Urinary System Flashcards
Specify the location and describe the function(s) of each of the organs of the renal system.
a) Kidneys
The kidneys occur in pairs, with one kidney on either side of the vertebral column, at the level between the last thoracic and third lumbar vertebrae. The kidneys are external to the peritoneal lining of the abdominal cavity on the posterior wall of the abdomen, and are partially protected by the eleventh and twelfth pair of ribs. The right kidney is slightly lower than the left, due to the large area occupied by the liver.
The functions of the kidney are to help control the composition and volume of the blood, to maintain the water (osmotic) balance, to maintain blood pH and to remove nitrogenous wastes from the blood.
b) Ureters
The human body has two ureters, one for each kidney. Each ureter is an extension of the pelvis of the kidney and extends 25 to 30 cm to the urinary bladder. At the point of widest diameter, the ureters measure less than 1.7 cm. The ureters enter the urinary bladder at the superior lateral angle of its base.
The ureters drain the urine from the kidney and move the urine by peristalsis to the urinary bladder.
c) Bladder
The urinary bladder is a hollow muscular organ situated in the pelvic cavity posterior to the symphysis pubis. In the male, it is directly anterior to the rectum. In the female, it is anterior to the vagina and inferior to the uterus. At the base of the bladder is the opening to the urethra, surrounded by the circular smooth muscles forming the internal sphincter. This sphincter is under involuntary control.
The urinary bladder functions as a storage organ for urine. The average capacity of the urinary bladder is 700 to 800 ml. When the amount of urine in the bladder exceeds 200 to 400 ml, stretch receptors transmit impulses initiating a desire to expel urine.
d) Urethra
The urethra is a small tube leading from the floor of the urinary bladder to the exterior of the body. In females, it lies directly posterior to the symphysis pubis and is embedded in the anterior wall of the vagina. Its length is approximately 3.8 cm. The urethra opens between the clitoris and the vaginal opening.
Surrounding the urethra below the bladder, is the external urethral sphincter. This sphincter is consists of skeletal muscle and is under voluntary control.
In males, the urethra is about 20 cm long. It passes vertically through the prostate gland immediately below the urinary bladder and then through the penis.
The urethra is the terminal portion of the urinary system. It functions as the passageway for discharging urine from the body. The male urethra also serves as the duct through which semen is discharged
Describe the gross anatomy of the kidney.
Shape and size
The kidneys are reddish organs that resemble kidney beans in shape. The average adult kidney measures about 10 to 12 cm long, 5 to 7.5 cm wide and about 2.5 cm thick. Each kidney is about the size of a fist. Its concave medial surface faces the vertebral column and blood vessels, lymph vessels, nerves, and ureter enter and exit the kidney in this region called the hilus
Describe the formation and composition of urine.
- Glomerular filtration
In glomerular filtration, all components of blood except blood cells and most proteins are filtered out of the glomerular capillaries into Bowman’s capsule. Thus, the glomerulus functions as a sieve and Bowman’s capsule collects the soluble products. At any given time, about one-fifth of the water and other dissolved substances, including glucose, amino acids, Na+, Cl-, K+, nitrogenous wastes and vitamins, are forced through the capillary membranes. Filtration is an entirely passive event caused by the fact that hydrostatic pressure in the glomerulus (created by contractions of the heart) is greater in the capillaries than the fluid pressure in the capsule. Consequently, this pressure forces fluid into the capsule at a rate of about 125 ml/min in the normal adult. The resulting fluid is called glomerular filtrate and passes into the proximal convoluted tubule. Proteins and blood cells are too large to be filtered out of the blood plasma through the membranes of the glomerulus and consequently are not normally present in the glomerular filtrate
2) Tubular reabsorption
During tubular reabsorption, about 99 % of the water and most of the solutes of the filtrate are returned to the bloodstream. identifies the regions of the nephron where water is reabsorbed during the production of urine. Solutes of importance to bodily nutrition are also reabsorbed, mostly in the proximal convoluted tubule. These include glucose, amino acids, some small proteins, and ions such as Na+ and K+ and vitamins. Some of these substances are absorbed by active transport (e.g. Na+). Others are absorbed either by simple diffusion or facilitated diffusion. Wastes such as urea are partially reabsorbed (about half of the filtered urea is reabsorbed from the filtrate. Water is reabsorbed by osmosis with about 65 % of the water of the filtrate being reabsorbed in the proximal convoluted tubule.
3) Tubular secretion
A few substances are actively secreted from the blood into the renal tubules for excretion. Substances that are actively secreted include: hydrogen ions, ammonia, potassium ions, uric acid and creatinine. Also, a number of drugs such as penicillin and phenol compounds used to test renal function are secreted.
Tubular secretion has two principal effects:
i) It controls the blood pH by secreting hydrogen ions
ii) It rids the body of certain toxic materials such as creatinine, ammonia and uric acid.
Active secretion of hydrogen ions occurs in the proximal convoluted tubules, distal convoluted tubules and collecting ducts.
Describe the nervous control of micturition.
Micturition is the process by which urine is expelled from the urinary bladder. In humans, micturition is caused by a combination of involuntary and voluntary nervous activity. The involuntary autonomic nervous system and its components, the sympathetic and parasympathetic nervous systems will be discussed during Week 6.
Describe the physical properties and chemical composition of urine.
Table 1 shows the composition of the glomerular filtrate collected in Bowman’s capsule, immediately after blood is filtered in the glomerulus, as well as the composition of urine after various substances are reabsorbed or secreted by the nephron.
Inorganic ions (e.g. sodium, chloride) and salts are almost completely reabsorbed. Although only about half the urea is reabsorbed (53 %), urea is 85 times as concentrated in urine compared to the glomerular filtrate and uric acid is 17 times as concentrated as in the filtrate as a result extensive reabsorption of water.
