The kidneys perform the primary function of the urinary system. They filter the blood and excrete waste and excess water as urine, regulating the blood pressure, volume, pH and salt balance.
The ureters, bladder and urethra are accessory structures used only to eliminate the urine from the body.
The kidneys are bean-shaped, reddish-brown organs, which are about 11cm long. They are found on the posterior abdominal wall either side of the vertebral column between T12 and L3. The right kidney lies slightly lower than the left, as it is restricted superiorly by the large right lobe of the liver. Each kidney is held to the posterior abdominal wall behind the parietal peritoneum and is partially protected by the lower ribs.
The lateral margin of each kidney is smoothly convex while the medial margin is concave and further indented in the middle as the hilum. It is through the hilum that the renal vessels and the renal pelvis enter and exit the kidneys.
The internals of the kidneys have the following three distinguishable regions;
In the medulla and cortex of each kidney are more than a million microscopic tubular structures called nephrons. The nephrons function to filter the blood so as to regulate the amount of water, salts, sugars and urea circulating the body. The filtrate that is produced is urine, which flows down the ureters to the bladder where it is voided.
Each nephron is composed of a glomerular capsule, glomerulus, proximal convoluted tubule, loop of Henle and distal convoluted tubule. The renal corpuscle includes the glomerular capsule and the glomerulus. The renal tubule is the part of the nephron that directs the filtrate away from the glomerular capsule and includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule and the collecting duct. The collecting duct is not considered part of the nephron as many nephrons drain into one collecting duct.
Glomerulus
Reabsorption & Secretion
Collecting tubule
(CT)
Reabsorption, Secretion & Transport
Filtration at the glomerulus is under pressure as the afferent arteriole is so close to the abdominal aorta. The fluid that passes through the wall of the glomerular capsule into the nephron is called the glomerular filtrate and is similar in composition to plasma. Blood and protein cannot pass into the filtrate but small waste molecules can.
600 ml of blood will pass through the glomerulus each minute, 125 ml of which will be absorbed into the nephron as glomerular filtrate.
The tubule of the nephron functions to reabsorb most of the glomerular filtrate. The cells of the tubule reabsorb vital nutrients and water back into the blood, while retaining the waste products that the body needs to eliminate. The plexus formed by the efferent arteriole (from the glomerulus) passes closely to the proximal convoluted tubule, allowing direct transfer into the blood. In the loop of Henle the filtrate is further concentrated. Water is absorbed by osmosis, being transported down its concentration gradient.
The amount of water reabsorbed is controlled by an anti-diuretic hormone secreted by the posterior lobe of the pituitary gland. The amount of salts reabsorbed is controlled by aldosterone secreted by the cortex of the suprarenal glands. These hormones are increased or decreased according to the needs of the body.
During active secretion, wastes that were not initially filtered out of the blood in the glomerular capsule such as ammonia and certain drugs and toxins are removed from the capillaries into the distal convoluted tubule.
The ureters are two tubes that drain urine from the renal pelvis into the trigone of the bladder. They are 25 to 30 cm long with a diameter of approximately 3 mm.
Each ureter descends on the surface of psoas major, behind the ovarian or testicular vessels before entering the lesser pelvis and running along its lateral wall, to finally turn medially and enter the trigone of the bladder.
The ureters have an outer fibrous layer, two muscular layers and an inner mucous layer. Urine is passed down to the bladder by peristaltic waves of the smooth muscle walls.
The urinary bladder is a hollow, muscular reservoir that functions to store urine until urination occurs.
The female bladder is in contact with the anterior vaginal wall. The male bladder is in contact with the rectum, seminal vesicles and ductus deferens.
When empty, the bladder lies in the lesser pelvis behind the pubic symphysis. When full, it pushes up into the false pelvis and may reach as far as the umbilicus. It can hold 500 ml urine, but will feel full holding only 250-300 ml. When the bladder is empty the internal surface falls into folds or rugae, apart from a triangular region called the trigone at the base of the bladder that always appears smooth. This is the point of entry of the ureters and remains rigid to avoid tearing these ducts.
The bladder is made up of thick muscular walled lined internally by an inner mucous layer; it is covered superiorly by peritoneum.
The urethra extends from the internal urethral orifice to the external urethral orifice. It leaves the neck of the bladder and passes through the pelvic diaphragm. It has muscular and internal mucosal layers. When the bladder is filled with 250-300 ml of urine, sensory impulses cause a reflex contraction of the bladder and relaxation of the urethral sphincter to allow urination.
The male urethra is 18-20 cm long and serves as an outlet for the reproductive as well as the urinary system. It is made up of four portions (although it is often described in three);
The female urethra is 4 cm long and unlike the male, only serves as an outlet for the urinary system. It passes downwards from the internal urethral orifice, embedded in the anterior wall of the vagina to terminate at the external urethral orifice.