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LAB 1 - Urinary System


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The urinary system is composed of paired kidneys and ureters, the urinary bladder and the urethra. Urine is produced in the kidneys and then drains through the ureter to the urinary bladder where the urine is stored. Urine is eliminated from the body thought he urethra.

The urinary system is a system of organs that produce and excrete urine, the liquid waste product of the body. In most vertebrates the two kidneys filter all substances in the bloodstream, and waste matter is produced continuously as urine, which flows through the two ureter's to the bladder, a membranous sac. After being stored in the bladder, urine is released into a tube called the urethra, which leads out of the body. Urine is passed by the involuntary opening of a muscular sphincter between the bladder and the urethra and also by the voluntary opening of a sphincter in the urethra. Kidneys regulate the levels of ions and pH in our extracellular fluids and eliminate metabolic wastes and toxins, thus contributing to the homeostasis of our internal environment.

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.


The Nephron of the Kidney
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The nephron is the structural and functional unit of the kidney.

Vascular components
phases.gif (93875 bytes) Afferent arteriole- carries blood to the glomerulus.
phases.gif (93875 bytes) Efferent arteriole - carries blood from the glomerulus.
phases.gif (93875 bytes) Glomerulus - consists of capillaries that filters a protein free plasma into the tubular component.
phases.gif (93875 bytes) Peritubular capillaries - supply the renal tissues involved in exchanges with the fluid in the tubular lumen.

Combined vascular/tubular components
phases.gif (93875 bytes) Juxtaglomerular apparatus - secretes substances involved in the conduct of kidney function.

Tubular components
phases.gif (93875 bytes) Bowman's capsule - collects the glomerular filtrate.
phases.gif (93875 bytes) Proximal tubule - uncontrolled reabsorption and secretion of selected substances occur here.
phases.gif (93875 bytes) Loop of Henle - establishes an osmotic gradient in the renal medulla that is important in the kidneys's ability to produce urine of varying concentration.
phases.gif (93875 bytes) Distal tubule and collection duct - variable controlled reabsorption of Na + and H2O and secretion of K+ and H+ occur here. Fluid leaving the collecting duct is urine, which enters the renal pelvis.

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Formation of urine by the kidneys involves three main processes, filtrate, reabsorption and secretion. The glomerulus acts as an ultra filter allowing passage of water, electrolytes, and small organic molecules such as glucose The first step in urine formation is glomerular filtration. The plasma is filtered by glomerular capillaries then it enters the Bowman's capsule. The fluid collection in the capsular space is called glomerular filtrate. The filtrate is composes of water, ions (sodium, potassium and chloride), nitrogenous wastes (urea, uric acid, creatinine), and organic molecules (glucose, amino acids). As the glomerular filtrate makes its way through the nephron, its original composition is quickly modified by reabsorption of various metabolites, ions, and water. The amount of filtrate formed in kidneys per minute is the glomerular filtration rate (GFR), which is ~125 ml fluid/min. The amount of urine formed = 1 ml fluid/min. The second step in urine formation is tubular reabsorption. As filtrate flows through tubules, substances of value to the body are returned to the peritubular capillaries and re-enter the circulatory system. Of the 180 liters of filtrate produced per day, about 178.5 liters are reabsorbed in this process, and the other 1.5 liters are ultimately eliminated as urine. The final step in urine formation is Tubular secretion. Tubular secretion is the selective transfer of substances form the peritubular capillary blood into the tubular system.

Filtration Pressures
Glomerular hydrostatic pressure
- the blood pressure in the glomerular capillaries.

phases.gif (93875 bytes)   Pushes ions, small organic molecules and water, out of the capillaries.
phases.gif (93875 bytes)   Promotes filtration.

Capsular hydrostatic pressure - back-pressure of filtrate already in the capsule.

phases.gif (93875 bytes)   Pushes ions, small organic molecules and water from capsule into capillaries.
phases.gif (93875 bytes)   Opposes filtrate.

Colloid osmotic pressure - is the result of suspended proteins since the concentrations of ions and small organic molecules are the same in the plasma and the filtrate.

phases.gif (93875 bytes)   Opposes filtrate.
phases.gif (93875 bytes)   Blood colloid osmotic pressure draws water from filtrate into plasma.
phases.gif (93875 bytes)   Capsular colloid osmotic pressure is generally zero unless glomerulus is damaged and plasma proteins enter the capsule.

Net filtration pressure - average pressure forcing water and solutes into capsule.

Average Pressure Values
phases.gif (93875 bytes) Glomerular hydrostatic pressure = 60mm Hg
phases.gif (93875 bytes) Capsular hydrostatic pressure = 15 mm Hg
phases.gif (93875 bytes) Glomerular osmotic pressure = 28 mm Hg
phases.gif (93875 bytes) Net filtration pressure = 60 mm Hg - (15mmHg + 28mmHg) = 17mmHg

Decreasing filtration rate the cl - ions reaching the macula densa decrease. Macula densa causes smooth muscle in the wall of the afferent arteriole to become dilated, and more blood flows into the glomerulus. The increase in glomerular pressure causes the GFR to rise, and juxtaglomerular cells release rennin. This causes the formation of angiotensin I, which is quickly changed into angiostensin II. This then causes the muscle in the efferent arteriole to constrict. This process is known as autoregulation.