Basic Information
-Lies in the abdominal + pelvic cavities
-Anatomically linked with the reproductive and referred to as the urogenital system
-Both systems share the urethra = runs through the male’s penis and joins the female’s vagina
Parts:
-Pair of Kidneys
-Pair of Ureters
-Bladder
-Urethra
Functions
-Regulate chemical composition and volume of body fluids (osmoregulation)
-Remove nitrogenous waste products + excess water from the body (excretion)
-Act as an endocrine gland (secretion of the hormone erythropoietin)
Kidney
- 2 in the cranial abdominal cavity, either side of midline, ventral to lumbar hypaxial muscles
- Closely attached to lumbar muscles by parietal peritoneum coverings (retroperitoneal)
- Right kidney is cranial to left kidney due to stomach’s evolution to position itself on the left side of the abdomen
Macroscopic:
- Characteristic bean shape, indented shape: hilus (blood vessels, nerves + ureter enter and exit the kidney)
- 4 layers = capsule > protective layer of irregular dense fibrous connective tissue closely attached to the cortex = cortex > contains renal corpuscles and convoluted tubules of nephrons = medulla > has triangular-shaped pyramids containing the collecting ducts and the tissue containing the loops of Henle of the nephrons inbetween = pelvis > made of fibrous connective tissue, urine found by the nephrons drains into the pelvis and out of the kidney by a single ureter
Blood Supply:
- Arterial blood from the aorta in a single renal artery to each kidney = carries out 20% cardiac output
- Renal Artery divides into several interlobar arteries (passing between the renal pyramids and into the cortex)
- Capillaries supply renal tubules, giving off numerous capillary networks (glomeruli), each supplying an individual nephron
- Recombination of the capillaries forms the interlobar veins, entering the single renal vein (venous blood to caudal vena cava)
- Blood carries O2, nutrients + waste products (CO2) but the glomerulus removes the nitrogenous waste
Microscopic:
- Kidney functional unit = nephron (millions, closely packed) + responsible for filtration of blood + urine production
- Nephron divided into several parts =
- Glomerular Capsule* encloses blood capillary network, the glomerulus (AKA Bowman’s capsule), together form the renal corpuscle. Inner surface has a basement membrane, close to the glomerulus capillaries’ endothelium, containing podocytes (pores) for passage of fluid + small molecules. Outer surface is continuous with the proximal convoluted tubule (PCT) epithelium. Fluid filtered here, inbetween both layers, moves onto the next part of the nephron.
- Proximal Convoluted Tubule (PCT)* long, twisted tube leading from the capsule neck and lying in the renal cortex. Lined in simple cuboidal or columnar epithelium and the side lying towards the lumen of the tubule is lined with microvilli, inc. SA for water and electrolyte reabsorption.
- Loop of Henle* U-shaped, leads from PCT to renal medulla. Lined with simple squamous epithelium, thicker in ASCENDING than descending loop.
- Distal Convoluted Tubule (DCT)* short, less twisted than PCT. Lies in renal cortex and lined with cuboidal epithelium but no microvilli.
- Collecting Duct* receives urine, conducting it through pyramids in renal pelvis. Lined in columnar epithelium.
Renal Function: Urine Formation
- Blood filtered by kidneys
- Filtrate undergoes modifications with renal tubules = urine (osmosis, diffusion, reabsorption + secretion)
Glomerulus:
- high blood pressure (blood straight from renal artery and aorta, smooth muscle in efferent arteriole able to constrict - under hormone RENIN = regulating glomerular pressure)
- high pressure forces fluid + small molecules out of the blood via podocytes but other, much larger molecules are retained by ULTRAFILTRATION
- filtrate known as glomerular filtrate or primitive urine, contains 99% H2O + 1% chemical solutes
Proximal Convoluted Tubule (PCT):
- reabsorption of H2O, Na+ (Na+ and Cl- ions actively reabsorbed from filtrate into blood), H2O by osmosis in response to Na+ ion movement (80% Na+ and Cl- ions reabsorbed at this point)
- reabsorption of C6H12O6 (Glucose) = glomerular filtrate contains glucose which is all reabsorbed (no glucose in urine) but only up to a certain level, renal threshold. -nitrogenous waste becoming concentrated = main waste product is urea (produced as a result of protein metabolism) and water reabsorption concentrates levels of urea.
- secretion of toxins + certain drugs = actively secreted into filtrate - penicillin + derivatives secreted from blood + carried to the bladder via urine (useful with bladder infection)
Loop of Henle:
-function is to regulate concentration and volume of urine, glomerular filtrate flows from descending loop to ascending loop, both in medulla
-descending loop in which walls are permeable to water but not ions, water drawn out by osmosis due t surrounding ions in medullar tissue
-ascending loop in which walls are permeable to ions but not water, contains sodium pumps for active reabsorption of Na+ into medullary tissue and capillaries, draws H2O from descending loop. Less concentrated due to removal of Na+ ions
[Dependent on animal’s hydration, H2O will be reabsorbed or lost)
Distal Convoluted Tubule (DCT):
- final adjustments made to chemical constituents of urine
- reabsorption of Na+, secretion of K+ = smaller amount of reabsorption of Na+ than in PCT > aldosterone controls varying reabsorption of H2O (osmosis) due to reabsorption of Na+ (produced by adrenal glands) - K+ excreted in urine to replace lost Na+
- regulation of acid/base balance by excretion of H+ ions > normal blood pH is 7.4, pH falls = kidney excretes H+ ions into urine via DCT, pH rises = kidney stops excreting H+ ions (retain in blood)
Collecting Duct:
- final adjustments made to volume of H2O in urine in response to ECF
- hormone > antidiuretic hormone (ADH) produced by posterior pituitary gland, able to change permeability of duct walls to water
- dehydrated = plasma and ECF dec., ADH released to inc. permeability. H2O drawn in by high concentration Na+ ions and into plasma + ECF
- ultrafiltrate = concentrated urine (repeated reabsorption + secretion)
Osmoregulation: Control of Renal Function
- Ensures volume of plasma + concentration of dissolved chemicals in plasma and other tissue fluids remains constant.
