CH26: The Urinary System Flashcards
What are the components of the urinary system
Two kidneys, two ureters, one urinary bladder and one urethra
List the various functions of the kidneys
Waste excretion: The kidney helps excrete waste through the formation of urine. Some wastes excreted in urine result from metabolic reactions, such as urea and ammonia from the deamination of amino acids, creatinine, uric acid from the catabolism of nucleic acids etc. Urea, ammonia, creatinine, uric acid, and urobilin, are collectively known as nitrogenous wastes due to them all having nitrogen. Other wastes excreted from urine include foreign substances that have entered the body, such as drugs and environmental toxins
Regulation of blood ionic composition: The kidneys help regulate the blood levels of several ions, mainly Na+, K+, Ca2+. Cl- and HPO42-. They do this by adjusting the amount of these ions excreted into urine.
Regulation of blood pH: The kidneys excrete a variable amount of hydrogen ions (H+) into the urine and conserve bicarbonate ions (HCO3-), which are an important buffer in the body. Both activities help regulate blood pH
Regulation of blood volume: The kidneys adjust blood volume by conserving or eliminating water in the urine. An increase in blood volume causes an increase in bp and vice versa
Regulation of blood pressure: The kidneys help regulate bo by secreting the enzyme renin, which activates the renin-angiotensin aldosterone pathway. Increased renin causes increase in blood pressure
Maintenance of blood osmolarity: By separately regulating loss of water and loss of solutes in the urine, the kidneys maintain a relatively constant blood osmolarity close to 300 milliosmoles per liter
Production of hormones: As we saw, the kidneys produce calcitriol, the active form of vitamin D and erythropoietin, which stimulates the formation of rbcs
Regulation of blood glucose level: Like the liver, the kidneys can use the amino acid glutamine in gluconeogenesis, the synthesis of new glucose molecules. They can then release glucose into the blood to help maintain a normal blood glucose level
What are the two parts to a nephron and their function
A renal corpuscle where blood plasma is filtered and a renal tubule into which the filtered fluid (glomerular filtrate) passes and is further regulated
Describe the parts of the renal tubule
The renal tubule has three main sections. In the order that fluid passes through them:
Proximal convoluted tubule (PCT), nephron loop (loop of Henle), and the distal convoluted tubule (DCT). Proximal and distal are in relation to the glomerular capsule. Convoluted means that the tubule is tightly coiled rather than straight
Describe the structure of the glomerular capsule
The glomerular capsule consists of visceral and parietal layers . The visceral layer consists of modified simple squamous epithelial cells called podocytes. The parietal layer of the glomerular capsule consists of simple squamous epithelium and forms the outer wall of the capsule
Fluid filtered from the glomerular capillaries enters the capsular space, the space between the two layers of the glomerular capsule, which is continuous with the lumen of the renal tubule
Describe the three processes involved in urine formation
To produce urine, nephron and collecting ducts perform three basic processes: glomerular filtration, tubular reabsorption, and tubular secretion
Glomerular filtration: In the first step of urine production, water and most solutes in blood plasma move across the wall of glomerular capillaries, where they are filtered and move into the glomerular capsule and then into the renal tubule
Tubular reabsorption: As filtered fluid flows through the renal tubules, and through the collecting ducts, renal tubule cells reabsorb about 99% of the filtered water and many useful solutes. The water and solutes return to the blood as it flows through the peritubular capillaries and vasa recta
Tubular secretion: As filtered fluid flows through the renal tubules and collecting ducts, the renal tubule and duct cells secrete other materials, like wastes, drugs and excess ions into the fluid. Notice that a tubular secretion removes a substance form the blood
Solutes and the fluid that drain into the minor and major calyces and renal pelvis constitute urine and are excreted. The rate of urinary excretion of any solute is equal to its rate of glomerular filtration, plus its rate of secretion, minus its rate of reabsorption.
