Kidney
Two bean shaped kidneys are attached to the posterior abdominal wall, one on each side of the vertebral column.
The kidneys have a tough fibrous capsule (irregular dense connective tissue) for protection. Otherwise they have very little connective tissue between the nephrons.
The kidney has a granular cortex (outer region). It has this appearance because it is full of ovoid filtration units.
And a medullar (inner region) which has a more striated appearance.
The kidney is organised into many lobes, organised in a pyramidal structure, where the outer portion is made up of cortex, and the inner portion is made up of the medulla.
The kidney contains about 1 million functional units called nephrons, which are continuous with a system of collecting tubules. The nephrons are responsible for filtration, excretion and resorption, and they regulate ion balance, water content, help to stabilise blood pressure.
This regulation of ion balance and water content (Osmoregulation) of the blood plasma means that all the other body fluids will also be regulated.
Urine leaves the kidney via the ureter for temporary storage in the bladder.
Renal Corpuscle
The renal corpuscle is formed when a mass of glomerular capillaries grows into the blind ending of a nephron.
The nephron encapsulates this mass of capillaries, and it invaginates to form Bowman’s capsule. (see diagram).
The epithelial cells on the inside (visceral layer) become closely associated with the capillaries. The outer layer of epithelium is called the parietal layer.
Blood is filtered out from the capillaries through the visceral layer of epithelium into the capsular (Bowman’s) space, and can move out from here into the proximal convoluted tubule.
Large amounts of glomerular filtrate are produced for two reasons:
First: The glomerular capillaries do not drain into veins, but into an efferent arteriole.
Both afferent and efferent arterioles have smooth muscle cells in their walls.
Contraction of this smooth muscle, maintains a relatively high hydrostatic pressure along the length of the capillaries.
Second: the glomerular capillaries are fenestrated, which enables blood to be filtered out.
Molecules from blood plasma must
- 1. pass through the fenestrated capillary epithelium (fenestrations are 50-100nm in diameter)
- 2. pass through the thick basement membrane (main filtration barrier)
- 3. pass through filtration slits between the podocyte feet
Nephron
The nephron consists of the renal corpuscle and the renal tubule.
The nephron is responsible for filtration, excretion and resorption. It filters out small molecules from plasma, selectively re-absorbs most of the water, and some of the molecules, and also secretes some secretory products into the filtrate.
The end product is urine.
This schematic diagram shows where the different parts of the nephron are found in the cortex and medulla of the kidney.
Filtration of the blood plasma takes place in the renal corpuscle.
This is the proximal end of the nephron, which is expanded into an ovoid structure. The renal corpuscles are always found in the renal cortex.
Here a compact mass of looped fenestrated capillaries called the glomerulus (latin for ‘small ball’) is encapsulated by the proximal end of the renal tubule (‘Bowman’s capsule).
After leaving the renal corpuscle, the filtrate passes through the renal tubule in the following order:
proximal convoluted tubule (found in the renal cortex)
loop of Henle (mostly in the medulla)
distal convoluted tubule (found in the renal cortex)
collecting tubule (in the medulla)
collecting duct (in the medulla)
The beginning of the distal convoluted tubule is found close to the renal corpuscle, in a structure known as the juxtaglomerular complex.
Ureter
- A thick, fibroelastic, lamina propria lies underneath the epithelium.
- This mucosa (epithelium and lamina propria) is protective.
- There are no mucosal or submucosal glands, and no submucosa.
- There is a layer of smooth muscle outside the mucosa:
- The upper two-thirds has two layers of smooth muscle: inner longitudinally arranged, and outer circularly arranged.
- The lower third has three layers of smooth muscle; Inner longitudinal, middle circular, outer longitudinal.
- Urine is squeezed into the bladder by peristalsis.
- Outer adventitial layer has fibroelastic connective tissue, with blood vessels, lymphatics and nerves.
- Folds of mucosa help to protect against reflux of urine when the bladder is full.
Bladder
The bladder has three layers of smooth muscle, and a transitional epithelium. It’s harder to make out the three layers, because the bladder is sac like, not a tube.
The mucosa is heavily folded – this helps to accommodate for large volume changes.
The transitional epithelial lining can stretch until it looks like stratified squamous epithelium.
Urethra
The urethra conveys urine from the bladder to the exterior of the body.
It is similar in structure to the ureter, though shorter.
The lumen is kept closed, unless urine is being passed.
The female urethra is lined by stratified columnar epithelium, with a few small mucosal glands that secrete mucus, and is bound to the anterior wall of the vagina by an external layer of fibrous connective tissue.
