Renal System: Chapters 25 & 26

• The renal system is covered in chapters 25 and 26.

Kidney Importance

• Kidneys regulate salt and water balance in the body.
• Kidneys regulate blood ion concentrations.
• Kidneys maintain acid-base balance.
• Kidneys excrete metabolic wastes, drugs, and toxins.
• Kidneys produce EPO (for RBC production) and renin (for blood pressure regulation).
• Kidneys convert vitamin D to its active form.

Anatomy of the Male & Female Urinary Systems

• Kidneys are bean-shaped and retroperitoneal.
• Kidneys are located in the superior lumbar region.
• The superior lumbar region of the kidneys extends from the 12th thoracic vertebra to the 3rd lumbar vertebra.
• Rib cage provides some protection, but the right kidney is lower due to the liver.
• Kidneys weigh approximately 150 g and measure 12 x 6 x 3 cm.
• Adrenal glands sit on top of the kidneys.

Internal Kidney Anatomy

• The concave medial side is the renal hilum, leading to the renal sinus internally.
• The three layers of supportive tissue are the renal capsule, perirenal fat capsule and Renal Fascia.
• The renal capsule is fibrous and adheres directly to the kidney surface.
• Perirenal fat capsule cushions the kidney and helps to hold it in place.
• Renal fascia is dense connective tissue that surrounds the adrenal gland and kidney, providing anchoring.

Medulla, Cortex and Pelvis

• Cortex is where several processes occur.
• Medulla is darker in color and contains medullary or renal pyramids.
• Renal pyramids appear striped and are separated by renal columns, which are indentations of cortical tissue.
• The Pelvis is a flat, funnel-shaped tube continuous with the ureter.
• The Pelvis contains major and minor calices; minor calices enclose papillae of pyramids.
• Calices collect urine.
• Walls of calyces, pelvis, and ureter contain smooth muscle and propel urine by peristalsis.

Circulatory System

• The Circulatory Pathway through the Kidney goes from the Renal artery to the Segmental artery to the Interlobar artery, then the Arcuate artery to the Cortical radiate artery and then to the Afferent arteriole.
• From the Afferent arteriole blood goes to the Efferent arteriole, then the Peritubular capillaries or vasa recta, then the Cortical radiate vein to the Arcuate vein, then the Interlobar vein and then the Renal vein

Blood and Nerve Supply

• Renal arteries receive about 1/4 of the total systemic cardiac output, approximately 1.2 L per minute.
• Arterial branches pass up between medullary pyramids to reach the cortex.
• Venous branches drain back via the same route as arterial branches.
• Nerve supply is provided by the renal plexus, consisting primarily of sympathetic fibers.
• Sympathetic fibers regulate renal blood flow by adjusting the diameters of renal arterioles.

Nephrons

• There is about 106 nephrons and thousands of collecting ducts per kidney.
• Describe the structure of a nephron and differentiate between cortical & juxtamedullary nephrons.

Filtration Components

• The glomerulus and Bowman's capsule form the renal corpuscle.
• Renal corpuscle has a fenestrated glomerular endothelium.
• Podocytes, pedicels, or foot processes form filtration slits.

Nephron Functions

• Proximal convoluted tubule is first.
• The next one is the nephron loop (loop of Henle)
• The Distal convoluted tubule is next.
• The Nephron then goes to the collecting duct.
• The last part is the papillary duct and then the minor calyx

Collecting Ducts

• Collecting ducts have principal cells that lack microvilli and are involved in salt and water balance
• Intercalated cells in collecting ducts have microvilli and are involved in acid-base balance.

Types of Nephrons

• There are two types of nephrons: cortical (85%) and juxtamedullary (15%).

Nephron Microcirculation

• Glomerulus is specialized for filtration and is fed and drained by arterioles.
• Arterioles are high resistance vessels, and the afferent arteriole has a larger diameter.
• Peritubular capillaries arise from efferent arterioles and drain into renal venules.

Juxtaglomerular Complex

• Juxtaglomerular complex is at the junction of the early DCT and afferent/efferent arterioles and regulate renal function
• Arteriole walls contain granular cells (JG cells), which are enlarged smooth muscle cells that act as mechanoreceptors and secrete renin.
• Tubule wall contains macula densa cells, which are chemo- or osmoreceptors that monitor filtrate and adjust GFR.
• The Juxtaglomerular Complex (JGC) regulates filtrate formation and systemic blood pressure.

