Biology 157- The Urinary System

The urinary system consists of: 2 kidneys- filter blood plasma and produce urine, 2 ureters- transport urine to the urinary bladder, 1 urinary bladder- temporarily stores urine until elimination, 1 urethra- conducts urine to the exterior
Urology- branch of medicine that deals with the male and female urinary systems, and the male reproductive system
Nephrology- study of the anatomy, physiology, and pathology of the kidneys
Functions of the kidneys: 1. excretion- removal of waste products from the body fluids. a. urea- products of amino acid breakdown b. urobilinogen- products of hemoglobin breakdown c. creatinine- products of the breakdown of creatine phosphate in muscle fibers. d. uric acid- products of nucleic acid breakdown. e. foreign chemicals- drugs and environmental toxins 2. Regulate blood homeostasis. a. blood volume. b. blood ionic composition. c. blood ph
Water balance in the body: the average human body contains about 40L of total body water in the form of intracellular fluid, interstitial fluid, and plasma
The nephron are the functional units of the kidneys. Each kidney contains approximately 1.25 million nephrons
Each nephron consists of 2 main parts: renal corpuscle and renal tubule. The renal corpuscle is the site of blood filtration and it is composed of two parts: the glomerulus- a capillary network and glomerular capsule- cup that surrounds the glomerulus. The renal tubule is the passageway where filtered fluid passes from the glomerular capsule. It is composed of various parts: a. the proximal convoluted tubule. b. loop of henle (descending limb and ascending limb) c. distal convoluted tubule
Blood arrives at the nephron through an afferent arteriole, passes into the capillaries of the glomerulus, then flows into an efferent arteriole to the peritubular capillaries and then it is collected in a vein.
Renal physiology: to produce urine, the nephrons and collecting ducts perform 3 basic functions: 1. filtration, 2. absorption, 3. secretion
The walls of each glomerulus form a filtration membrane between the blood and the capsular space. The filtration membrane is composed of 3 layers: 1. the cells on the inside of the glomerulus have large fenestrations that permit all solutes to pass, except blood and platelets 2. the lamina densa is a matrix of fibers that prevents the filtration of large plasma proteins. 3. The outside of the glomerulus is covered with cells called podocytes which have many foolike projections called pedicles that wrap around the glomerular capillaries. The spaces between the pedicles are filtration slits
The kidneys are capable of filtering a large volume of blood for 3 reasons: 1. glomerular capillaries present a large surface area for filtration 2. the filtration membrane is thin and porous 3. glomerular capillary blood pressure is high (55 mmHg instead of 30 mmHg in most other systemic capillaries in the body) because the efferent arteriole is smaller in diameter than the afferent arteriole
The glomerular filtration rates (GFR) is the amount of filtrate that the kidneys produce each minute. A normal GFR is 125ml/min.
Renal clearance tests are renal function tests that are routinely used to assess glomerular filtration rate to determine if renal function has been compromised
Inulin (not insulin) is an inert polysaccharide, and often used as the standard to calculate renal clearance because inulin is not reabsorbed or stored or secreted by the kidneys
Glomerular filtration rate can be modified by altering blood pressure in the afferent and efferent arterioles by vasodilation or constriction. This can be induced by: Auto regulation- baroreceptors in the afferent arterioles monitor stretching of the vessel walls. Hormonal regulation- renin can increase blood pressure and increase GFR. Autonomic regulation- sympathetic activation can cause vasoconstriction of afferent arterioles and reduce GFR.
Proximal convoluted tubule- about 60-70% of the filtrate is reabsorbed here. Loop of henle- about 15-20% of fluid is reabsorbed in the loop of henle. Distal convoluted tubule- only about 15-20% of the intial filtrate reaches the DCT.
Collection ducts- the collecting ducts collect tubular fluid from several nephrons
Reabsorption of sodium is associated with the loss of potassium. a. therefore, prolonged aldosterone stimulation can produce hypokalemia a dangerous reduction of blood potassium concentration. b. this is one reason why it is preferable to drink water-electrolyte solutions with during heavy or extended exercise, rather than plain water.
Most of the remaining water is reabsorbed by facultative water reabsorption, which is regulated by the antidiuretic hormone (ADH). a. ADH increases the permeability of the collecting duct by stimulating the addition of water channels in cell membranes. b. If homeostasis is disrupted and blood volume or water content declines, ADH is released and stimulates water conservation by increasing water reabsorption by the collecting ducts. c. in the absence of ADH, a large volume of dilute urine is produced. A person suffering from diabetics insipidus may excrete up to 20 L/day
Diuretics- substances that slow renal reabsorption of water and cause an elevated urine flow, which in turn reduces blood volume
The kidenys play an important role in re-establishing blood pressure, water and electrolyte homeostasis. When renal blood flow declines juxtaglomerular cells in the juxtaglomerular apparatus detect this and respond by secreting renin. The JGA consists of a special group of epithelial cells of the DCT that are near the renal corpuscle and the juxtaglomerular cells which are special smooth muscle fibers in the walls of the afferent and efferent arterioles
Dialysis- blood is cleansed artificially by dialysis if kidneys are so impaired by disease or injury that they are unable to function properly
1. Hemodialysis- blood is redirected to flow through tubing made of selectively permeable dialysis membrane
2. Peritoneal dialysis- blood is cleansed inside the body, through the peritoneum
Kidney stones- sometimes the salt crystals present in the urine precipitate and solidify into insoluble stones. Excessive calcium intake, low water intake, and hyperparathyroidism can lead to kidney stones. When a stone lodges in a narrow passageway the pain can become intense. Treatments include dissolving drugs, shock-wave therapy, or surgery
Urine is collected from collecting ducts in the kidneys and transported to the urinary bladder through the ureters. The urinary bladder is a hollow, muscular organ that can change shape drastically depending on the volume of urine it contains. The urethra is the tube leading from the floor of the bladder to the exterior.
Discharge of urine from the urinary bladder is called micturition. It occurs by a combination of voluntary and involuntary contractions
Urinary incontinence- lack of voluntary control over micturition usually due to a weakening of muscles or damage to nerves

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