What factors promote/oppose glomerular filtration
Glomerular filtration depends on three main pressures. One pressure promotes filtration and two oppose it
Glomerular blood hydrostatic pressure (GBHP) is the blood pressure in glomerular capillaries. Generally, GBHP is about 55 mmHg. It promotes filtration by forcing water and solutes in blood plasma through the filtration membrane
Capsular hydrostatic pressure (CHP) is the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and renal tubule. CHP opposes filtration and represents a “back pressure” of about 15 mmHg.
Blood colloid osmotic pressure (BCOP), which is due to the presence of proteins such as albumin, globulins, and fibrinogen in blood plasma, also opposes filtration. The average BCOP in glomerular capillaries is 30 mmHg.
Describe the myogenic mechanism
The myogenic mechanism occurs when stretching triggers contraction of smooth muscle fibers in the walls of afferent glomerular arterioles. As blood pressure rises, GFR also rises because renal blood flow increases. However, the elevated blood pressure stretches the walls of the afferent glomerular arterioles. In response, smooth muscle fibers in the wall of the afferent glomerular arteriole contract, which narrows the arteriole’s lumen. As a result, renal blood flow decreases, thus reducing GFR to its previous level. Conversely, when arterial blood pressure drops, the smooth muscle fibers are stretched less and thus relax. The afferent glomerular arterioles dilate, renal blood flow increases, and GFR increases. This is an example of a negative feedback loop, and it helps keep a constant rate of GFR over a wide range of systemic pressures
Describe tubularglomerular feedback
The second contributor to renal autoregulation, tubuloglomerular feedback, is so named because part of the renal tubules, the macula densa, provides feedback to the glomerulus.
When GFR is above normal due to elevated systemic blood pressure, filtered fluid flows more rapidly along the renal tubules. As a result, the proximal convoluted tubule and nephron loop have less time to reabsorb Na+, Cl−, and water.
Macula densa cells are thought to detect the increased delivery of Na+, Cl−, and water and to inhibit release of nitric oxide (NO) from cells in the juxtaglomerular apparatus. Because NO causes vasodilation, afferent glomerular arterioles constrict when the level of NO declines. As a result, less blood flows into the glomerular capillaries, and GFR decreases. When blood pressure falls, causing GFR to be lower than normal, the opposite sequence of events occurs, although to a lesser degree. Tubuloglomerular feedback operates more slowly than the myogenic mechanism.
How does neural regulation maintain GFR
This causes vasoconstriction of 𝛼1 receptors, which are plentiful in the smooth muscle fibers of afferent glomerular arterioles. At rest, sympathetic stimulation is moderately low, the afferent and efferent glomerular arterioles are dilated, and renal autoregulation of GFR prevails
With moderate sympathetic stimulation, both afferent and efferent glomerular arterioles constrict to the same degree. Blood flow into and out of the glomerulus is restricted to the same extent, which decreases GFR only slightly. With greater sympathetic stimulation, however, as occurs during exercise or hemorrhage, vasoconstriction of the afferent glomerular arterioles predominates. As a result, blood flow into glomerular capillaries is greatly decreased, and GFR drops. This lowering of renal blood flow has two consequences: (1) It reduces urine output, which helps conserve blood volume. (2) It permits greater blood flow to other body tissues
How do hormones maintain GFR
Two hormones contribute to regulation of GFR. Angiotensin II reduces GFR; atrial natriuretic peptide (ANP) increases GFR.
Angiotensin II is a very potent vasoconstrictor that narrows both afferent and efferent glomerular arterioles and reduces renal blood flow, thereby decreasing GFR. Cells in the atria of the heart secrete atrial natriuretic peptide (ANP). Stretching of the atria, as occurs when blood volume increases, stimulates secretion of ANP. By causing relaxation of the glomerular mesangial cells, ANP increases the capillary surface area available for filtration. Glomerular filtration rate rises as the surface area increases.
