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Nephroptosis is the inferior displacement (dropping) of the kidney, usually as a result of the adipose capsule not maintaining appropriate placement of the kidney, which is common in those with very low body fat percentage. Occasionally, nephroptosis can occur as a result of severe trauma. Regardless of the cause, displacement of the kidney can cause the ureter to kink, resulting in a backup of the flow of urine. This results in additional pressure on the kidneys which can ultimately cause damage to the tissues surrounding the kidneys.

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Paired, bean-shaped organs located superior to the waist, with right kidney sitting slightly lower than the left. The kidneys are partially protected by the eleventh and twelfth ribs.

The kidneys are responsible for filtering the blood, with most of the water and solutes being returned to the bloodstream. The remaining contents compose urine and pass through the ureters, is stored in the urinary bladder and exits the body through the urethra.

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The renal fascia is a layer of thin, dense, irregular connective tissue which secures the kidney to surrounding structures and the abdominal wall.

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A layer of fatty tissue which helps to secure the kidneys in place and protects from trauma

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The renal capsule is a smooth layer of dense, irregular connective tissue continuous with the adventitia of the ureter, which helps to maintain the shape of the kidney and provide protection. (1)

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The renal cortex is the most superficial portion of the kidney,

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The renal medulla is the deepest portion of the kidney and consists of several renal pyramids and columns.

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Several renal pyramids form the renal medulla with the base of each renal pyramid facing the renal cortex and the apex of each renal pyramid forming the renal papillae.

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Urine is passed from collecting duct into several hundred papillary ducts.

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Each minor calyx receives urine from one renal pyramid, which then flows to a major calyx.

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Urine is emptied into a major calyx from several minor calyces, before continuing into the renal pelvis.

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The renal column extends into the renal medulla and separates the renal pyramids from each other.

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Urine is emptied into the renal pelvis from several major calyces. The urine is then transported from the renal pelvis, into the ureter.

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A renal artery enters each of the kidneys at the renal hilum, branches in several small vessels which deliver unfiltered blood to the kidney via the afferent arteriole.

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Filtered blood is removed from the kidney through the renal vein and delivered back to the heart via the inferior vena cava

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Nephrons are the functional unit of the kidney and are responsible for filtering blood to make urine which is then drained through renal papillae. A nephron is made up of the renal corpuscle and renal tubule. It's long, winding shape increases the surface area for reabsorption of nutrients. (4)

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The renal corpuscle consists of the glomerulus and the glomerular capsule.

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The renal tubules consist of the proximal convoluted tubule, the nephron loop and the distal convoluted tubule and is responsible for tubular reabsorption and tubular secretion.

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The adventitia is the outermost layer of the ureter consisting of areolar connective tissue which secures the ureters in place. Within the adventitia, blood vessels and nerves can be found.

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The muscularis is the middle later of the ureter formed by smooth muscle, with an inner layer of longitudinal muscle and and outer layer of circular muscle. To facilitate the strong waves of peristalsis occurring within the ureters, the distal third of these organs also contain an additional layer of longitudinal muscle.

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The mucosa is the deepest layer of the ureter formed by transitional epithelium. Within the mucosa are goblet cells which secrete mucus that acts as a protective barrier, preventing the urine from touching the mucosa cells.

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The ureters are paired organs characterised as narrow tubes with thick walls varying in diameter along their course. The ureters transport urine from each kidney to the urinary bladder via gravity and peristalsis.

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The glomerulus is a cluster of capillaries which acts as a microscopic filter (1) facilitating filtration of the blood through the presence of large pores which allow water, ions and other small molecules to pass into the glomerular capsule.

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The glomerular capsule surrounds the glomerulus and collects the contents filtered out of the blood. This contents is then transported to the proximal convoluted tubule.

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The afferent arteriole is surrounded by a filtration membrane and receives unfiltered blood from the cortical radiate arteries before feeding it into the glomerulus. Due to the larger diameter of the afferent arteriole, compared to that of the efferent arteriole, blood enters the glomerulus quicker than it exits, resulting in high pressure in the glomerular capillary bed.

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The efferent arteriole is surrounded by a filtration membrane and is responsible for transporting filtered blood from the glomerulus, to the peritubular capillaries.

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The proximal convoluted tubule receives filtrate from the glomerular capsule. It's long, curly shape, microvilli and mitochondria aid in the function of reabsorption and secretion of water and a number of solutes.

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The nephron loop consists of the ascending limb and the descending limb and differs in thickness along it's course. The thin portion of the nephron loop consists of simple squamous epithelium, enabling diffusion. The thick portion contains many mitochondria and is formed by columnar epithelium, allowing secretion and reabsorption, as well as facilitating active transport of ions.

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The distal convoluted tubule sits in the renal cortex and is formed by simple cuboidal epithelium with very little microvilli allowing for more secretion to occur, and less absorption. The distal convoluted tubule is considered the final part of a nephron and pass filtrate into the collecting duct.

