7- Urinary System Microanatomy- Pt 1.docx

- **Urinary System: General Info** - The **kidneys** excrete nitrogenous waste, conserve body fluids and electrolytes, and reabsorbs solutes and water. - The **ureters** convey urine to the bladder. - The **urinary bladder** stores urine. - The **urethra** expel...

- **Urinary System: General Info** - The **kidneys** excrete nitrogenous waste, conserve body fluids and electrolytes, and reabsorbs solutes and water. - The **ureters** convey urine to the bladder. - The **urinary bladder** stores urine. - The **urethra** expels urine from the body. - **Urine** is an ultrafiltrate of blood. - Clinical signs of renal and non-renal disease relates to the microanatomy and function of the kidneys. - These signs could be from abnormal urine or primary renal disease. - A **dog's kidney** has a normal dark red/brown cortex color. - The kidney's cortex being pale red would be concerning. - A **cat's kidney** has a normal pale red cortex color, due to their high adipose tissue components. - The 4 components of the **blood-urinary barrier** are: fenestration, basement membrane, endothelial cells, and podocytes. - Proteins always travel to the basement membrane because filtration happens here. - **Urine descent** takes place in the following order: proximal convoluted tubules, Loops of Henle, and then distal convoluted tubules. - **Kidney: Functions** - The kidneys help maintain water and electrolyte homeostasis by doing the following: - Filtration of cellular waste form blood - Selective reabsorption of water and solutes - Regulation of fluid balance - Maintaining electrolyte homeostasis/ acid base balance - The kidneys also excrete metabolic waste product, bioactive substances (including drugs), and water. - The kidneys produce the hormones erythropoietin and renin. - Erythropoietin is involved in targeting bone marrow cells to increase RBC (red blood cell) production. - For this reason, kidney damage can result in anemia. - The kidneys regulate blood pressure via the juxtaglomerular apparatus. - The kidneys also activate vitamin D. - Vitamin D aids in calcium metabolism. - **Kidney: Structure** - The kidneys are composed of a: capsule, renal lobe/pyramid, outer cortex, inner medulla, papillae/crest, calices, and renal pelvis. - The **calices** are dilations of the renal pelvis. - The **renal pelvis** is a dilation of the proximal ureter. - The renal hilus is the area of the kidneys where arteries and veins enter/leave. - It is normal to see adipose connective tissue in the renal sinus in low quantities. - The **nephron** of the kidneys is known as the "functional unit" of the kidneys, and it is composed of the renal cortex and medulla. - The nephrons are the site of osmoregulation via: - Filtration of water and small molecules form blood plasma to form a filtrate. - Selective reabsorption of most of the water and other molecules from the filtrate. - The kidneys are composed of lobes that may be single, multiple, or fused. - **Unilobular** kidneys are typically found in carnivores. - **Multilobular** kidneys of large ruminants lacks a renal pelvic, and the kidneys surface has each lobe distinctly outlined by deep grooves. - Multilobular kidneys of pigs have a smooth surface. - The **capsule** of the kidney is composed of collagen fibers, smooth muscle, and blood vessels. - The **cortex** of the kidneys is composed of renal corpuscles, (distal and proximal) convoluted tubules, collecting tubules, and peritubular capillary plexuses. - Proximal convoluted tubules have a brush border. - Distal convoluted tubules (DCT) lack a brush border. - The **medulla** of the kidneys is composed of the Loop of Henle, collecting ducts, and vasa recta. - The **renal pelvis** is composed of urothelium, submucosae, smooth muscle, and adventitia. - The **renal corpuscle** is composed of the glomerulus and Bowman's capsule. - The renal corpuscle has the main function of protein conservation. - The **glomerulus** is loops (tufts) of capillaries with fenestrated endothelium within the bowman's capsule. - **Mesangial cells** lie between fenestrated capillaries. - Mesangial cells are phagocytic, contractile, and act as support. - The **mesangium** is composed of mesangial cells and matrix. - The glomerulus is the site of ultrafiltration. - The glomerulus retains proteins. - [Deep sea fish lack a glomerulus], and instead, they have renal tubules surrounded by well-developed renal portal system (rather than by a glomerulus) and form primitive urine by tubular secretion. - The **Bowman's capsule** is composed of podocytes (visceral epithelium), glomerular basement membrane, urinary space, squamous cells (parietal epithelium), a vascular pole, and a urinary (tubular) pole. - Podocytes do not lie between