Lecture 30 - The ureters, bladder and urethra Flashcards
Describe the gross internal structure of the kidneys, including a brief overview of the path of blood towards the site of filtration
The kidneys have a renal pelvis which receives urine via channels called calyces, and empties into the ureter. The inner part of the kidney is arranged into medullary pyramids, surrounded by an outer cortex which is continuous and separates each medullary pyramid via renal columns. One pyramid and all of the cortex that surrounds it is called a lobe
The primary function of the kidney is to filter waste from the blood. From the abdominal aorta, renal arteries enter the hilum of the kidney. From here they branch into smaller and smaller arteries until they reach the cortex, where the renal corpuscle is located. Here the blood and nephron meet. Afferent arterioles carry blood into the glomerulus, a ball of capillarities specialised for filtration
Describe the structures present in the renal corpuscles that allow filtration from the blood to occur, and which vessels receive reabsorbed filtrate
From the glomerulus, water and solutes pass across the filtration barrier into the nephron. The first layer of the filtration barrier is the fenestrated endothelium of the capillary. Next is a fused basement membrane. The third component is the filtration slits between the pedicels of the podocytes. The filtrate will then be inside the capsular space of the nephron
As the filtrate passes through the lumen of the nephron, some will be reabsorbed back into the blood. Blood leaves the glomerulus via efferent arterioles, which branch to form peritubular capillaries which are specialised for reabsorption. These wrap around the renal tubules and take back any filtrate that is reabsorbed. Vasa recta are found associated with juxtamedullary nephrons alongside the nephron loops, where they are involved in maintaining a medullary gradient that allows production of more concentrated urine
The cells that form the visceral layer of the glomerular capsule are called
Podocytes - unusual, branched epithelial cells
The cells of the JGA on the afferent arteriole are called
Juxtaglomerular
What type of epithelial cells would you find in the nephron loop?
Cuboidal with microvilli brush border at part closest to PCT, simple squamous, cuboidal with few microvilli at part closest to DCT
Name the two capillary beds associated with the nephron
Glomerular and peritubular
Which type of nephron is important for the production of concentrated urine?
Juxtamedullary
Transitional epithelium
In the urinary system ….
Stratified, rounded cells
Flatten when stretched (and you want it to still form a barrier when stretched)
For protection
Ureters
Arise from each renal pelvis at each hilum
Slender tubes that carry urine from kidneys to bladder
Descend retroperitoneally through abdomen, vertical from hill
Perstaltic waves moves urine to the bladder, this means that there must be a muscularise layer to allow for this to occur
Ureters are very slender tubes running vertically down through the abdomen and behind the peritoneum (retroperitoneal) and they do have some bends as they run past other structures and they run down and empty into the urinary bladder
Run obliquely through the wall of bladder at its posterolateral corners - acts as a sphincter/valve: compressed by increased bladder pressure to prevent back flow
The angle it is at acts as a valve/sphincter to prevent back flow
Ureter histology
Three layers
Transitional epithelium
Muscularis (inner longitudinal, outer circular) (smooth muscle layer)
Adventitia - outer covering of fibrous connective tissue
Folded protective protein plaques on inner surface
Is the muscularis of the ureter the same as the GI tract?
