Renal and Urinary Learning Objectives

Outline the major structures (and their functions) of the urinary system. What are the major functions of the urinary system? Describe the gross features of the kidney.

The urinary system consists of a pair of kidneys located in the retroperitoneal space, serving as dynamic filters of the blood, ureters which drain urine from the kidneys to the urinary bladder where urine is stored, and the urethra which releases urine to the external environment. 

Each kidney has a superior and an inferior pole, a convex lateral border, and a concave medial border, which hosts the hilum. The hilum is the region of the kidney where neurovasculature (e.g. renal artery and sometimes accessory renal a., renal v., and the renal plexus) and renal pelvis are conducted between the parenchyma and the ureter. 

The parenchyma of the kidney is organized into two tissues: cortex and medulla.

The medulla is cone-shaped, with the apex of the cone pointed toward the hilum. When viewed in sections, the parts of the medulla are called pyramids, as a cone section would resemble a pyramid. The apex of the pyramid is the renal papilla, which is the point of no return physiologically between renal filtrate (proximally) and urine (distally).

Cortex surrounds the pyramids with renal columns on each side of the pyramid and cortical arches superficial to the bases of the pyramids. 

Within the parenchyma of each kidney are millions of functional units called nephrons. Nephrons (and surrounding cells) work with surrounding vasculature for three major functions of the urinary system:

• Maintenance of extracellular volume & osmolarity

  • Concentrations of ions: Na+, K+, Ca2+, Cl-, & HPO42-

  • pH: secretes H+, conserves HCO3-

  • Volume: H2O

  • Concentration of blood Glucose: glutamine  gluconeogenesis, or excrete

• Endocrine:

  • Calcitriol: increase blood Ca2+ concentrations (PCT)

  • Erythropoietin: erythrocyte production (peritub.)

  • Renin: increase blood pressure (juxtaglomerular apparatus)

• Excretion: 

  • Catabolic wastes, bioactive substances

Describe the relative locations and surroundings of the kidney in the retroperitoneal space.

The kidneys are located within the perirenal space of the retroperitoneum. 

Retroperitoneal Space

The peritoneum is a serous membrane lining the abdominopelvic cavity, consisting of two layers: a parietal peritoneum that lines the walls of the cavity, and a visceral peritoneum which covers the viscera suspended within the cavity. The retroperitoneal space is the region posterior to the parietal peritoneum and anterior to the posterior wall of the abdomen and vertebral column. Structures found within the retroperitoneal space are said to be 'retroperitoneal.'

The kidneys (as well as their associated suprarenal glands and ureters) are retroperitoneal. Each kidney is found within the ipsilateral perirenal space, a space surrounded by (peri)renal fascia and also filled with perirenal adipose tissue (fat). (Peri)renal fascia is often called Gerota’s fascia. There is much variation of the perirenal space, but it is best conceptualized as an inverted cone, with a superior base and an inferior apex, which tapers along the course of the ureter. Surrounding a perirenal space are the ipsilateral pararenal spaces. The anterior pararenal space is found between the parietal peritoneum and the peri(renal) fascia. The posterior pararenal space is found between the peri(renal) fascia and the investing fascia of posterior and lateral muscles (e.g. quadratus lumborum m., internal abdominal oblique m., transversus abdominis m., etc.). The pararenal spaces are typically filled with pararenal adipose tissue (fat).

Relative Spatial Relationships

Kidneys are located high in the abdomen on the posterior wall inferior to the diaphragm in the region of the lower ribs. The left kidney is located slightly higher than the right kidney – roughly from T11-L2 vertebral level, while the right kidney spans T12-L3 vertebral levels. The superior 1/3 of the posterior surface of the kidney lies against the diaphragm. The remaining 2/3 is inferior to the diaphragm, and is adjacent to the quadratus lumborum muscle. The subcostal nerve (T12) and vessels run posterior to the kidneys. And the 11th and 12th ribs run behind the upper portion of both kidneys. At rest, the hila of the kidneys coincide with the transpyloric plane, the midpoint between the jugular notch and the pubic symphysis, which corresponds to where the pyloric part of the stomach meets the duodenum (approximately at the L1 vertebral level).

