Urinary system or renal system is the principal system in the human excretory system without which the body cannot excrete the wastes. Urinary system is also known as renal system compassing 2 kidneys, 2 ureters, bladder and urethra. Urinary system along with other system performs many roles however, excretion of the waste products and the regulation ofblood pressureare the primary functions of the renal system. Starting from the kidneys (lumbar region), the ureters, bladder and urethra pointing downwards to reach till the urethra. Urinary system produces urine -a material rich in nitrogenous wastes, ammonia and uric acid. A healthy adult eliminates approximately 800 mL to 2,000 mL per day. Together, the urinary system, skin and respiratory system help in the removal of all the wastes from our body. In addition to the bodily wastes, some of the drugs, alcohol, toxins and other inorganic material is also being excreted out of the system through various parts of the urinary system.
Embryonic development of the urinary system
Urinary system begin to develop as early as the first trimester of the pregnancy. Urinary system is closely linked with genital or reproductive system. Mesoderm is the region from which the urinary and reproductive organs are developed. Mesoderm is branched out to result in the pronephros, the mesonephros and the metanephros of the kidney along with the Wolffian and Müllerian ducts that form the sex organ. Anatomically the renal system is divided into kidneys, ureters, bladder and urethra.
Anatomy and physiology of kidneys
Kidneys are the bean-shaped organs found in a pair. They are anatomically located in the lumbar region( lower back) of the spinal cord. On top of each kidney, there is adrenal gland or suprarenal gland help in secreting adrenaline hormone. Nephrons are the structural and functional units of kidneys. Approximately there are 1 million nephrons in each kidney. Each kidney is measured around 4 or 5 inches long with a slight difference between the left and the right kidney. Each kidney has 3 major regions, they are, Renal cortex, Renal medulla and Renal pelvis. The renal cortex is the space between the inner medulla and the outer capsule. Most of the nephron parts are embedded within the renal medulla helping to filter the fluid entering into the kidneys through renal arterial supply. Erythropoietin is a hormone secreted by the renal cortex. Erythropoietin aid in the RBC production. The renal pelvis connects the kidney with the circulatory system and the nervous systems. The medulla is the inner region of the renal parenchyma. Renal medulla is made up of many pyramidal tissues called the renal pyramids. Renal medulla has a dense network of nephrons to provide the triangular pattern in the kidneys. The renal pelvis has the hilum –a concave part where blood vessels and nerves enter in and exit out of the kidney.
Urinary system - image from wiki commons
Anatomy and physiology of Nephrons
A nephron is the basic structural and functional unit of the kidneys assist in the formation of urine. Urine formation takes place through a series of secretory, absorptive and excretory processes. Kidney filters the blood that comes through afferent arterioles, thereby help in regulating the water and electrolyte balance. Nephrons also play a vital role in the homeostasis (balance) of blood volume, blood pressure, and plasma osmolarity. Renal functions are regulated by antidiuretic hormone, aldosterone, and parathyroid hormones. Each nephron has a glomerulus, Bowman`s capsule, proximal convoluted tubule (PCT), loop of Henle and the distal convoluted tubule (DCT). Parts of the nephron are explained as below.
Structure of Nephron- Image from Wiki Commons
1. The glomerulus
It is a bunch of capillaries receiving the blood supply through an afferent arteriole. The filtered blood from the glomerulus is transferred into a space called Bowman’s capsule. Glomerulus has specialized cells known as juxtaglomerular apparatus (JGA). JGA secrete the enzyme called renin. Renin helps in regulation of blood pressure.
2. The Bowman’s capsule
It is composed of 2 types of epithelium, the inner visceral layer and the outer layer parietal layer. Inner layers are made up of simple squamous epithelial cells and the outer parietal layer has simple squamous epithelial cells. The Bowman`s capsule help in filtering the Red blood cells and large proteins ( serum albumins) entering into the bladder. RBCs and proteins are filtered as they are larger molecules that cannot pass through the capsule.
