Ans: 

• To regulate fluid balance by filtering out water for excretion in urine and by reabsorbing water in the nephron when blood volume is low
• To maintain electrolyte balance by filtering out electrolytes for excretion in urine and by reabsorbing them in the nephron when they are needed in a greater amount within the body
• To regulate acid–base balance through the excretion and reabsorption of hydrogen and bicarbonate ions as necessary
• To excrete waste products in urine (e.g. urea, creatinine)
• To produce/release hormones such as renin and erythropoietin (Chapters 7 and 12)

Ans: The renal system consists of the following organs:

• Two kidneys
• Two ureters
• One urinary bladder
• One urethra

Ans: 

• Nephrons are the functional units of the kidney. There are approximately 1.25 million nephrons in each kidney. The nephron has the following components:
  
  • Glomerulus: a cluster of capillaries that sits within Bowman’s capsule. They are located within the renal cortex. Blood enters by afferent arterioles and exits by efferent arterioles. Filtration is influenced by hydrostatic pressure and their semi-permeable membranes.
  • Proximal convoluted tubule: In this part of the nephron, located in the renal cortex, water, ions and all organic nutrients are reabsorbed. Cell membranes here are freely permeable, making this part of the nephron excellent for reabsorption.
  • Loop of Henle: the descending limb of the loop allows the reabsorption of water, whereas the ascending limb permits sodium and chloride ions to be reabsorbed. These loops, along with blood vessels and collecting tubules, form the renal pyramids in the renal medulla.
  • Distal convoluted tubule: this part of the nephron is located in the renal cortex and is where ions, acids, drugs and toxins are secreted. It is here that variable reabsorption of water, sodium, hydrogen and bicarbonate occur.
  • Collecting duct: collects the filtrate from a number of nephrons.

Ans: The pH of body fluid is maintained between 7.35 and 7.45. Remember that hydrogen ions determine the acid–base balance of the body and, in excess, contribute to acidosis in the body, that is, reabsorbing these will lower pH and excreting them will raise pH. Bicarbonate ions contribute to alkalosis, that is, reabsorbing them will raise pH and excreting them will lower pH. Hydrogen ions and bicarbonate ions can be excreted and reabsorbed in the nephron as necessary to regulate pH.

Ans: Fluid balance is controlled at two points:

• Amount of fluid consumed – consciously controlled and stimulated by the thirst centre through the renin-angiotensin-aldosterone system (RAAS) (Chapter 12).
• Amount of urine excreted – regulated by hormones.
  
  • Anti-diuretic hormone (ADH), released from the posterior pituitary, increases the volume of water reabsorbed back into blood from the nephron. This occurs when osmolality is raised and when low blood volume triggers the release of ADH through the RAAS.
  • Aldosterone, released through the RAAS when blood volume is low, increases the amount of sodium reabsorbed back into blood from the nephron. When sodium is reabsorbed, water is automatically reabsorbed as water follows sodium.
  • Atrial Natriuretic Peptide (ANP) is released in response to the stretching of the walls of the heart by increased blood volume or pressure. This results in dilation of the afferent arteriole and constriction of efferent arteriole, increasing glomerular pressure and therefore glomerular filtration rate (GFR). This increases the volume of fluid in the nephron and thus urine excreted, reducing blood volume and therefore blood pressure.

Ans: As urine is formed in the nephrons, it is collected in the collecting ducts and goes to the bladder via the ureters. On average, it can store 600–800 ml of urine, but micturition occurs normally between 150 and 300 ml. Once the bladder starts to get full, stretch receptors in the bladder wall are activated as the bladder wall stretches to accommodate the volume. Sympathetic nerves are stimulated, initially inhibiting contraction of the detrusor and constricting the neck of the urethra, preventing micturition. Continued contraction of the external sphincter is maintained by the pudendal nerves promoting continence. Once pressure rises within the bladder, small contractions of the detrusor occur, increasing pressure further. This further rise in pressure leads to contraction of the detrusor and relaxation of the internal urethral sphincter. At an appropriate place and time, voluntary relaxation of the external urethral sphincter then permits micturition.