The anterior relationships are different on each side, except that both kidneys are capped superomedially by a suprarenal (adrenal) gland. The right kidney contacts the descending (2nd) portion of the duodenum, the right lobe of the liver, the right colic (hepatic) flexure, and some coils of jejunum. The left kidney contacts the pancreas, the stomach, the spleen, the left colic (splenic) flexure, and some coils of jejunum.

The medial border of the kidney contains the hilum – the space into which the renal arteries and veins pass and the renal pelvis and ureter emerge. The kidneys do not sit with their borders and surfaces in a strict anatomical position. For example, the medial borders of the kidneys are oriented anteromedially, and the lateral borders are oriented posterolaterally.

Describe the proximal pathway of urine conduction, from the renal papillae to the ureters.

Distal convoluted tubules of nephrons → collecting ducts → Minor calices (calyces) @ renal papillae* → Major calices* → Renal pelvis* → Ureter* → Urinary bladder* → Urethra*               [*Grossly visible]

Minor calices 

• There are typically 3-4 minor calices for each major calyx.

• Each minor calyx surrounds a renal papilla, an indentation of the apex of the renal pyramid, where urine is excreted.

  • This is where renal secretion & absorption concludes and urine is complete

Major calices

• There are typically 2-3 major calices in a kidney, which receive urine from the minor calices, and drain into the renal pelvis.

Renal pelvis

• The renal pelvis is the superior expansion of the ureter which receives urine from the major calices.

Describe the flow of blood to and through the kidney. Understand left renal vein entrapment syndrome and the relationships of the vessels that contribute to this susceptibility.

Each kidney is supplied with blood from a renal artery which branches directly from the abdominal aorta, slightly inferior to the origin of the superior mesenteric artery. Renal arteries travel relatively posterior to the ipsilateral renal vein. Renal veins drain directly into the IVC. Because the aorta runs slightly to the left of midline, and the IVC is significantly right of midline, the arteries and veins have different lengths depending on if they are associated with the right or left kidney. Notably, the left renal v. is relatively long.

Another functional way of subdividing a kidney is by renal lobes. Renal lobes consist of a single renal pyramid, the cortical arch associated with that pyramid, and portions of the cortical columns (containing the nephrons) associated with that pyramid. Renal lobes may be subdivided into renal lobules. A renal lobule is a collection of cortex containing all the nephrons associated with a single medullary ray and a single collecting duct.

Each renal artery typically hosts five segmental arteries. Segmental arteries give rise to interlobar aa., which transition into arcuate aa. as the vessels move from the lateral surfaces of renal pyramids to the bases of renal pyramids. Both interlobar and arcuate aa. give rise to cortical radiate aa. (interlobular aa.), and cortical radiate aa. give rise to afferent arterioles, which feed glomeruli. Glomeruli are drained by efferent arterioles, which feed into peritubular capillaries and vasa recta. Because efferent arterioles are vessels connecting two capillary beds (glomerulus to the peritubular capillaries), we refer to those systems as the renal portal system.

The veins follow a similar pattern, with multiple tributaries coming out of the hilum of the kidney to coalesce to form the renal vein.

As the left renal v. is approximately 3x longer than the right renal v., a variety of veins draining structures on the left side of the body frequently drain into the left renal v. These include the:

• L. inferior phrenic v.

• L. suprarenal v.

• L. gonadal v.

The left renal v. occupies a narrow space, posterior to the superior mesenteric a. (SMA) and anterior to the abdominal aorta. An aneurysm in either artery (typically in the wall of the aorta) may compress the left renal v., jeopardizing the return of blood from the left kidney, suprarenal gland, gonad, and inferior hemidiaphragm. This is known as left renal vein entrapment syndrome (aka nutcracker syndrome).

To summarize:

Discuss extrinsic control of renal function. Map the autonomic innervation of the kidneys.

Most control of the kidney is intrinsic (coming from hormones within the tissues of the kidney itself), but there are two sources of extrinsic control:

• Hormones

• Autonomic nervous system

  • Renal plexus

    • Found in association with renal a., and fibers follow arterial supply

    • Most significant function is sympathetic

      • Vasoconstriction

      • Juxtaglomerular apparatus stimulation (renin)

The renal plexuses:

• Are mixes autonomic plexuses

• Have small ganglia at the origins of the renal aa.

