Tag Archives: ultrafiltration

The Kidneys and the Ionic Composition of the Extracellular Fluids

The regulation of homeostasis within the body involves the renal system – which comprises of 2 kidneys connected to a urinary bladder by independant ureters. From the bladder, urine passes out of the body via the Urethra.

The urine from the bladder can tell us many things about the condition of the body – from ionic content to water content to illnesses and disease. There are 4 different properties to look at:

  1. Colour – eg Red/Black might indicate red blood cells present & therefore Malaria
  2. Clarity – is it opaque, translucent…
  3. Odour
  4. Taste – eg A sweet taste might indicate glucose presence and therefore Diabetes

Kidney Function

The kidney allows homeostatic regulation of the water and ion content in the blood. This iincludes:

  • Regulation of extracellular fluid (ECF)
  • Regulation of blood osmolarity
  • Regulation of ion concentrations – eg keeping Na+, K+, Cl-, Ca2+ within normal ranges.
  • Regulation of blood pH with H+/HCO3-

Following the regulation of these systems the waste fluids and substances (such as urea) are excreted. The renal system is also involved in the production of some hormones such as Vitamin D hormone.

To do all of this, the Kidney is highly specialised and first filters substances out of the blood before selectively reabsorbing what is needed by the body. Anything not reabsorbed is excreted.

Kidney Structure

The functional unit within the kidney is the nephron, which spans the cortex and medulla of the kidney.

Diagram of a Kidney with Nephron closeup

Diagram of a Kidney with Nephron closeup – From HowStuffWorks

Looking at the Kidney above, we have:

A – Renal Vein
B – Renal Artery
C – Ureter
D – Cortex/Medulla
E – Renal Pelvis
F – Capsule

In fact, this illustration is not great – all of the tissue between the pelvis and capsule forms the medulla and cortex. The medulla surrounds the pelvis with a structure containing Renal Pyramids (shown in red on the picture above). Surrounding this is the thinner cortex which does not share this unique structure.

Spanning the medulla & cortex there are millions of nephrons. And looking at the nephron (listed in the order filtrate passes through):

5 – Renal corpuscle, comprised of the glomerulus and the bowmans capsule
4 – Prominal Convoluted Tubule
2 – Descending loop of Henle
1 – Ascending loop of Henle
6 – Distal Convoluted Tubule
3 – Renal Capillaries – these do surround all of the above but are only illustrated surrounding several areas.

The distal convoluted tubule then leads on to the collecting duct which also has an important role in Kidney function. The collecting duct leads to the ureter.


Filtration of substances out of the blood happens at the Renal Corpuscle.

Renal Corpuscle

Renal Corpuscle – Modified from original: Gray’s Anatomy figure 1130

The entire unit is the renal corpuscle, which areas in red are the glomerulus and pink the bowmans capsule.

A – Afferent Arteriole
B – Efferent Arteriole (leaving the glomerulus)
C – Fenestrated Endothelium of Glomerular Capillary
E – Basal Lamina
D – Bowman’s Capsule Epithelium
F – Beginning of Proximal Convoluted Tubule

The blood enters the glomerulus and molecules/fluid are filtered out of the blood along a net filtration gradient (~17mmHg). This is driven by the high (60mmHg) hydrostatic pressure which is resisted by the capsule fluid pressure and glomerular osmotic pressure. This net filtration pressure forces molecules through 3 barriers:

  • Glomerular Capillary Endothelium
  • Basal Lamina
  • Bowman’s Capsule Epithelium

This means molecules are sieved and have to be able to ‘fit’ through ‘slits’ in the bowman’s capsule epithelium – so larger molecules like cells and larger proteins (eg red blood cells) stay in the blood. The standard glomerular filtration rate is 125ml/min or 180L/day.

Autoregulatory systems ensure the hydrostatic pressure remains constant even when blood pressure or heart rate increases by altering blood flow through the glomerular capillaries. Smooth muscle contracts or relaxes depending on the need.