3. Proximal Convoluted Tubule( PCT)
PCT is the site where most of the water and salt is reabsorbed. The reabsorption of the solutes is the result of the combination of many factors such as pressure gradient, selective reabsorption of the tubules, osmosis and the active mineral transport( of Na+/K+/ATPase pumps) initiated by the renal system. PCT has approximately 2/3rds of the total composition of water and 100% of the glucose are reabsorbed. The filtrate from here moves into the loop of Henle.
4. The Loop of Henle or Henle`s loop
It is a U-shaped tube consisting of a descending limb, a loop and a straight ascending limb. It acts as a junction between the proximal and the distal convoluted tubule. The filtrate that comes from PCT is received into the U-shaped loop highly permeable to water but completely impermeable to essential ions. This helps in the reabsorption of most of the essential minerals into the body. The fluid osmolarity in the loop goes up to 1200 mOSm/L. On the other hand, the ascending limb of Henle’s loop is highly impermeable to water but highly permeable to ions causing a sudden drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L. Such a difference is helpful in the quick transport of the urine into the bladder.
5. Distal Convoluted Tubule and Collecting Duct
DCT is the final segment of reabsorption in the nephron. DCT is generally impermeable to water and permeable to ions. The filtrate from DCT reaches the collecting duct. Collecting ducts will redirect the filtrate into the tips of ureters that enter the bladder through the posterior region.
Blood supply and Nerves supply to the Kidneys
The kidneys receive blood supply from the renal arteries – branched out from the aorta. The renal artery is divided into right and left branches to enter into their respective right and left kidneys. Similarly, renal veins drain the venous blood. From the smaller renal veins, blood is collected towards major renal vein. From thereon, venous blood moves into the abdominal veins to reach the superior vena cava to drain into the right atrium. The kidneys are supplied by both the parasympathetic and sympathetic nerves. Renal nerves help in the contraction and relaxation of the bladder as well as closing and opening of the sphincter muscles.
Anatomy and physiology of ureters
The ureters are the channel of communication that drain the urine from the kidneys to the bladder. The 2 ureters enter into the left and right side of the bladder from behind. (i,e superior and posterior region of the bladder). The ureters run retroperitoneally along the roof of the abdominal cavity towards the bladder-a balloon-like sac. The ureters are the hollow muscular tubes made up of smooth muscle fibres. Smooth muscles help in the peristaltic movement of ureters causing movement of urine. In a normal adult, the ureters measure about 25–30 cm long and 3–4 mm in diameter. The ureter is lined by urothelium, a type of transitional epithelium.
Anatomy and physiology of the urinary bladder
The urinary bladder is a hollow muscular organ work as a temporary reservoir of urine. Bladder is smooth, distendable and has a highly flexible sheet of cells. The typical bladder can hold approximately 300 to 500 mL of urine. A bladder has many segments like fundus, body, an apex and a neck. The detrusor muscle is a layer found in the urinary bladder wall. Detrusor muscles are made up of smooth muscle fibres arranged in spiral, longitudinal, and circular fashion causing an easy contraction and relaxation of the bladder.
Anatomy and physiology of urethra
The urethra is a single tube that drains the urine out of the urinary bladder. The urethra has a urinary meatus from which the bladder will eject the urine. Urethra has a very rich nerve supply to respond when the bladder is full. When the bladder is full, it will compress the nerves found in the bladder region. The compressed bladder sends the signals to the brain to open the bladder. Male and female urethra are anatomically distinguished as the male urethra is longer than the female's. The 2 sphincter muscles act as a valve between the bladder and ureters. The external urethral sphincters are made up of striated and the internal sphincter is formed by the involuntary smooth muscles. The tip of the urethra has an opening that helps in urination.
Microanatomy of the urinary system
Under the ultra-microscope, we can see a thin and unique lining of cells called the urothelium. It is a type of transitional epithelium. Transitional epithelium has the ability to get flatten and distend as and when required. Urothelium covers most of the urinary system, including the renal pelvis, ureters, and bladder.