• Are Predominantly sympathetic in composition

  • Stimulation leads to:

    • Vasoconstriction

    • Renin secretion (RAA pathway) → increased blood pressure

• Have direct inputs from:

  • Celiac ganglia & plexus (mixed)

  • Aorticorenal ganglion (mixed)

  • Least splanchnic n. (sympathetic)

  • 1st Lumbar splanchnic n. (sympathetic)

  • Intermesenteric plexus (mixed)

What is a nephron? Describe the structures and functions of a nephron. Describe the structure of a renal corpuscle. Describe glomerular filtration as a process. How does a nephron regulate water balance? What hormones work on the kidneys to regulate water balance and Ca2+ homeostasis? How do the PCT & DCT work in the reabsorption and secretion of materials? Describe how the juxtaglomerular apparatus controls renal blood flow and GFR. What are diuretics? How do they work? What are the major classifications of nephrons, and what features distinguish them?

Nephrons

The nephron is the fundamental functional unit of the kidney, consisting of:

• Renal corpuscle

  • Glomerulus

    • Specialized capillary to convert blood to filtrate

    • Supplied by afferent arteriole

    • Drained by efferent arteriole to peritubular capillaries (renal portal system)  

  • Glomerular capsule

    • Receives filtrate, conducts to tubules

• Renal tubules

  • Proximal convoluted tubule (PCT)

  • Nephron loop (of Henle)

  • Distal convoluted tubule (DCT)

Renal tubules are surrounded by peritubular capillaries and there are many processes of tubular secretion (moving materials from the blood to the tubules) and tubular reabsorption (moving materials from the tubules to the blood).

Renal Corpuscle

The renal corpuscle consists of the glomerulus and the surrounding glomerular capsule. The glomerulus is a modified capillary bed surrounded by a specialized layer of cells called podocytes, which can help regulate the amount of material filtered. 

As blood moves through the glomerulus, the process of glomerular filtration produces filtrate, which is captured by the capsule and moved distally into the proximal convoluted tubule (PCT). The rate at which filtrate is produced is called glomerular filtration rate (GFR), which is typically proportional to blood pressure. Branches of the renal plexus innervate the smooth muscle of afferent arteriole, thus controlling GFR. 

Thinking about the capsule as a serous membrane, the capsule is analogous to the parietal component of the membrane, whereas the podocytes are analogous to the visceral component of the membrane. Supporting the podocytes and the glomerulus are mesangial cells, which are smooth muscle and help to control GFR by relaxing or contracting.

Proximal Convoluted Tubule (PCT)

The PCT receives filtrate from the capsule and has two visually distinguishable portions, the pars convoluta and the pars recta. Throughout both parts, processes of tubular secretion and tubular reabsorption occur. The PCT is THE PRINCIPAL SITE OF TUBULAR REABSORPTION, especially for substances such as water, glucose, and a number of other molecules and ions.

Distal Convoluted Tubule (DCT) & Collecting Ducts

Filtrate moves from the nephron loop into the DCT, where the processes of tubular secretion and reabsorption continue. 

Of particular consideration are processes facilitated by two types of cells:

• Intercalated cells

  • Found in both the DCT and collecting ducts

  • Responsible for regulation of pH

    • H+ and HCO3- movement

• Principal cells

  • Found in both the DCT and collecting ducts

  • Reabsorption of water in response to aldosterone (RAA system)

  • Reabsorption of water in response to vasopressin (antidiuretic hormone/ADH)

Reabsorption of calcium in response to parathyroid hormone (PTH)

Juxtaglomerular Apparatus (JGA)

While the lumen of DCTs are very distal to the renal corpuscle, the DCTs cross adjacent to corpuscles on their way to collecting ducts. This relationship is the basis for intrinsic control of renal function: the juxtaglomerular apparatus (JGA). 

The portion of the wall of the DCT (and nephron loop) which contacts the corpuscle is known as the macula densa, a densely packed cluster of epithelial cells. These cells are sensitive to the concentration of salts (e.g. NaCl), and a decrease in NaCl concentration (which would signal a low GFR) in the filtrate causes two important changes that affect blood flow:

1. the relaxation of smooth muscle of the afferent arteriole (vasodilation), which increases GFR, and 

2. the secretion of renin from juxtaglomerular cells, which initiates the RAA pathway, increasing blood pressure and GFR.

An increase in NaCL concentration (a high GFR) would have the opposite effects (vasoconstriction of the afferent arteriole and inhibition of renin secretion).