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The collecting duct extends from the renal cortex to the renal medulla and is responsible for receiving filtration from several distal convoluted tubules and transporting this to the renal papillae. The collecting duct maintains electrolytes and control reabsorption of water through the presence of epithelial cells with microvilli.

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The descending limb is the portion of the nephron loop that travels down, further into the renal medulla. It is permeable to water, allowing water to exit, and impermeable to solutes, ensuring they remain within.

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The ascending limb continues from the descending limb and travels back up towards the renal cortex. It is impermeable to water, and permeable to solutes, using active transport to pump sodium, potassium and chloride out of the renal tubule, into the renal medulla.

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The nervous system and urinary system are interconnected in regulating the glomerular filtration rate (GFR) through the vasoconstriction if the afferent arteriole which decreases the GFR

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The urinary bladder is a hollow organ located within the pelvic cavity, posterior to the pelvis synthesis and is responsible for receiving urine from the ureters and storing this until an appropriate to urinate. When empty, the bladder folds in on itself and is much smaller, however when it fills with urine, it is able to expand, due to the presence of rugae, and is spherical in shape.

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The urethra is a small, hollow tube leading from the base of the urinary bladder to the exterior of the body and is responsible for expelling urine. In males, the urethra also transports semen.

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Clinical Connection 2: Interstitial Cystitis

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Clinical Connection 2: Interstitial Cystitis

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Interstitial cystitis is inflammation and irritation of the urinary bladder. When the walls of the bladder are inflamed, the signals sent to the brain by the stretch receptors become confused, often leading to chronic pelvic pain, frequent urination of small amounts and/or pain when the bladder is filling. This condition can also cause the bladder wall to stiffen, limiting the amount of urine the bladder is able to store. The direct cause of interstitial cystitis is unknown, however there are many causes that are suspected to contribute including infection, hereditary conditions, and a defect in the epithelium of the bladder, allowing toxic substances within the urine to come in contact with the bladder wall, causing irritation (2). Treatment of this condition can be difficult to get right and occasionally takes a culmination of treatment options to be effective. These include oral medications, physical therapy, nerve treatment or surgery (3).

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The mucosa is the deepest layer of the urinary bladder formed by transitional epithelium. Within the mucosa are goblet cells which secrete mucus that acts as a protective barrier, preventing the urine from touching the mucosa cells. In addition, the mucosa also contains rugae, enabling the bladder to expand as it fills with urine.

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The muscularis is the middle later of the urinary bladder formed by smooth muscle, with an inner layer of longitudinal muscle, a middle layer of circular muscle, and an outer layer of longitudinal muscle.

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The endocrine system and urinary system are interconnected in regulating blood osmolarity through the effect of antidiuretic hormone on principal cells within the collecting ducts and distal convoluted tubules.

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The posterior pituitary gland releases ADH in response to an increase in blood osmolarity.

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ADH acts on the principal cells in the distal convoluted tubule and the collecting duct by increasing permeability to water. ADH controls the concentration of urine being produced.

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ADH acts on the aquaporin storage vesicle cells within the principal cells of the distal convoluted tubule and collecting duct, to insert the aquaporins into the cell membrane. This allows water to pass through the principal cells, through the interstitial fluid and back into the bloodstream. in doing so, ADH increases the cell's permeability to water, which increases water reabsorption, which ultimately results in more water being retained in the blood and less being excreted through urine. (1)

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The adventitia is present at the posterior and inferior surface of the urinary bladder, consisting of areolar connective tissue which secures the organ in place. Within the adventitia, blood vessels and nerves can be found. The adventitia of the urinary bladder is continuous with the adventitia of the ureters.

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The serosa is present at the superior portion of the bladder and is a layer of peritoneum, which is the internal lining of the abdomen and pelvis.

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The internal urinary sphincter is smooth muscle formed by circular muscle fibers and controls the involuntary passage of urine out of the bladder and into the urethra.

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The external urinary sphincter is skeletal muscle formed by circular muscle fibers and controls the voluntary passage of urine out of the bladder and into the urethra.

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Cells and Tissue: Epithelial Tissue in the Proximal Convoluted Tubule

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Cells and Tissue: Epithelial Tissue in the Proximal Convoluted Tubule

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Tissue Type: Epithelial Tissue

Epithelial tissue consists of both epithelial and connective tissue components and performs a variety of functions.

Cells that form this tissue: Simple cuboidal epithelial cells (1)

The presence of a microvilli (brush) border provides a much greater surface area for reabsorption of water and nutrients back into the bloodstream. (1)

Through the presence of copious intracellular mitochondria, ATP is generated to provide energy for absorptive and secretory functions of the proximal convoluted tubule. (1)

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In males, the urethra passes through the prostate to the perineum.

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The urethra then passes through the deep muscles of the perineum, to the penis.

