Nothing greater than albumin is filtered out of the blood, meaning larger solutes such as proteins never enter the renal tubule.

Other regions of the nephron include the proximal convoluted tubule, the nephron loop (with its entities), the distal convoluted tubule and finally, the collecting duct:

Proximal convoluted tubule

Reabsorbs water, ions and all organic nutrients

Nephron loop

Thick descending limb: reabsorption of Na+ and Cl- ions

Thin descending limb: freely permeable to solely water

Thick ascending limb: creates high solute concentrations in peritubular fluid

Distal convoluted tubule

Actively secretes ions, acids, drugs and toxins

Reabsorbs Na+ and Ca2+ ions

Reabsorbs water to concentrate tubular fluid
  • The distal convoluted tubule travels between the afferent and efferent arterioles surrounding the renal corpuscle. With the afferent arteriole, the macula densa cells of the DCT form the juxtaglomerular complex. This complex functions using a combination of signalling cells to maintain blood pressure and filtration rate at the glomerulus. To help in this adjustment, the juxtaglomerular complex secretes renin and erythropoietin.

Collecting Ducts

From the renal medulla to the renal pelvis

Variable reabsorption of water concentrates tubular fluid

Variable reabsorption or secretion of Na+, K+, H+ and HCO3- ions


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