Urinary System Overview and Kidney Function

Urinary System Overview

• The urinary system is a major excretory organ
• It maintains the body's internal environment by regulating water volume, solute concentration, ion concentrations in extracellular fluid, acid-base balance, and excreting metabolic wastes, toxins, and drugs
• It produces renin, regulating blood pressure and erythropoietin for red blood cell production
• It also activates vitamin D, and performs gluconeogenesis if needed

Kidney Function

• Kidneys filter blood plasma, removing waste products and excess water
• They produce urine
• They cleanse and adjust blood composition
• They deliver about 1/4(1200 ml) of cardiac output per minute
• Blood flow: renal → segmental → interlobar → arcuate → cortical radiate (interlobular)
• Venous flow: cortical radiate → arcuate → interlobar → renal veins
• There are no segmental veins
• Nerve supply: via sympathetic fibers from renal plexus

Kidney Anatomy

• Kidneys are positioned against the posterior body wall, behind the parietal peritoneum
• They lie retroperitoneal, between the twelfth rib and the iliac crest.
• The internal anatomy includes the renal cortex, medulla, major calyx, papilla of pyramid, renal pelvis, minor calyx, ureter, renal pyramid, and renal column.
• The fibrous capsule surrounds the kidney. There are supportive tissue layers called renal fascia, and perirenal fat capsule

Nephrons

• Nephrons are the structural and functional units that form urine in each kidney.
• There are greater than 1 million nephrons per kidney
• Two main parts:
• Renal corpuscle (where filtration occurs)
• Renal tubule (where reabsorption and secretion happen)

Filtration Membrane

• Composed of three layers, the capillary endothelium, basement membrane, podocyte foot processes
• It acts as a filter, allowing smaller substances like water, glucose, amino acids, salts, and some ions to pass into the tubule. Larger molecules, like proteins remain in the blood vessels.

Glomerular Filtration Rate (GFR)

• GFR is the volume of filtrate formed by both kidneys per minute (normal = 120-125 ml/min)
• GFR is directly proportional to the net filtration pressure (NFP), total surface area for filtration, and filtration membrane's permeability
• NFP is determined by hydrostatic pressure within the capillaries, and capsular hydrostatic pressure, and colloid osmotic pressure.

Kidney Function Regulation (intrinsic)

• Intrinsic controls (autoregulation)maintain nearly constant glomerular filtration rate (GFR) when mean arterial pressure is between 80-180 mm Hg.
• Two types of autoregulation:
  • Myogenic mechanism- The afferent arterioles constrict when stretched, decreasing blood flow into the glomerulus.
  • Tubuloglomerular feedback mechanism- The macula densa cells detect changes in NaCl content of filtrate and signal to the afferent arterioles.

Kidney Function Regulation (extrinsic)

• Extrinsic mechanisms regulate GFR by adjusting the systemic blood pressure, thus adjusting GFR
• Two types of extrinsic control are the sympathetic nervous system, and hormones, like renin - angiotensin-II—aldosterone system (RAAS).

Tubular Reabsorption

• Most of the filtrate is reabsorbed back in to the bloodstream through the tubular membranes
• Key substances are reabsorbed by primary active transport (Na+) which provides the energy for secondary active transport for other nutrients: water, glucose, amino acids, salts.

Tubular Reabsorption of Water

• Water's reabsorption depends on ADH released from the posterior pituitary; with higher ADH levels, more water is reabsorbed into the bloodstream.

Tubular Secretion

• This is opposite of reabsorption
• Substances like drugs, metabolites, H+, NH4, creatinine and organic acids enter the filtrate through the tubular cells; thus, it's important for eliminating unwanted substances, regulating blood pH, or for eliminating excess K+

Urine Characteristics

• Physical characteristics: Color, transparency, odor, pH, and specific gravity
• Variations in these characteristics can indicate various conditions or diseases

Urine Concentration and Volume

• Two countercurrent mechanisms: the countercurrent multiplier and countercurrent exchanger in the juxtamedullary nephrons
• The gradient causes the collecting ducts to concentrate or dilute the urine as needed

Urinary Bladder

• Urine storage; collapses when empty, rugae appear; expands and rises superiorsly during filling without significant rise in internal pressure
• Holds 500 ml (1 pint)
• Capacity can double if necessary
• Can burst if excessively distended

Urethra

• Carries urine out of the body
• Males have a longer urethra
• Three named regions:
• Prostatic urethra, Intermediate, Spongy urethra
• Sphincters help control urination
• Internal urethral sphincter (smooth muscle): involuntary, Contracts to open

Renal Clearance Rate

• Measures the volume of plasma cleared of a substance by the kidneys per minute
• Renal clearance of inulin is approximately equal to the GFR.
• If the renal clearance rate is less than the estimated GFR, then the substance is reabsorbed or not filtered.
• If the renal clearance rate is greater than the estimated GFR, then the substance is actively secreted.

Other

• Other substances like electrolytes are also found in blood such as -Na+, K+, PO₄³¯, and SO42-, Ca2+, Mg2+, and HCO3−; abnormally high concentration or abnormal components could indicate pathology.

This quiz covers the essential functions and anatomy of the urinary system and kidneys. Explore how the kidneys filter blood, regulate bodily functions, and produce urine. Test your knowledge on the systemic role of the urinary system in maintaining homeostasis.