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The urethra finally passes through the penis, where it opens to the exterior of the body where urine and semen are expelled.

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In females, the urethra is located directly posterior to public synthesis, with the opening between the clitoris and the vaginal opening where urine is expelled.

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Urine formation is the process of filtering blood through glomerular filtration, reabsorbing substances through tubular reabsorption, and the removal of waste products through tubular secretion.

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Glomerular filtration begins with blood arriving at the glomerulus via the afferent arteriole. Facilitated by blood pressure, the glomerulus filters blood plasma and dissolved substances into the glomerular capsule before the glomerular filtrate is removed via the efferent arteriole.

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Tubular reabsorption is the process of the reabsorption of substances into the blood through the peritubular capillaries. This reabsorption occurs in the collecting duct and renal tubules, primarily within the proximal convoluted tubule.

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Tubular secretion is the process of removing waste products from the blood via peritubular capillaries and transporting them into the renal tubules to be excreted from the body in urine.

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The peritubular capillaries wrap around sections of the renal tubules. These capillaries are responsible for reabsorbing substances from the tubules and back into the bloodstream, and secreting substances into the tubules. Numerous peritubular capillaries come together to form the cortical radiate veins.

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Glomerular filtration rate (GFR) is the amount of filtrate that the two kidneys form per minute. This filtration is caused by the pressure of the blood inside the glomerulus and is controlled by the following pressures.

1. Glomerular Blood Hydrostatic Pressure, which is the blood pressure inside the glomerular capillaries, forces substances through the filtration membrane, encouraging filtration.

2. Capsular Hydrostatic Pressure, which is the pressure applied by the fluid in the capsule, against the filtration membrane which opposes filtration.

3. Blood Colloid Osmotic Pressure is the pressure caused by water being pulled into glomerular capillaries via osmosis, due to the presence of proteins within the blood plasma which opposes filtration.

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(1) Chruscik A, Kauter K, Windus L, Whiteside E. Urinary System. In: Fundamentals of Anatomy and Physiology. Australian Edition. Place of publication not identified. University of Southern Queensland; 2021.

(2) Mayo Clinic Staff. Interstitial cystitis: Symptoms & causes. Mayo Clinic. September 29, 2021. Last update unknown. Accessed September 22, 2024. https://www.mayoclinic.org/diseases-conditions/interstitial-cystitis/symptoms-causes/syc-20354357

(3) Mayo Clinic Staff. Interstitial cystitis: Diagnosis & treatment. Mayo Clinic. September 29, 2021. Last update unknown. Accessed September 22, 2024.  https://www.mayoclinic.org/diseases-conditions/interstitial-cystitis/diagnosis-treatment/drc-20354362

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The micturition reflex is the process of releasing urine from the bladder and occurs via both involuntary and voluntary contraction of the internal and external sphincters respectively. This reflex is initiated upon the bladder volume exceeding normal range (200-400mls), which increases the pressure within the bladder. Stretch receptors within the bladder are stimulated by this pressure increase, and send nerve impulses to the spinal cord, which then travel to the micturition center in the pons and finally initiate the reflex. Once the reflex has been triggered, parasympathetic impulses instruct the muscularis of the bladder to contract and internal sphincter muscles to relax. Simultaneously, the micturition center in the pons impedes the motor neurons that direct skeletal muscle of the external sphincter, causing it to relax.

This reflex can be overridden due to the sensation of fulness felt when the bladder fills, leading to the conscious desire to urinate before the reflex occurs, and the body can learn to control this voluntarily.

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The glomerular filtration membrane surrounds the glomerulus and consists of 3 layers: fenestrated endothelium, basement membrane and podocyte. Together, these layers allow water and small substances to be filtrated and prevent the filtration of larger substances such as plasma proteins, blood cells and platelets.

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As the fenestrated endothelium is very porous, most solutes are able to pass through this barrier, with the exception of blood cells and platelets.

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The basement membrane consists of a negatively charged gel with collagen fibers and proteoglycans present, which prevents negatively charged ions from passing through.

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Podocytes have extensions called pedicles which surround the glomerular capillaries, however not completely, leaving gaps between each pedicle which are considered filtration slits. There is a thin membrane between each filtration slit which allows passage of very small substances.

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Regulation of Glomerular Filtration Rate (GFR)

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Regulation of Glomerular Filtration Rate (GFR)

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The sympathetic division of the autonomic nervous system facilitates regulation of the glomerular filtration rate (GFR) through the release of norepinephrine by sympathetic fibers supplying the kidneys. Norepinephrine signals the afferent arteriole to vasoconstrict resulting in less blood flow to the glomerular capillaries which ultimately decreases GFR. This is important in situations where sympathetic stimulation is increased, such as exercise or blood loss, as it allows for redirection of blood flow to tissues more in need.

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The kidneys are supplied by sympathetic fibers of the sympathetic nervous system.