Exploring the urinary system, and its relationship to the cardiovascular system.

The urinary system consists of four major components: kidneys, ureters, urinary bladder, and urethra.

Urinary system
Figure 1. Urinary system. Adapted from "Urinary system ver 2," by March i Nogué, 2010, retrieved from https://commons.wikimedia.org/wiki/File:Urinary_system_ver_2.svg. CC BY-SA 3.0.

Organ System and Location

The kidneys are two dark red organs shaped like kidney beans. The average kidney is about 12cm long, 6cm wide, and 4cm thick. The kidneys are located in the superior lumbar region, and lie in a retroperitoneal position against the dorsal body wall. That is approximately below the bottom of the rib cage. Note that the right kidney is slightly lower than the left.

The ureters are two slender tubes each about 25-30cm in length and 6cmm in diameter. Ureters run behind the peritoneum, connecting the renal hilum to the posterior of the urinary bladder.

The urinary bladder is a muscular sac that is both smooth and collapsible. The urinary bladder is located posterior to the pubic symphysis and retroperitoneally in the pelvis. The urethra is a thin tube that provides a path for urine to exit the body.

Urinary system
Figure 2. Kidney structure. Adapted from "KidneyStructures PioM," by Piotr Michał Jaworski, 2006, retrieved from https://commons.wikimedia.org/wiki/File:KidneyStructures_PioM.svg. CC BY-SA 3.0.

Kidney Physiology

The kidneys are the most important part of the urinary system, because they help cleanse the blood of toxins and maintain acid-base balances, releasing the remaining filtrate into the urine.

As the blood enters the kidneys through the renal veins, it separates out into segmental arteries, interlobar arteries, and then arcuate arteries. The blood then passes through the glomerulus of nephrons where the filtering process takes place by using the high pressure to force substances out of the blood. Substances needed in the blood are then reabsorbed from the filtrate. Additional substances are secreted into the remaining filtrate to help the body dispose of it and maintain pH balance.

The cleansed blood with its reabsorbed substances is sent back to the heart through a series of arcuate and interlobar veins. It exits the kidneys through the renal vein and enters the inferior vena cava. Collecting ducts in the renal pyramids collect all the urine produced by the nephrons after the filtering process, so that it can be transported to the renal pelvis.

The renal pelvis consists of many calyces, which are cup-shaped areas that collect urine from the pyramids. The urine is then all funneled towards the ureters as it exits the kidneys.

Histology

Nephrons are structural and functional units in the kidneys and produce urine. Each kidney contains over a million of them. Each nephron consists of a glomerulus, which is responsible for filtering the blood. Unlike any other capillary bed in the body, the glomerulus is both fed and drained by arterioles, leading to a much higher blood pressure than other capillary beds. Thousands of collecting ducts collect fluid from several nephrons.

The walls of the bladder are smooth muscles called the detrusor muscle, and have a special mucosa called the transitional epithelium.

Urinary system
Figure 3. Basic kidney. Adapted from "Urinary system ver 2," by March i Nogué, 2010, retrieved from https://commons.wikimedia.org/wiki/File:Urinary_system_ver_2.svg. CC BY-SA 3.0.

Interactions with Cardiovascular System

The kidneys are supplied with blood from the cardiovascular system through the renal artery. The blood vessels carry vital nutrients and oxygen that are needed by the kidneys.

Kidneys dispose of nitrogenous wastes, toxins, and other drugs found in the blood, while maintaining an acid-base and electrolyte balance. Filtered blood from the kidneys is then returned to the heart through the vena cava.

Around 1/4 of the total blood in your system passes through the kidneys every minute.

Homeostasis through Blood Pressure

Systemic blood pressure is necessary for glomerular filtration to take place. If blood pressure is low, the pressure in the glomerulus will not be sufficient to force substances out of blood, and filtration stops.

The kidneys help regulate blood pressure by altering blood volume. When blood pressure is too high, they release more water into urine to lower blood pressure. When blood pressure is too low, more water is retained in order to increase blood pressure.

Homeostasis through Reabsorption

The heart secrets atrial natriuretic peptide, which helps prevent reabsorption of sodium ions in the kidneys and subsequently causes more absorption of chloride and potassium ions. However, too little sodium remaining in the blood can cause the blood to be too diluted and may flow out into tissue, potentially shutting down the entire system.

If blood pressure is getting too low, however, the kidneys can also release renin, which results in the formation of angiotensin II, and stimulates the adrenal cortex to release aldosterone. Once the aldosterone makes its way back to the kidneys, it will temporarily increase sodium ion reabsorption in the kidneys.

Renal Failure

Renal failure occurs in two flavors. Chronic renal failure usually develops alongside other chronic conditions such as diabetes. In our society, diabetes mellitus and hypertension (or high blood pressure) are frequent causes of chronic renal failure, accounting for 44% and 28% of all cases in adults. Acute renal failure is usually caused by things like repeated kidney infections, physical trauma, or chemical poisoning and can sometimes be reversed through treatment.

Details

During renal failure, filtration in the kidneys slows or stops completely, preventing toxic wastes being removed from the blood. A damaged kidney will cause higher blood pressure, which recursively causes the kidneys to be further impaired by damaging blood vessels and inherently reducing blood circulation.

High blood pressure, pain while urinating, and swollen hands or feet may indicate there’s a problem with a kidney. A simple and cheap urinalysis test by a doctor can detect renal problems well in advance of any symptoms actually appearing.

Implications

Because the kidneys are unable to filter toxins from the blood, a process of hemodialysis is needed to cleanse the person’s blood using an artificial kidney apparatus. The process of hemodialysis can cause further complications with the patient, such as thrombosis, infection, ischemia, or hemorrhage.

In cases where the damage is not reversible, a kidney transplant is the only way to solve the problem. After a transplant, the patient requires immunosuppressants for the remainder of their life (unless the kidney comes from an identical twin) in order to prevent rejection of the transplanted kidney.

One out of every 15 Americans has some degree of kidney dysfunction, and nearly 4 percent require dialysis or a kidney transplant just to stay alive.
(Marieb, 2015)

Other Renal Diseases

  • Kidney stones (renal calculi) form when urine becomes extremely concentrated and solutes begin forming crystals.
  • Urinary tract infections can occur when bacteria enter the renal system, usually through the urethra due to poor toileting habits.
  • Hydronephrosis occurs when the protective fatty tissue around the kidneys decreases, causing one or both kidneys to drop to a lower position. Urine then backs up in the kidneys and exerts pressure because it can no longer pass through the ureters.

Jaworski, P. M. (2006, June 8). KidneyStructures PioM [Digital image]/ Retrieved from https://commons.wikimedia.org/wiki/File:KidneyStructures_PioM.svg. March i Nogué, J. (2010, July 10). Urinary system ver 2 [Digital image]. Retrieved from https://commons.wikimedia.org/wiki/File:Urinary_system_ver_2.svg. Marieb, E. N. (2015). Essentials of Human Anatomy & Physiology. (11th ed.). [Pearson eText version].