- Homeostasis maintained, body functions normally
- Factors involved in osmoregulation control:
- amount of water lost from body
- amount of salt lost from body
Control Water Loss:
Low Blood Pressure (baroreceptors) + Inc. Osmotic Pressure** (osmoreceptors) = Inc. ADH secreted by posterior pituitary gland > permeability in CD inc. > inc. reabsorption of H2O from ducts into medulla and associated blood vessels> volume of ECF inc. > rise in blood pressure
THIRST CENTRES ARE ACTIVATED
Control Sodium Levels:
Low Na+ levels in plasma > Low arterial blood pressure > renin released from glomeruli > angiotensin [FROM ANGIOTENSINOGEN] > aldosterone released > acts on DCT > stimulates inc. Na+ reabsorption > H2O reabsorbed by osmosis + enters blood vessels = rise in blood pressure (also from vasoconstriction due to angiotensin)
Excretion
-Removal of waste products
-Formed within the tissues > result of metabolic processes + are useless (surplus to body requirements or potentially harmful)
>Water< varying amounts dependent on volume of ECF + controlled by osmoregulatory processes
>Inorganic Ions< amount depends on osmotic pressure of the blood + other fluids, also controlled by osmoregulatory processes
>Nitrogenous Waste Products< result from metabolism of protein taken into the body by food - protein is broken down by digestion into amino acids (converted by the liver into body protein). Deamination = surplus AA broken down
Ornithine Cycle = urea formed
CO2 + NH3 = CO(NH2)2 + H2O
CO(NH2)2 = urea = excreted in urine
>Products of Detoxification< hormones, certain drugs + poisons inactivated within the liver + excreted by the kidney
Other parts of the Urinary System
Ureters:
- urine formed by nephrons leaves via a single ureter at the hilus
- narrow muscular tube running caudally towards bladder, each side of the dorsal abdomen’s midline
- visceral peritoneum fold (mesoureter) suspends ureter
- peristaltic waves push urine towards bladder
- transitional epithelium lines ureters, allows for expansion as urine passes along
Bladder:
- each ureter enters close to neck of bladder, underrunning bladder mucosa before opening into lumen (acts as valve, preventing urine backflow)
- 2 ureters enter bladder > trigone, due to 3 structures enter/exiting bladder (ureters + urethra)
- rounded end points cranially, narrow/neck end ponts caudally, lying in the pelvic cavity
- cross-section > bladder = transitional epithelium (wall expansion), submucosal layer of elastic tissue + smooth muscle in folds (expansion)
- cranial end (abdomen) covered in peritoneum but rest surrounded by connective tissue (pelvic)
- bladder neck ends in bladder sphincter (controls flow of urine down urethra) > internal: smooth muscle, involuntary > external: ring of striated muscle, voluntary
Urethra:
-conveys urine caudally from bladder to outside
-structure varies based on sex of animal and, with males, if cat or dog
>Female< short tube, opens into floor of reproductive tract (junction of vagina + vestibule) = external urethral orifice, located in the centre of a small ridge (urethral tubercle) > useful landmark when catheterising a bitch, seen with a speculum
>Male Dog< close to neck of bladder, 3 openings = prostate gland + 2x vas deferens - runs caudally through pelvic cavity, over ischial arch + joined by erectile tissue. Penile urethra opens at its tip
>Tomcat< short length of urethra, cranial to 3 openings, known as preprostatic urethra, not found in dog. Continues caudally and opens to outside in perineal area, ventral to anus, but no outer penile urethra. Close to urethra’s end are the openings from the bulbourethral glands.
Conveys both urine + sperm (once the vas deferens join)
Micturition + Urinalysis
Micturition:
-act of expelling urine from the bladder (normally a reflex activity but can be voluntary)
>Bladder distends with urine from kidneys
>Stretch receptors in bladder’s smooth muscle are stimulated, send nerve impulses to centres in appropriate segment of spinal cord
>Nerves transmitted via parasympathetic nerves + back to smooth muscle, initiating contraction
>Impulses also stimulate relaxation of internal bladder sphincter - urine expelled
-voluntary control develops alongside maturity, puppies + kittens not fully developed until 10 weeks old
Urinalysis:
- derived from the ultrafiltrate of plasma (reflecting on helath status of whole animal)
- diagnostic tool > normal urine (water, salts, urea)
Clinical Parameters > daily volume, appearance, pH, specific gravity, protein + crystals/casts
Normal Values>
DV: Dogs = 20ml-100ml/kg, Cats: 10-12ml/kg
A: clear, yellow, smell
pH: 5-7
SG: Dogs = 1.016-1.060, Cats= 1.020-1.040
P: None
C + C: Small quantities
Comments:
- poly/olig (uria) > DV
- anuria > DV
- tomcat (unpleasant) > A
- carnivore = acid, herbivore = alkaline > pH
- reflects the concentration of urine > pH
- proteinuria = presence of protein > P
- large quantities = calculi, stones, block tract > C + C
