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The Urinary System

The average person urinates six to eight times a day. While urinating may seem like an inconvenience at times, it is important because it is the body’s way of excreting wastes. In order to excrete waste, the body needs two kidneys, two ureters, a bladder, and a urethra. Kidney filter the blood of any wastes and prepares it for excretion, while returning useful nutrients back into the bloodstream. The ureters transport the waste filtered by the kidneys into the bladder. The bladder stores urine until it has reached capacity and expels it through the urethra. These organs work together to ensure that nutrients and blood are returned to the body, or reabsorbed, while wastes are excreted in the form of urine.

The Kidneys

The main organs of filtration are the kidneys. Although kidneys are the size of a bar of soap, they have many functions. Kidneys filter blood plasma, regulate osmolarity, and clear drugs and toxins from the body. The kidneys are divided into two regions, the renal cortex and renal medulla. The renal cortex is the outer portion of the kidney while the renal medulla is in the inner portion of the kidney. The renal medulla is made up of pyramids and columns.  Pyramids are a cone shaped structure that tapers into a point called the renal papilla. The renal papilla leads into the minor calyx, which collects urine. The minor calices converge to form a major calyx which eventually leads urine into the renal pelvis. The renal pelvis leads urine into the ureters for excretion.

Renal Circulation

Blood that needs to be filtered follows a path to the kidney. Blood that is at the heart begins its journey to the kidneys through the aorta, the main artery of the body. The aorta leads oxygenated blood into the renal artery, located at the hilum of the kidney. The renal artery diverges to form branches called segmental arteries. The segmental arteries lead blood in between the renal pyramids through vessels called interlobar arteries. The interlobar arteries go around the pyramid becoming arcuate arteries. The arcuate arteries form small branches called interlobular arteries which become afferent arterioles. The afferent arterioles bring blood into nephrons, the filtration unit of the kidney. Blood that has been returns to the heart deoxygenated as a result of the peritubular capillaries and vasa recta which removes the oxygen from the blood. The deoxygenated follows a similar pattern as the oxygenated blood, the key difference being that it is in the reverse order and it travels through the veins rather than the arteries. The deoxygenated blood leaves the glomerulus through the efferent arterioles. The efferent arterioles lead blood into the interlobular veins and through the arcuate veins. The arcuate veins travel down the column through the interlobar veins. The interlobar veins converge to form renal veins. The blood leaves the kidneys from the renal vein and enters the heart through the inferior vena cava, which will be oxygenated.

Nephron

The nephron is the small filtration unit of the kidney. Each kidney has about 1.2 million nephrons that are located both in the medulla and the cortex. Nephrons located in the cortex are called cortical nephrons while nephrons located in the medulla and cortex are called juxtamedullary nephron. The nephron is made up of the renal corpuscle and the renal tubule. The renal corpuscle is made of a glomerular capsule that encircles the glomerulus. The glomerular capsule is composed of two layers, a parietal layer made up of simple squamous epithelium and a visceral layer that is made up of podocytes that prevent blood from being excreted. The renal tubule is a duct that leads filtrate into the renal papilla. The renal tubule is composed of a proximal convoluted tube, nephron loop, distal convoluted tubule, and collecting duct.

Urine Formation

  1. Glomerular filtration: Glomerular filtration is the process in which fluid passes through the 3 barriers of the filtration membrane and water and solutes enter the capsular space. The first barrier is the fenestrated endothelium which contains pores that prevent blood from being excreted. The second barrier is the basement membrane that has proteoglycan gel that prevents large and negatively charged particles from being filtered. The third barrier are the filtration slits created by the pedicles of podocytes. The podocytes form an obstacle for large anions. The glomerular filtration rate is the amount of filtrate formed per minute by two kidneys. It is directly related to blood pressure. The rate is regulated renal autoregulation, sympathetic control, and hormonal control. Renal autoregulation is the kidney’s own way to control GFR. It uses the myogenic mechanism which constricts arterioles to decrease blood flow when it is stretched too much. Tubuloglomerular filtration, the other mechanism to renal autoregulation, occurs when the glomerulus receives feedback on the amount of fluid flowing and adjust accordingly. Sympathetic control of the GFR involves the renin angiotension aldosterone mechanisms which uses renin and angiotensin converting enzyme to make angiotensinogen II. The active form of angiotensinogen is received by the hypothalamus, cardiovascular system, and kidney. The hypothalamus stimulates thirst and drinking, the cardiovascular system constricts the vessels, and kidney retains sodium and water to increase blood pressure.

  2. Tubular Reabsorption: Tubular reabsorption is the process in which water and solutes are reclaimed and returned to the bloodstream. In the proximal convoluted tubule, reabsorption takes place through the transcellular route and the paracellular route. When substances pass through the cytoplasm, reabsorption takes place through the transcellular route. The peritubular capillaries, located around the nephron loop of a cortical nephron, reabsorb through osmosis. Osmosis is affected by the accumulation of fluid on the basal side of the membrane, narrowness of efferent arterioles, and an elevated colloid osmotic pressure.

  3. Collecting Duct: The collecting duct receives urine. As urine flows through the collecting duct, the urine is concentrated and water is reabsorbed. This occurs through the countercurrent exchange system and the countercurrent multiplier. The countercurrent multiplier multiplies the osmolarity of the medulla by reabsorbing water and salt. The descending loop reabsorbs water and the ascending loop reabsorbs salt. By releasing chlorine and sodium, the ascending loop maintains the high osmolarity of the medulla while keeping the urine dilute. The counter current exchange system occurs in the vasa recta, preventing any changes in osmolarity from occurring in the medulla. As blood flows down, water diffuses out of capillaries while salt diffuses in. The opposite occurs when blood flows up, water diffuses in while salt diffuses out.

     

     

     

     

     

     

     

     

     

The Urinary Bladder

The urinary bladder is a muscular sac that holds urine until it is ready to be excreted. The muscle of the urinary bladder is called detrusor, and is composed of three layers of smooth muscle. The ureters lead into two openings creating a triangular shape with the opening of the urethra. This triangle form is known as the trigone. The wrinkles in the bladder, rugae, are stretched when the bladder is full and return to its wrinkled form when it is empty. When urine is ready to be released, the internal urethral sphincter relaxes, releasing urine to the urethra. The internal urethral sphincter is involuntary and when urine can be released in a timely manner, the voluntary external urethral sphincter is relaxed.

What Your Urine Is Telling You

If your urine is clear…then you are drinking too much water. The body has too much urine and is released in the urine in order to maintain a healthy osmolarity. The excretion of the excess water dilutes the color of the urine.

If your urine is a pale yellow or transparent yellow…then you are excreting normal urine.

If your urine is dark yellow…then you may be dehydrated. There isn’t enough water in your system to dilute the urine, causing the urine to be concentrated.

If your urine is white…then you may have an infection such as a urinary tract infection. The white color of the urine is caused by pus, the body’s mechanism to fight off infection. A urinary tract infection can be treated with antibiotics and drinking cranberry juice.

If your urine is red or pink…then it may be an affect of your diet. Eating colored foods such as beets, blueberries, or rhubarb can cause the urine to turn red or pink. Urine can also be red or pink if blood is present.

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