Diuretics

Diuretics are substances which increase the formation of urine. The process of diuresis may be enhanced by anything which: increases tubular secretion of water, decreases tubular reabsorption of water, or increases GFR. Diuretics are commonly prescribed for conditions such as hypertension (high blood pressure) to slightly reduce blood volume (and thus pressure) via diuresis. 

Two commonly used (and perhaps abused) freely available diuretics are caffeine and alcohol. Caffeine enhances diuresis by increasing GFR. Alcohol enhances diuresis by inhibiting vasopressin/ADH production, which thus slows tubular reabsorption of water in the DCT and collecting ducts.

Nephron Types

There are two types of nephrons:

• Cortical nephrons

  • Most common and ubiquitous

  • Corpuscles high in the cortex

  • Lower glomerular filtration rate (GFR)

    • GFR = rate of conversion of blood to filtrate

      • Positively correlated to blood pressure

  • Short nephron loops

• Juxtaglomerular nephrons

  • Less common

  • Corpuscles close to the medulla

  • Higher GFR

  • Longer nephron loops

    • Vasa recta (straight vessels)

      • Reclaim more water

      • More concentrated urine

Describe the structures and processes involved in the movement and storage of urine, in particular the ureters, urinary bladder, and urethra. Understand the autonomic innervation and blood flow to the urinary bladder. Describe the features of the female and male bladder and urethra.

Ureters

Ureters are continuations of renal pelves that conduct urine to the urinary bladder.

The ureters are retroperitoneal and found mostly within the perirenal spaces. Ureters are tripartite, consisting of:

• Abdominal part

  • Perirenal space

  • Anterior to psoas major m.

• Pelvic part

  • Begins at pelvic brim, adjacent to bifurcation of common iliac a.

• Intramural part

  • Wall of urinary bladder

  • Very short

Urinary bladder

The urinary bladder is a distensible, urine-storing, subperitoneal organ.

The apex of the urinary bladder is supero-posterior to the superior portion of the pubic symphysis.

Inferior from the apex is the anterior surface of the bladder, which lies adjacent to the pubic symphysis.

Posterior to the apex is the superior surface of the bladder. The superior surface is covered by peritoneum.

The base/fundus of the bladder is the posterior-most wall (opposite the apex). It may be closely associated with either the vagina (women), or the rectum (men).

The wall of the base contains the intramural parts of the ureters, which open into the lumen of the bladder via ureteric orifices.

The neck is the inferior-most portion of the bladder, and contains the internal urethral meatus. In males, the neck of the bladder is superiorly adjacent to the prostate gland, and in females, the neck is very closely associated with vagina. The neck is held in place anteriorly and anterolaterally by either the pubovesical ligament or puboprostatic ligament.

The triangular space defined by the ureteric orifices and the internal urethral meatus is called the trigone.

Urethra

The urinary bladder voids urine to the external environment through the urethra. The urethra is substantially sexually polymorphic.

The typical assigned female at birth (AFAB) urethra begins at the internal urethral meatus, travels anteroinferiorly between the pubic symphysis and vagina, and terminates in the external urethral meatus. The external urethral meatus is typically found in the vestibule of the vagina between the glans clitoris and the opening of the vagina.

The typical assigned male at birth (AMAB) urethra is multipartite, its parts including:

• Intramural (preprostatic) part: neck of bladder to prostate

• Prostatic part: begins at the internal urethral meatus, traverses the prostate gland, where it receives secretions of the prostate as well as from the ejaculatory ducts (this is the point of entry of semen into the urethra)

• Membranous part: traverses the pelvic diaphragm

Spongy/penile part: traverses the corpus spongiosum of the penis; ends in the external urethral meatus on the distal ventral portion of the glans penis.

The urinary bladder is served by the superior and inferior vesical aa. of the internal iliac (aka hypogastric) a. Sex is on a continuum, arterial branches are named for their pelvic targets (eg the uterine a. supplies the uterus, vesical aa. supply the bladder, etc.).

The urinary bladder is innervated by the vesical plexus, a mixed autonomic plexus supplied by anterior brs. of the inferior hypogastric plexus. The inferior hypogastric plexus has the major inputs including:

• Pelvic splanchnic nn.,

  • Parasymp. fibers from S2-4 

• Sacral splanchnic nn., &

  • Sacral sympathetic ganglia (S1-2)

• Hypogastric nn. (mixed).

  • Superior hypogastric plexus (mixed)