Urinary System Spring 2010

Function   The kidneys regulate blood volume and composition, help regulate blood pressure and pH, produce two hormones, and excrete wastes. The ureters transport urine from the kidneys to the urinary bladder. The urinary bladder stores urine and expels it into the urethra The urethra discharges urine from the body.

Which organ of the urinary system does most of the work to form urine?

Regulation of ion levels in the blood. The kidneys help regulate the blood levels of several ions, most importantly sodium ions (Na+), potassium ions (K+), calcium ions (Ca2+), chloride ions (Cl-), phosphate ions (HPO42-) and bicarbonate ion (HCO3-)

Regulation of blood volume and blood pressure. The kidneys adjust the volume of blood in the body by returning water to the blood or eliminating it in the urine. They help regulate blood pressure by secreting the enzyme renin, which activates the renin–angiotensin–aldosterone pathway by adjusting blood flow into and out of the kidneys, and by adjusting blood volume.

Regulation of blood pH. The kidneys regulate the concentration of H+ in the blood by excreting a variable amount of H+ in the urine. They also conserve blood bicarbonate ions (HCO3-), an important buffer of H+. Both activities help regulate blood pH. NH3 picks up the excess H+ ions Urine pH is 4.5-8.0 Blood pH is 7.4   

Production of hormones. The kidneys produce two hormones. Calcitriol, the active form of vitamin D, helps regulate calcium homeostasis, and erythropoietin stimulates production of red blood cells   

Excretion of wastes. By forming urine, the kidneys help excrete wastes—substances that have no useful function in the body. Some wastes excreted in urine result from metabolic reactions in the body. These include ammonia and urea from the breakdown of amino acids; bilirubin from the breakdown of hemoglobin; creatinine from the breakdown of creatine phosphate in muscle fibers; uric acid from the breakdown of nucleic acids(nucleotides). Other wastes excreted in urine are foreign substances from the diet, such as drugs and environmental toxins.

Urea

Where in the kidney are the renal pyramids located?

How much blood enters the renal arteries each minute?

A water molecule has just entered the proximal convoluted tubule of a nephron. Which parts of the nephron will it travel through (in order) to reach the renal pelvis in a drop of urine?

Function of the Nephron

To produce urine, nephrons and collecting ducts perform three basic processes—glomerular filtration, tubular reabsorption, and tubular secretion See Figure 10.6

Filtration   is the forcing of fluids and dissolved substances smaller than a certain size through a membrane by pressure. Glomerular filtration is the first step of urine production: Blood pressure forces water and most solutes in blood plasma across the wall of glomerular capillaries, forming glomerular filtrate. Filtration occurs in glomeruli just as it occurs in other capillaries See figures 10.4, 10.5 a and b  

Tubular Reabsorption Tubular reabsorption occurs as filtered fluid flows along the renal tubule and through the collecting duct: Tubule and duct cells return about 99% of the filtered water and many useful solutes to the blood flowing through peritubular capillaries.

Tubular Secretion also takes place as fluid flows along the tubule and through the collecting duct: The tubule and duct cells remove substances, such as wastes, drugs, and excess ions, from blood in the peritubular capillaries and transport them into the fluid in the renal tubules.

Aldosterone It regulates the reabsorption of sodium and water in the distal convoluted tubule Is a hormone secreted by the adrenal glands. When blood pressure is too low, renin is secreted by the juxtaglomerular apparatus renin stimulates the release of aldosterone, see figure 10.7 It promotes the excretion of K+ and the reabsorption of Na+.

What are Aquaporins Water channels located in the distal convoluted tubules and collecting ducts. ADH is secreted by the posterior pituitary ADH opens the aquaporins located in the distal convoluted tubules and collecting ducts. Water is reabsorbed into the peritubular capillaries to conserve water

ANH Atrial natriuretic hormone Secreted by the atria of the heart when cardiac cells are stretched due to increased blood volume. ANH inhibits the secretion of renin by the juxtaglomerular apparatus and the secretion of aldosteron (adrenal glands) It promotes the excretion of Na+; less water will be reabsorbed

Countercurrent mechanism The Loop of the Nephron Descending limb contains aquaporins Because of the osmotic pressure (hypertonic) Water leaves the descending limb in Salt NaCl diffuses out of the lower and upper portions of the ascending limb. See figure 10.8

Acid-Base Balance of Body Fluids pH scale measure concentration of the H+ ions A person may develop alkalosis if the blood pH rises above 7.4 A person may develop acidosis if the blood pH decrease below 7.4 Alkalosis and acidosis are abnormal conditions that may need medical attention

Buffers The pH of the blood stays near 7.4 because the blood is buffered. A buffer is a chemical or a combination of chemicals that can take up excess hydrogen ions or excess hydroxide ions. The most important buffers are H2CO3 (carbonic acid) and bicarbonate ion HCO3- . See figure 10.9

Read disorders with kidney function Urine analysis-Health focus