Biology · Chapter 8
Study notes aligned to the official NEB syllabus.
Excretion is the process by which nitrogen-containing waste products of metabolism are eliminated from the body. The main nitrogenous waste products are ammonia, urea, and uric acid.
Based on the excretory product formed, the following modes of excretion are known in animals.
Ammonotelism is the elimination of nitrogenous waste in the form of ammonia. Such organisms are called ammonotelic.
$$\text{Amino acid} + \text{oxygen} \rightarrow \text{keto acid} + \text{ammonia}$$
Ureotelism is the process of elimination of nitrogen mainly in the form of urea. Such animals are called ureotelic.
$$\text{Ammonia} + CO_2 \xrightarrow[\text{(via arginine)}]{\text{enzyme}} \text{Urea} + \text{water}$$
Uricotelism is the process of elimination of nitrogen in the form of uric acid. Such organisms are called uricotelic.
The human urinary system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
The kidneys are paired structures and the main excretory organs in mammals. They are dark red, slightly flattened, bean-shaped organs, about 10-12 cm long, 5-7 cm wide, and 2-3 cm thick. They are placed against the back wall of the abdominal cavity, just below the diaphragm, one on either side, opposite the last thoracic vertebra (T12) and the first three lumbar vertebrae.
Blood vessels (renal artery and renal vein), nerves, and the ureter enter or leave the kidney through the hilum. The kidney excretes nitrogenous waste in the form of urea.
There are two ureters, one exiting from each kidney. Each ureter is a thin, muscular tube about 25 cm in length, arising from the renal pelvis in the hilum region and opening into the urinary bladder. Its inner wall is lined by transitional epithelium, the middle layer is muscular, and the outer layer is fibrous. Urine moves through the ureter by the peristaltic waves of its walls.
The urinary bladder lies in the pelvic cavity and is a pear-shaped, muscular, hollow, sac-like structure. It collects urine from both kidneys via the ureters. It has a strong muscular wall with the ability to stretch. The bladder has three openings, two from the ureters and one for the urethra, forming a triangle called the trigone.
The wall of the urinary bladder is made up of three layers: an inner layer of smooth muscle, a middle layer of circular fibres, and an outer layer of longitudinal fibres. It stores urine temporarily.
The urethra is a canal-like, membranous tube that extends from the neck of the bladder and leads to the exterior. It carries urine at intervals from the urinary bladder to the outside.
In the female, it opens by the urethral orifice in front of the vaginal aperture, whereas in the male, the urethra is much longer and opens out at the tip of the penis by the urogenital aperture. Thus, in the male, the urethra carries both urine and semen. There is a urethral muscular sphincter present close to the junction of the urethra and bladder, which regulates urination.
Excretion is the process by which nitrogen-containing waste products of metabolism are eliminated from the body. The main nitrogenous waste products are ammonia, urea, and uric acid.
Based on the excretory product formed, the following modes of excretion are known in animals.
Ammonotelism is the elimination of nitrogenous waste in the form of ammonia. Such organisms are called ammonotelic.
Ureotelism is the process of elimination of nitrogen mainly in the form of urea. Such animals are called ureotelic.
Uricotelism is the process of elimination of nitrogen in the form of uric acid. Such organisms are called uricotelic.
The human urinary system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.
The kidneys are paired structures and the main excretory organs in mammals. They are dark red, slightly flattened, bean-shaped organs, about 10-12 cm long, 5-7 cm wide, and 2-3 cm thick. They are placed against the back wall of the abdominal cavity, just below the diaphragm, one on either side, opposite the last thoracic vertebra (T12) and the first three lumbar vertebrae.
Blood vessels (renal artery and renal vein), nerves, and the ureter enter or leave the kidney through the hilum. The kidney excretes nitrogenous waste in the form of urea.
There are two ureters, one exiting from each kidney. Each ureter is a thin, muscular tube about 25 cm in length, arising from the renal pelvis in the hilum region and opening into the urinary bladder. Its inner wall is lined by transitional epithelium, the middle layer is muscular, and the outer layer is fibrous. Urine moves through the ureter by the peristaltic waves of its walls.
The urinary bladder lies in the pelvic cavity and is a pear-shaped, muscular, hollow, sac-like structure. It collects urine from both kidneys via the ureters. It has a strong muscular wall with the ability to stretch. The bladder has three openings, two from the ureters and one for the urethra, forming a triangle called the trigone.
The wall of the urinary bladder is made up of three layers: an inner layer of smooth muscle, a middle layer of circular fibres, and an outer layer of longitudinal fibres. It stores urine temporarily.
The urethra is a canal-like, membranous tube that extends from the neck of the bladder and leads to the exterior. It carries urine at intervals from the urinary bladder to the outside.
In the female, it opens by the urethral orifice in front of the vaginal aperture, whereas in the male, the urethra is much longer and opens out at the tip of the penis by the urogenital aperture. Thus, in the male, the urethra carries both urine and semen. There is a urethral muscular sphincter present close to the junction of the urethra and bladder, which regulates urination.