.MICTURITION
– Gradual filling of urinary bladder causes stretching. As a result, stretch receptors on its wall send impulses to CNS.
The CNS passes on motor messages. It causes the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter.
It results in micturition (release of urine).
– The neural mechanism causing micturition is called micturition reflex.
– An adult human excretes 1 to 1.5 litres of urine (25-30 gm urea) per day.
– Urine is a light yellow colored watery fluid, slightly acidic (pH-6.0), and has a characteristic odor.
– Various conditions can affect the characteristics of urine.
– Analysis of urine helps in clinical diagnosis of many metabolic disorders and malfunctioning of the kidney.
E.g. Glycosuria (presence of glucose) and Ketonuria (ketone bodies) in urine indicates diabetes mellitus.
Role of Lungs, liver & skin in Excretion
Lungs: Remove CO2 (200 mL/minute) and water.
Liver: Secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. Most of them pass out along with digestive wastes.
Skin (Sweat glands & sebaceous glands): Sweat contains water, NaCl, small amounts of urea, lactic acid, etc.
The primary function of sweat is to give a cooling effect on the body surface. Sebaceous glands eliminate sterols, hydrocarbons, waxes, etc. through sebum. Sebum provides a protective oily covering for the skin.
Saliva eliminates small amounts of nitrogenous wastes.