11th Class Biology Excretory Products and their Elimination Urine

The fluid and dissolved waste substances excreted by the kidneys constitute urine.

Quantity : An adult man normally passes about 1 to 1.8 litres of urine in 24 hours. The volume of urine depends upon (i) the fluid intake, (ii) level of physical activity, (iii) type of food taken and (iv) environmental temperature increase urine output. Less fluid intake and profuse sweating due to heavy physical work and high temperature reduce urine output. Certain substances, such as tea, coffee and alcohol, increase urine output. These are said to be diuretic.

Physical properties : Urine is transparent yellowish fluid, but becomes turbid (cloudy) on standing, its colour depending on its concentration. Its colour is due to a pigment urochrome derived from the breakdown of haemoglobin from the worn-out RBCs. Colour of the urine is altered by certain materials taken such as beet, vitamin B complex and some drugs diseases. It is hypertonic to blood plasma. Its specific gravity ranges between 1.001 to 1.035, being slightly higher than that of water. Its pH is 6. It depends on the diet. High protein food and fruits increase acidity whereas vegetables increase alkalinity. Urine has a characteristic unpleasant odour. If allowed to stand, urea is degraded by bacteria to ammonia which imparts a strong smell to urine.

Chemical composition : Urine consists of water and organic and inorganic substances. Water alone forms about 95% of it, other substances form only 5%. The organic substances are mainly nitrogenous organic compounds include urea, uric acid, creatinine and hippuric acid. Of these, urea is the principal component of human urine. The non nitrogenous organic compounds include vitamin C, oxalic acid, phenolic substances include ammonia, and mineral salts such as chlorides, sulphates and phosphates of sodium, potassium, calcium and magnesium. Sodium chloride is the principal mineral salt of the urine. Urine also contains some other substances, such as pigments and drugs, and some epithelial cells, leucocytes, mucin, enzymes, and hormones.

Abnormal materials : Presence of proteins (albumins), bile salts, bile pigments, ketone bodies, blood, pus, microbes and more than a trace of glucose in the urine is pathological condition. Presence of glucose, protein, blood, ketone bodies and pus in the urine is called glucosurea, proteinuria, haematuria, ketonuria and pyuria respectively.

Renal threshold : A negligible amount of glucose is present in the urine. The highest concentration of a substances in the blood upto which it is fully reabsorbed from the glomerular filtrate is called its threshold. If its concentration in the blood exceeds its renal threshold, some of the filtered out substance is not reasborbed and is excreted in the urine. For example, the renal threshold of glucose is 180 mg. per 100 ml. of blood. If its blood level exceeds 180 mg., some of the filtered out glucose is not reabsorbed and is passed in urine.

Conduction of urine and Micturition : Urine is produced and drained continuously by the nephrons into the renal pelvis. From here, it is carried down the ureters by peristaltic waves into trigonum vesicae and then into the body of the urinary bladder. The bladder serves to store the urine temporarily and also to pass it out at suitable intervals. The process of passing out urine from the urinary bladder is called urination or micturition, As urine collects, the muscular walls of the bladder distend to accommodate it. Distension of its walls stimulates the sensory nerve endings in the bladder wall and this sets up reflexes, which cause an urge to pass out urine. During the discharge of the urine, the bladder and urethral sphincters relax and the smooth muscles of the bladder wall gradually contract. This slowly drives the urine from the bladder through the urethra to the exterior. Reflux of the urine into the ureters is prevented because the terminal parts of the ureters pass obliquely through the bladder wall and are consequently closed when the bladder wall contracts around them. Relaxation and contraction of the urinary bladder are caused by impulses from the sympathetic and parasympathetic nerve fibres.

Micturition may be voluntarily postponed for some time until the pressure in the bladder rises too high to control. Micturition may also be voluntarily achieved even before sufficient urine has accumulated in the bladder. Normally an urge for micturition starts when the bladder is a little more than halffull of urine.

Urine constituants in man (in gram)

Urine constituents in man (in %)