No because it is flipped the other way round in terms of the inner and outer layers and the way to think about the reason for this is about peristalsis - peristalsis in the GI system is about moving from one part of the tract to the other so moving all the way along and for the most part we have sphincters and valves so it does not matter if there is a little back flow because it will just mean more digestion BUT you do NOT eat back flow happening with the ureters because the urine could sit on the kidney tissue and do some damage so the in the ureter when the outer circular contracts it will causes the inner longitudinal to become smaller and effectively shut the transmission off as the lumen is now closed
Urinary bladder
Collapsible muscular sac
Stores and expels urine
When empty, the bladder collapses along folds (rugae)
When full, the bladder expands without great increase in pressure (approx 500mL)
Rugae
Folds that allow the bladder to expand
Urethra
Carries urine out of the bladder
Trigone
Triangular region between 2 openings of entry of ureters and 1 opening for urethra
Bladder wall
Contains muscle for expulsion of urine
Empty bladder
Pyramidal
Lies within the pelvis
As bladder fills
Becomes more spherical
Expands superiorly into abdominal cavity
Can be palpated above pubic symphysis
Location of male bladder
Anterior to rectum
Superior to prostate gland (wraps around urethra)
Location of female bladder
Anterior to vagina and uterus
Urinary bladder wall
Folded into rugae for expansion
Mucosa of transitional epithelium + protein plaques
Has a little submucosa that we don’t really distinguish
Thick smooth muscle layer called detrusor (muscularis layer)
Longitudinal, circular and oblique fibres
Contractions to expel urine from bladder into urethra during urination
Name three structures that protect the urinary tract
Transitional epithelium
Protein plaques
Entrance to bladder prevents back flow (oblique entrance)
Name the muscle that forms the wall of the bladder. What is the function?
Detrusor muscle
Expels urine via contraction
Urethra structure
Thin walled muscular tube, a little bit thinner and more variable across its length
Drains urine from the bladder out of the body to the external environment
Epithelium changes:
Transitional near bladder
Columnar
Stratified squamous near the external opening
Mucus glands to protect epithelium from urine
Male vs female urethra
Significant differences between males and females although they have the same function
Female
Shorter (approx 5cm)
Separate from the reproductive system
Male
Longer (approx 25cm) (because urethra runs through the penis)
Part of reproductive system - can carry seminal fluid during ejaculation
Initial section surrounded by prostate gland (produces seminal fluid)
3 sections - prostatic, membranous, spongy/penile
Urethral sphincter
Internal urethral/urinary sphincter
Junction of bladder and urethra
Detrusor muscle - extension of a part of this muscle that contracts a little differently so when the bladder is empty and all the muscles are contracting this will relax
It is smooth muscle so it is under involuntary control
External urethral/urinary sphincter
Located where urethra passes through the urogenital diaphragm
Skeletal muscle
Under voluntary control
Urination
Bladder fills with urine and expands
Action potential from stretch receptors to brain
Urgency increases as signals increase
Internal sphincter relaxes
Conscious relaxation of external sphincter
What motility pattern moves urine down the ureter? How does the muscle in the ureter differ from the GI tract?
Peristalsis
Opposite to the GI tract - inner longitudinal, outer circular
Name three places in the urinary tract you would find transitional epithelium
Ureters, urinary bladder, initial part of the urethra
Compare and contrast the structure of the epithelium lining the proximal convoluted tubule with the structure of the epithelium lining the ureters. How do the different cellular structures of these epithelia relate to their function?
The PCT is the site of bulk reabsorption of the filtrate. The cells located here are cuboidal epithelium with a dense brush border of microvilli, which increases the surface area for increased absorption. These cells also have numerous mitochondria to provide energy for active transport
The ureters transport urine from the kidneys to the bladder. To protect the wall of the ureters from damage by the urine, they are lined with transitional epithelium. This epithelium consists of stratified cells which are rounded when relaxed, but become flattened when stretched, providing a barrier between the urine and tissue underneath
Renal anatomy summary
We have the afferent arteriole bringing blood in to the glomerulus where we have filtration occurring across the filtration barrier and into the capsular space
From here passing through the different parts of the nephron so passing through the proximal convoluted tubule which is the site of bulk reabsorption reflected by cuboidal cells with their dense brush border and are very leaky and also lots of mitochondria for the active transport and then into the nephron loops where there is simple squamous epithelium in the thin parts and then up into the distal convoluted tube which is fine tuning where we go back to cuboidal epithelium with less mitochondria and less microvilli because we don’t have that bulk reabsorption anymore and then the collecting duct which is the last chance to change the content that is in the filtrate
Now empty out into the minor calyces, then major calyces, renal pelvis and then the ureter
Urine carried with ureters and uses peristalsis
