Human Physiology Quiz: Kidney Regulation

Renal System Functions

• The renal system has six functions
• Regulates extracellular fluid volume and blood pressure
• Regulates body fluid osmolarity
• Maintains ion balance in the body fluids
• Plays a role in homeostatic regulation of body fluid pH
• Excretes wastes
• Produces hormones

Renal System Anatomy

• Kidneys: Filter blood and produce urine
• Ureter: Muscular tube connecting the kidney to the urinary bladder, conveying urine
• Urinary Bladder: Stores urine until excretion
• Urethra: Tube conveying urine from the bladder to the external body
• Nephrons: Small tubules in the cortex and medulla, functional unit of the kidney
• Cortex: Outer layer of the kidney
• Medulla: Inner layer of the kidney
• Renal Pelvis: Upper part of the ureter, receives urine from the cortex and medulla

Renal System Anatomy: The Nephrons

• The nephrons are tubules associated with blood vessels
• They extend through the cortex and medulla and open into the renal pelvis
• Filtration of blood and processing the filtrate into urine is the function of the nephrons

Renal System Anatomy: Blood Vessels

• Each nephron has two arterioles and two sets of capillaries
• Afferent arteriole: Receives blood from the renal artery and supplies blood to the glomerulus
• Efferent arteriole: Connects to glomerulus/network of capillaries
• Glomerulus (capillaries): Network of capillaries associated with Bowman's capsule
• Peritubular capillaries: Network of capillaries surrounding other parts of the tubule
• Juxtaglomerular apparatus (JA): The JA plays a role in regulating the glomerular filtration rate (GFR)

Renal System Anatomy: Tubular Element

• Bowman's capsule: surrounds glomerular capillaries
• Proximal convoluted tubule (PCT): where most reabsorption occurs
• Loop of Henle: divided into descending and ascending limb
• Distal convoluted tubule (DCT): connects to a collecting duct
• Collecting duct: empties into renal pelvis

Nephron Functions: Filtration, Reabsorption, Secretion, and Excretion

• Filtration: The movement of blood plasma into the lumen of the nephron, across the walls of the glomerular capillaries and Bowman's Capsule. About 20% of the blood plasma volume filters into the Bowman's Capsule. Filtration occurs by bulk flow, with everything except blood cells and proteins passing freely. Glomerular capillaries have small pores to prevent blood cells and proteins from passing. Filtrate consists of water and dissolved solutes (ions, glucose, amino acids, etc.).

• Forces influencing filtration: Hydrostatic pressure (blood pressure) created by blood flow through glomerular capillaries, colloid osmotic pressure (higher inside glomerular capillaries due to proteins in plasma), and fluid pressure (filtrate pushing back on the glomerulus).

• Net Filtration Pressure: 10mmHg, resulting from the interactions described above.

• Glomerular Filtration Rate (GFR): The amount of fluid that filters through the glomeruli into Bowman's capsules per unit time. Normal GFR = 180L/day (125mL/min).

• Mechanisms regulating GFR: Autoregulatory mechanisms (myogenic response and tubuloglomerular feedback), autonomic nervous system (sympathetic), and hormones.

• Reabsorption: The movement of water and solutes from the filtrate back into the blood. Two main types: bulk reabsorption (in proximal tubule and loop of Henle), and regulated reabsorption (in distal tubule and collecting duct). Bulk reabsorption is a process by which glucose, amino acids, and Na+ ions are recovered from the filtrate. It uses active transport mechanisms where the concentration of solutes in the filtrate is the same as that of the interstitial fluid. The energy needed is for maintaining concentration gradients. Regulated reabsorption is hormonally regulated and involved in the selective reabsorption of water and Na+ ions.

• Passive Reabsorption: Some solutes moved passively through cell membranes. Urea diffuses in.

• Transcytosis: The movement of water or solutes from the filtrate to interstitial fluid. It uses vesicles and protein receptors on membrane, in proximal tubule.

• Excretion: The remaining solutes after filtration, reabsorption, and secretion; the elimination of these solutes in the urine. Renal clearance of a substance can help tell what the kidneys are doing with it. Inulin is a substance excreted without reabsorption or secretion, and its rate is equivalent to GFR.

Kidney Function: Electrolyte and Acid-Base Balance

• The kidneys maintain electrolyte balance by regulating Na+ and K+ reabsorption/secretion.
• Aldosterone regulates Na+ reabsorption via principal cells in the distal and collecting tubules.
• Two primary factors regulating aldosterone synthesis are K+ concentration in interstitial fluid and decreased blood pressure.
• ANP increases the excretion of Na+ and water. It acts on adrenal glands, hypothalamus and cardiovascular center of medulla obligata to decrease aldosterone synthesis and blood pressure.
• Acid-base balance is regulated by the kidneys by secreting or reabsorbing H+ and HCO3−.

Summary

These notes cover the anatomy and function of the renal system in detail, including filtration, reabsorption, secretion, and excretion functions as well as mechanisms for regulating electrolyte and acid-base balance.