Chapter 15: The Urinary System
Lecture Notes taken from:
Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC
Function: Urinary system works with other organ systems to manage water and electrolyte (mineral ion) balance, maintain blood pH, and keep blood pressure, as a consequence, at normal levels. In order to accomplish these very important tasks, nitrogenous wastes and certain drugs and toxins are eliminated in the urine.
The Kidney (2 of these) – very complex organ that adjusts the volume and chemical composition of blood. Constantly filters water and solutes; reclaims needed water and solutes; excretes waste as urine. The kidneys are protected by 3 outer membranes that include an intermediate fat layer for protection and a water tight inner membrane to prevent water loss. They are located in the dorsal region of the abdominal cavity, just deep to the 12th ribs. The right kidney is slightly lower and more exposed than the left in order to make room for the liver. Adrenal glands sit atop each kidney and major blood vessels as well as the ureters enter/exit each kidney from the concave region known as the renal hilum. The internal structure of each kidney includes the renal cortex – outer region and renal medulla – the inner region that houses triangular structures known as medullary pyramids separated by columns of cortex tissue known as renal columns. Internally at the renal hilum region, urine is collected in a structure known as the renal pelvis (open to the ureters). Oxygenated, unfiltered blood arrives at the kidney via the renal artery. Oxygen-poor, filtered blood exits the kidney via the renal vein. In between, capillaries known as the glomerulus mark the site at which blood is filtered.
More on the kidney: Kidney is THE filtering organ of the urinary system (all other anatomy just guides and stores urine produced by the kidneys). The filtering unit of the kidney = nephron. More than a million nephrons with associated blood vessels (peritubular capillaries) manage the functions of each kidney. Each nephron is composed of a glomerulus and a renal tubule.
Tracking fluid movement through the nephron and the formation of urine (3 steps):
Blood delivered to the nephron via the afferent (‘carrying toward’) arteriole and enters the glomerulus where the blood is filtered (step 1). The glomerulus is a leaky bundle of capillaries inside the Glomerular capsule (a.k.a. Bowman’s capsule) – the entrance to the nephron that receives the plasma (filtrate) from the glomerulus. The filtrate (about 20% of the blood plasma) then travels through the proximal convoluted tubule (PCT) in the nephron. Critical blood components like water, glucose, and electrolytes are passed through the proximal tubule and back into the blood stream (peritubular capillaries) in a process called reabsorption (step 2). Most nephrons are cortical nephrons found completely within the cortex of the kidneys; however, to remove even more valuable water and electrolytes, the filtrate of juxtamedullary nephrons passes through the loop of Henle. The loop of Henle is in the very salty medulla of the kidney. This salty tissue helps drive water out of the nephron and active pumps push salts out. From the loop of Henle, filtrate moves into the distal tubule and into a collecting duct that empties to the ureter.
As filtrate travels through the nephron, peritubular capillaries secrete toxins (step 3) like H+, K+, and urea (as well as other waste products) that are taken up by the nephron and carried away in the urine. The secretion of H+ helps maintain proper pH levels as well.
In this manner, 2 normally functioning kidneys can process ¼ of your entire blood supply each minute!!! The resulting urine is normally a mix of Na+ and K+ ions, nitrogen containing waste (urea, uric acid, creatinine, ammonia), and bicarbonate ions. Urinalysis: urine contains traces of many substances which can be used to detect drugs as well as diagnose illness and disease. Some examples: glucose in urine might be diabetes; white blood cells indicate urinary tract infection; red blood cells can indicate infection, cancer, or injury; high protein levels may indicate kidney disease or high blood pressure.
In 24 hours, 1 – 1.8 liters of urine are formed and represents only about 1% of the total volume that flows through the renal tubules. In other words, 99% of what leaves the blood stream in the kidneys is returned to the blood.
Ureter (2 of these) – drain the kidneys of urine and deliver it to the bladder. Inside the kidneys, collecting ducts receive urine from multiple nephrons; multiple collecting tubules flow into calyces; calyces empty into the renal pelvis = beginning of ureters.
Urinary bladder – stores up to 1 liter of urine, expands greatly.
Urethra – empties urine from the bladder to outside the body. The urethra of females is relatively short and close to the anus. As a result, urinary tract infections (UTIs) are more common in females; usually caused by bacterial infections; can lead to inflammation of the bladder or even worse, infection can spread to the kidney. Male urethras are much longer and function as both a pathway for urine and semen, so it is considered part of the reproductive system and urinary system in males.
Urination is a reflex response. As bladder fills, tension on the walls stimulates the internal urethral sphincter to relax and you feel like going. Thankfully, you have control over an external urethral sphincter that allows us to exercise control over actual urination.
Water balance is controlled by hormone signals from the brain and the adrenal glands. Osmoreceptors in the hypothalamus detect water concentration in the blood. If it is too low, thirst sensations are triggered and ADH (anti-diuretic hormone) is released from the posterior pituitary. ADH targets the kidneys and results in reabsorption of more water than usual. A second pathway involves the detection of low blood pressure by sensors in large blood vessels or low levels of filtrate passing through the kidneys (both indicate low blood volume). Through a series of chemical reactions, aldosterone is released from the cortex of the adrenal glands. Aldosterone also targets the kidneys and promotes an increase in water reabsorption as well as an increase in Na+ reabsorption. Both pathways conserve water. Kidneys automatically generate dilute urine; hormones only act to concentrate it in times of water stress. High salt concentrations in the blood also reduce the amount of saliva in your mouth. A dry mouth stimulates nerves that signal the brain to make you ‘thirsty’.
The pH of blood is also under the longterm control of the kidneys. Buffers (chemicals that prevent large changes in pH) are managed by the kidneys. For example, bicarbonate (HCO3- , the way CO2 travels in the blood) is a weak base that is in constant equilibrium with carbonic acid (H2CO3): HCO3- + H+ « H2CO3. If the blood becomes too acidic, bicarbonate removes H+ by becoming carbonic acid and vice versa. Depending on the pH of the blood, kidneys will remove H+ or bicarbonate as needed. As a result, urine can vary greatly with pH (4.5 to 8.0). Much of this depends on diet: high protein diet = acidic urine; vegetarian diet = alkaline urine. The end result is a blood pH that stays within the critical range of 7.35 – 7.45!!