2. Mammalian Urinary System

Fig 3 Components of the urinary system

2.1.1 Kidney

• Site of urine formation.

• Left and right kidneys that are attached to the dorsal wall of abdominal cavity, one on each side of vertebral column.

• Each kidney weighs about 150g and is about the size of a clenched fist.

Functions of kidneys:

(a) Excretion of metabolic waste products through formation of urine.

(b) Regulation of water and ions (e.g. Na⁺, Cl^-, K⁺, Ca²⁺, H⁺, Mg²⁺ and PO₄^3-) balance.

(c) Regulation of arterial pressure by excreting variable amounts of water and sodium ions to maintain a constant blood volume and sodium ion concentration.

(d) Secretes erythropoietin that stimulates the production of red blood cells at the bone marrow.

(e) Like the liver, kidneys are also able to synthesise glucose from amino acids during prolonged fasting. The process is known as gluconeogenesis.

2.1.2 Ureter

• • Urine that is formed in each kidney passes through the ureter to the urinary bladder.

2.1.3 Urinary bladder

• • Elastic muscular bag made up of smooth muscles that stores urine.

  • The process by which the urinary bladder empties when it is filled is known as micturition. Contraction of the smooth muscles of the urinary bladder causes the urine in the urinary bladder to empty.

  • At low volumes of urine in the urinary bladder, emptying of bladder is under voluntary control. When the urinary bladder is completely filled, emptying of urinary bladder is under a nervous reflex known as micturition reflex. For elderly and individuals with neurological disorder, emptying of urinary bladder is only under micturition reflex. This means that this group of individuals have no control over their emptying of the urinary bladder.

2.1.4 Urethra

• • Duct through which urine passes from urinary bladder to outside environment.

  • For males, urethra is the common site in which semen and urine pass through at different times.

2.1.5 Sphincter muscles

• • Located at the bottom of the urinary bladder.

  • When the bladder is full, sensory neurones send impulses to the brain and thereafter, impulses from the brain are sent to the sphincter muscles. Causes the sphincter muscles to relax to allow urine to flow through urethra and out of the body.