the capillaries of the glomerulus in the sense od being positioned directly next to them. Rather, they are part of the glomerulus structure and surround the capillaries within the glomerulus. - **Ultrafiltration at the Glomerulus** - Vascular pole - Blood arrives at and enters the glomerular capillaries via the (main) afferent arteriole. - Blood exits at the efferent arteriole. - Ultrafiltration at the glomerulus - Blood pushed through the filtration barrier which is composed of fenestrated endothelium, glomerular basement membrane, and podocyte foot processes. - The ultrafiltrate enters the urinary space. - Urinary pole - The ultrafiltrate enters the proximal convoluted tubule. - **Filtration Barrier** - The filtration barrier has the following composition: - Endothelium of glomerular capillary loops with fenestrations. - Glomerular basement membrane which is made of fused basal laminae of capillaries and podocytes. - The glomerular basement membrane has the following 3 layers: lamina rara interna, lamina densa, and lamina rara externa. - The **lamina densa** of the basement membrane acts as the exclusion barrier for molecules the size of IgG and albumin. - Podocytes with pedicles (foot processes) - The outcome of the filtration barrier is that albumin and large molecules are retained, and smaller molecules cross freely with ultrafiltrate. - Filtration slits are 25-60nm in size, allowing the following filtrates to pass: water, glucose, amino acids, ions, urea, hormones, vitamin B and C, ketones, and very small amounts of protein. - **Endothelium: Types** - Continuous endothelium is a barrier system that controls the transfer of molecules across the cell - Fenestrated endothelium allows for controls transfer of small molecules and limited amounts of protein across the fenestrae (filtration mechanism). - Discontinued endothelium has gaps which allow for "free" transfer of plasma proteins, red and white blood cells, water, and most molecules across endothelial cells. - **Urine Formation: Steps** - Step 1: Primary/glomerular filtrate is produced by ultrafiltration of blood in the renal corpuscle. - The composition of ultrafiltrate is similar to blood plasma- it does not contain most proteins. - Step 2: In the renal tubules, 98% of filtrate is reabsorbed. - Most water and sodium, ALL of the glucose, and ALL of the amino acids from the ultrafiltrate are reabsorbed. - Step 3: Tubular secretion of potassium, hydrogen, NH4+, bile salts, drugs, and metabolites takes place. - Waste molecules and some water remains in the tubular system, and eventually will empty into the ureter. - Step 4: Urine is stored in the bladder pending voiding/micturition. - **Proximal Convoluted Tubule (PCT)** - The proximal convoluted tubules begin at the urinary pole of the renal corpuscle, and it is only located in the [cortex] of the kidney. - The proximal convoluted tubule consists of a single layer of cuboidal epithelial cells with microvilli (forming the Brush border), and a basement membrane. - The purpose of the brush border/microvilli is to increase the surface area. - The microvilli are non-motile. - The proximal convoluted tubules have highly metabolic active cells with many mitochondria that cat as: sodium-potassium pumps, aquaporins, peroxisomes, endosome, and lysosomes. - The proximal convoluted tubules **function** to reabsorb glucose, Na+/H2O, amino acids, peptides, and low molecular weight proteins; as well as activating vitamin D. - Low molecular weight proteins may normally be in the urine to some extent, but they are undetectable. - Otherwise, protein should not normally be detected in the urine. - Specifically, the PCT will absorb 85% of sodium and water from the glomerular filtrate. - Additionally, the PCT will absorb 100% of the glucose and amino acids present in the glomerular filtrate. - The PCT has selective reabsorption of anions, cations, and urea. - The **lateral borders** of the proximal convoluted tubule have inter-digitations of lateral cell process, making the cell limits indistinct. - The basal surface of the PCT have a folded membrane, forming basal striations. - The **basal striations** are composed of folds of the plasma membrane with ATP driven sodium pumps from the PCT and DCT - The mitochondria provides ATP for the sodium pumps. - **Loops of Henle (Nephron Loop)** - The Loops of Henle continues from the PCT, and is only located in the [medulla] of the kidney. - The Loops of Henle are U-shaped with segments/portions/limbs. - Thick descending is composed of cuboidal epithelium - Thin segment is composed of squamous epithelium - Thick ascending is composed of cuboidal epithelium - The **vasa recta** assists in ion and water exchange. - Blood vessels (capillary loop) run parallel with the vasa recta.