Urinary System: Anatomy and Function

• The urinary system is a subsection of the excretory system.
• The body also excretes things via sweat, exhaled gases, and feces.

Basic Anatomy

• Key structures of the urinary system are the kidneys, ureters, bladder, and urethra.
• Kidneys are retroperitoneal.
• The right kidney is cranial, except in pigs and grasscutters.
• Visceral peritoneum is next to organs.
• Parietal peritoneum is next to the body wall.

Renal Biopsy

• A renal biopsy can be performed using laparoscopy.
• During the procedure, a biopsy needle is carefully advanced into kidney cortex to extract a tissue sample.

Renal System Function

• The basic function of the renal system is homeostasis, and blood balance
• The renal system balances fluids, pH, electrolytes, and nitrogenous wastes in the blood.
• The kidneys filter blood.
• Kidneys reabsorb good substances from the filtrate.
• Kidneys secretes substances into the filtrate that need to be rid of.
• The renal system decides how much water is sent with filtrate
• The filtrate becomes urine when voided.

Hormone Production

• Side hormones that are produced include, erythropoietin (EPO), and prostaglandins (PGs).
• Renin, an enzyme, regulates BP through water and sodium retention.
• The kidneys affect and are affected by ADH and aldosterone.

Renal Anatomy

• Renal cortex and medulla make up the kidney.

Nephron

• The functional unit of the kidney is the nephron.
• Blood vessels play an important role in how the kidney functions.
• The nephron associates with the bloodstream to function.

Renal Blood Vessels

• Blood moves through the kidney via glomerular capillaries.

Renal Vasculature

• Cortical radiate artery brings blood into the arcuate artery.
• The arcuate artery takes blood to the afferent arterioles.
• Blood flows through the glomerular capillaries, the efferent arterioles, and then the peritubular capillaries.

Filtration, Reabsorption, Secretion

• Renal corpuscle consists of the glomerulus and Bowman's capsule.
• The glomerulus is a nest of blood vessels between two arterioles, and has high blood pressure.
• The diameter of the afferent arteriole is greater than that of the efferent arteriole.
• Glomerular capillaries have lots of fenestrations to regulate function.
• Podocytes make up the inner layer of Bowman's capsule.
• Glomerular Filtration Rate (GFR) is measured in mL/min.
• A 10 kg dog processes approximately 250 mL of blood through kidneys each minute.
• During filtration alone, 60L of urine would be produced per day.
• Glomerular damage can be caused drug use, diabetes, lupus, infection, and toxins.
• During filtration, Na+, K+, Ca2+, Mg2+, glucose, amino acids, Cl-, bicarbonate, and water move to the kidney structure.
• Osmosis and diffusion assist in kidney function.
• Active transport is active in kidney function.

Reabsorption

• 65% of Na, Cl, H2O, all glucose, and all amino acids are reabsorbed by kidney.
• Sodium is actively pumped out via transport protein.
• Glucose and amino acids hitch a ride via passive transport.
• The descending portion loop of Henle is permeable to water.
• The ascending portion loop of Henle is impermeable to water, and sodium is pumped out.
• The ascending portion Loop of Henle creates a salty (hyperosmotic) medulla.
• In the distal convoluted tubule (DCT), sodium leaves in exchange for H+, NH4+, and K+.
• Aldosterone controls the DCT.
• In the Collecting Duct, water is reabsorbed under the influence of ADH.
• If substances are not filtered but still need to leave the body, secretion processes are activated.
• The DCT is the most important region for secretion.
• Secreted elements include H+, K+, NH3, urea, penicillins, sulfonamide antibiotics, drugs and toxins.

Reabsorption and Secretion Visualization

• Visualize Reabsorption and Secretion at separate portions of the kidney

Water Regulation

• Anti-diuretic hormone (ADH) opens water pores in the collecting ducts.
• ADH is made in the hypothalamus and stored in the posterior pituitary.
• Aldosterone increases sodium reabsorption in DCT and collecting duct.
• If ADH is present, sodium moving out of tubules leads to water following the path.
• Aldosterone is made by the adrenal gland.

Blood Pressure Regulation

• Juxtaglomerular cells are mostly on afferent arteriole.
• Juxtaglomerular cells monitor BP, and secrete renin if needed (i.e. if BP falls).
• Macula densa are specialized cells in ascending thick Limb of Henle where it meets the DCT.
• Macula densa monitor sodium concentration of filtrate.
• Macula densa cells release prostaglandins, vasodilating afferent arteriole and telling JG cells to release renin.
• Macula densa senses decreased Na+ flowing by and indicating GFR is going down and Blood Pressure going down.

Renin-Angiotensin-Aldosterone System (RAAS)

• JG cells monitor blood pressure (BP).
• If JG cells detects low Blood Pressure, OR the macula densa detects less Na, JG cells release RENIN.
• Renin converts angiotensinogen to angiotensin I.
• Angiotensin I converts to angiotensin II via ACE (angiotensin converting enzyme).
• The arteries constrict and aldosterone and ADH is released to increase H20 reabsorption.
• Blood pressure increases due to the RAAS system operation.
• ACE inhibitors such as benazepril effect RAAS.
• Look to Lasix (Furosemide) slide previously in the presentation for understanding diurectics.

Ureters

• The ureters are composed of fibrous layer, smooth muscle, transitional epithelium and a lumen.
• Ureters use peristalsis to continuously move urine.
• Ureters enter the bladder obliquely so no back flow occurs.
• Ectopic ureters can occur.

Urinary Bladder

• The urinary bladder stores urine
• It consist of a muscular sac with a neck.
• The trigone is the arrangement of openings of the ureters into the bladder, and the opening from the bladder into the urethra
• The urinary bladder is lined with epithelium
• There are 3 layers of smooth muscle in the urinary bladder
• Detrusor muscle make up the bladder.
• An empty bladder is round and sits in pelvic canal.
• As bladder fills, pear shaped and extends cranially to abdominal cavity.

Micturition

• Urine accumulates, stretch receptors fire, spinal reflex sends motor signal to detrusor muscle, creating a sensation of "gotta pee", internal sphincter relaxes and smooth muscle operates function.
• External sphincter exist at the neck of the bladder, and is skeletal muscle controlled.
• The brain sends inhibitory signals to detrusor muscles and external sphincter until ready.
• The system will eventually relaxes if bladder is full enough.
• Sphincters open and detrusor muscle is activated.

Urethra

• In females, the urethra is short and straight, opening on floor of vestibule of vulva and is made of transitional epithelium.
• In males, the urethra is longer and curved, and runs along ventral aspect of penis and shares with reproduction system, carrying urine and semen and composed of transitional is epithelium.

Kidney Disease

• Kidney disease can be categorized as either acute or chronic.
• SDMA and creatinine is used to detect disease.