Micturition

• Micturition pathways in males versus females can be differentiated

Trigone

• The trigone triggers the urge to go to the toilet.
• Moderately full bladder holds 500 ml and measures ~12.5 cm long.
• The bladder can hold up to about double that volume.

Renal Physiology

• Nephron is the functional unit of the kidney.
• The kidney is a major excretory organ and are perfect example of homeostatic organs.
• The kidneys filter several liters of fluid from bloodstream daily.

Filtrate and Urine

• Toxins, metabolic wastes, and excess ions leave the body in urine.
• Materials still needed by the body are returned to the bloodstream.
• Blood passes through gomeruli at 1000-1200 ml per minute; this forces 120-125 ml plasma into renal tubules per minute.
• This action filters the entire plasma volume >60 times a day.
• Filtrate is plasma minus proteins.
• Urine is filtrate minus nutrients, essential ions, and most water.
• Kidneys process ~180 L fluid per day, and about 1% becomes urine (~1.5 L).
• The three parts of the process are the glomerular filtration, tubular reabsorption and Tubular Secretion.

Glomerular Filtration

• Glomerular filtration is passive and nonselective, forcing fluids and solutes through capillaries by hydrostatic pressure.
• Glomerulus is a very efficient filter because its filtration membrane is 1000sX more permeable than other cap membranes. Glomerular blood pressure is higher then in other cap beds.(Glomerular bp - 55 mm Hg vs < 26 mm Hg other areas).
• Allows the bp to be maintained at such a high level.
• 180 L filtrate is formed by kidney caps vs 2-4 L by other cap beds combined!

Functional Anatomy

• Functional components are located between blood and glomerula capsule which come in three layers.
• First is fenestrated capillary endothelium, second is the basement membrane and lastly the visceral membrane of glomerular capsule(which are the foot processes of podocytes).

Filtration

• Water, glucose, amino acids, and N-wastes are molecules < 3 nm that pass easily through the filtration membrane.
• Molecules 3-5 nm pass with greater difficulty.
• Molecules > 5 nm are usually not filtered.
• Basement membrane proteins that have negative charges restrict the passage of larger plasma proteins.
• Retention of plasma proteins maintains colloid osmotic pressure and blood flow.
• Presence of proteins or RBCs in urine suggests filtration membrane damage.

Net Filtration Pressure

• Net Filtration Pressure is the pressure responsible for filtrate formation.
• NFP = HPg – (OPg + HPc)

GFR

• Glomerular Filtration Rate (GFR) is the total amount of filtrate formed per minute by both kidneys (~125 ml/min).
• Depends on total surface area for filtration, filtration membrane permeability and net filtration pressure.
• Glomerular caps have a huge surface area.
• A decrease in glomerular bp of only ~15% completely stops filtration!
• GFR is directly proportional to NFP.

GFR Intrinsic and Extrinsic Influences

• Renal autoregulation (intrinsic)
• Neural controls (extrinsic)
• Renin-angiotensin system (extrinsic)

Renal Autoregulation, Intrinsic Mechanism

• Kidney keeps GFR constant by determining its own rate of flow and adjusting nephron blood flow.
• Renal Autoregulation regulates the diameter of afferent and efferent arterioles, but is more effective on Afferent arterioles.
• Its two controls are the Myogenic Mechanism and Tubuloglomerular Feedback mechanism

Myogenic mechanism

• The Myogenic Mechanism responds to changes in the bp of blood vessels (focus = afferent arteriole).
• Vascular smooth muscle tends to contract when stretched.

Tubuloglomerular feedback mechanism

• Tubuloglomerular Feedback mechanism is directed by macula densa cells of JGC, which monitor NaCl content of filtrate.
• High osmolarity of fast flowing filtrate releases vasoconstrictors (e.g. ATP)
• Low osmolarity of slow flowing filtrate allows vasodilation by releasing less ATP
• Renal autoregulation can maintain a constant GFR with 80-180 mm Hg systemic bp.
• If systemic bp drops below 80mm Hg (hypovolemic shock) renal autoregulation and filtrate formation shuts down and extrinsic regulation kicks in.