Renal System: Functions and Morphology

Role of the Kidneys in the Body

• Regulate the volume and composition of the ECF by maintaining a balance between intake and output of water and electrolytes in the body.
• Excrete and eliminate waste products of metabolism, such as urea, creatinine, and uric acids, as well as various toxins like drugs and food additives.
• Act as endocrine glands, producing hormones like erythropoietin and renin.
• Play a dominant role in the long-term and short-term regulation of arterial blood pressure.
• Contribute to acid-base regulation along with the respiratory system.
• Synthesize glucose from amino acids and other precursors.

Excretion

• Defined as the removal of waste products from the body.
• Organs involved in excretion:
  • Kidneys: excrete water and water-soluble waste products.
  • Lungs: excrete carbon dioxide, water vapor, and other volatile substances.
  • Skin: excretes water and salts mainly in the form of sweat.
  • Gastrointestinal tract: excretes undigested food.

Structure of Nephron

• Nephron is a structural and functional unit of the kidney.
• Each nephron is capable of forming urine.
• Two types of nephrons:
  • Cortical nephrons (86%): glomeruli are present near the surface of the kidneys; main function is absorption of sodium.
  • Juxtamedullary nephrons (14%): glomeruli lie at the junction of cortex and medulla; main function is to increase concentration of medullary interstitial fluid.
• Two major parts of a nephron:
  • Glomerulus: a tuft of capillaries that connect afferent arteriole with an efferent arteriole.
  • Renal tubule: consists of three parts - proximal convoluted tubule, loop of Henle, and distal convoluted tubule.

Blood Flow to Kidneys

• Rate of blood flow to kidneys: 1200 ml/min.
• Peculiarities of renal circulation:
  • Very high blood supply (21% of cardiac output).
  • Two sets of capillaries: glomerular capillaries and peritubular capillaries.
  • Glomerular capillary bed has a high hydrostatic pressure (60 mmHg) due to a smaller diameter of the efferent arteriole compared to the afferent arteriole.
  • Peritubular capillary bed is a low-pressure bed (13 mmHg).
  • Vasa recta (loop-shaped vessels) in juxtamedullary nephrons receive only 1-2% of blood flow, which is very sluggish.
  • Renal blood flow shows remarkable constancy in the face of blood pressure changes due to autoregulation.
• The kidneys regulate the hydrostatic pressure in both capillary beds by adjusting resistance of the afferent and efferent arterioles.