Equine Urinary System - Horse Anatomy

Kidneys

The kidneys of the horse are both enclosed in a fat capsule. Dorsally they rest against the psoas muscle and against the diaphragm. The left kidney is bean-shaped and the right kidney is heart-shaped. The right kidney lies cranial to the left kidney.

The right kidney is to be found ventrally to, and between, the last 2 ribs and first lumbar transverse process. Cranially it touches the liver and caudally it is attached to the pancreas and the base of caecum. The duodenum winds around its lateral and then ventral surfaces. Medially is the caudal vena cava and adrenal gland.

The left kidney is between the last rib and 3rd transverse process. Its ventral surface is almost completely covered by the peritoneum and contacts the small intestine and and small colon. The spleen contacts it cranioventrally. Medially is the left [[Equine Endocrine System - Horse Anatomy#Adrenal Glands|adrenal gland] and aorta.

The Basic Components of the Kidney

Outer fibrous capsule

A tough outer capsule surrounds the parenchyma and this prevents the kidney expanding.

Renal Cortex

The renal cortex is the outermost layer of parenchyma, it lies just beneath the renal capsule. The renal cortex is comprised of two parts, peripheral zone and the inner juxtamedullary zone. The cortex contains the following parts of the nephron:

• Renal corpuscle
• Proximal convoluted tubule
• Distal convoluted tubule
• Medullary Rays

Renal Medulla

The renal medulla is the inner layer of renal parenchyma. In the horse, medullary pyramids and their papillae are completely fused to form a renal crest; hence they are said to have a unilobar type of kidney. The medulla can be split into two parts, the outer and the inner. Different parts of the nephron reside in these areas.

Outer Medulla

The outer medulla can be further divided into the outer and inner stripe.

• Outer Stripe: This section located just inside the cortex contains the following parts of the nephron:
  • Proximal straight tubules
  • Distal straight tubules
  • Thin descending and thick ascending limbs of the loop of henle
  • Outer medullary collecting ducts
  • Vasa recta
• The Inner Stripe: Located inside of the outer stripe this section contains the following parts:
  • Thin ascending and thick ascending limbs of the loop of henle
  • Outer medullary collecting ducts

The outer stripe contains the straight proximal tubules and the inner does not. However the inner contains thin ascending limbs and the outer does not. This difference makes up the anatomical demarcation between the two stripes.

Inner Medulla

The inner medulla only contains the following parts:

• Loop of henle
• Collecting ducts
• Vasa recta

Renal Pelvis

The renal crest opens into the renal pelvis. The renal pelvis is an expansion where the proximal ureter begins. The renal pelvis is located within the renal sinus. The renal sinus is located within an indentation on the medial side of the kidney. The renal sinus is a potential space, which is occupied by the ureter, branches of the renal artery and vein, lymphatics and nerves that enter the kidney at the hilus. In the horse, the renal pelvis consists of a central cavity and two large terminal recesses. The terminal recess is a long tube-like structure that collects urine from the poles of the kidneys into the renal pelvis. Most of the papillary ducts open into the terminal recesses. The renal pelvis is lined with transitional epithelium and contains mucous glands in the horse; giving urine a frothy appearence.

Vascular Supply

The kidney receives approximately 25% of cardiac output. Each kidney is supplied by a renal artery, which is a branch of the abdominal aorta. The renal artery subsequently divides at the hilus of each kidney into interlobar arteries, which run to the corticomedullary junction. Here they branch into arcuate arteries. The arcuate arteries then give rise to interlobular arteries which radiate into the renal cortex. The interlobular arteries become the afferent arteriole and subsequently the capillary loops of the glomerulus. These capillary loops then unite to become the efferent arteriole, which supplies a capillary network around the nephrons. This capillary network drains blood from the renal cortex into the interlobular veins, arcuate veins, then interlobar veins. The interlobar veins drain into the renal veins, which subsequently drain into the caudal vena cava.

The renal capsule is supplied by capillary branches of the interlobular arteries. Venous drainage of the capsule is provided by the stellate veins, which subsequently drain into the interlobular veins.

The vasa recta form a series of straight capillaries that lie parallel to the loop of Henle within the renal medulla. The vasa recta branch off the efferent arterioles of juxtamedullary nephrons. Blood flow within the vasa recta is very slow, allowing the countercurrent exchange mechanism to occur within the loop of Henle by mainaining a meduallary concentration gradient. The maintenance of this concentration gradient is one of the components responsible for the kidney's ability to produce concentrated urine.

Innervation

The kidney receives sympathetic and parasympathetic fibres from the solar plexus. These fibres travel with renal arteries. Sympathetic fibres synapse in coeliac ganglion and cranial mesenteric ganglion.

Lymphatic Drainage

Lymphatic vessels are satellite to the blood vessels. The drain first to the renal lymph nodes, before terminating in the lumbar lymph nodes.

Ureters

Histology section of a normal ureter (© RVC 2008)

Histology section of a normal ureter (© RVC 2008)

The ureters convey urine from the renal pelvis to the bladder. There are two of them, one for each kidney. The ureters run retroperitoneally along the roof of the abdominal cavity and then enters the pelvis. Once entering the pelvis it moves medially in the broad ligament of the female or the genital fold of the male. It ends at its junction on the dorsolateral surface of the bladder within the lateral ligament. The ureter then runs intramurally (within the bladder wall) before opening into the bladder lumen at slit-like openings known as ostia.

Wall

The wall of the ureters has an internal mucosa layer formed from transitional epithelium. This provides protection against the urine. The middle layer is a muscularis layer. This is well developed for peristalsis, though can enter into spasm on irritation. The outer layer is composed of adventitia. In the horse, the wall in the proximal urethra contains mucous-producing glands.

Junction with the Bladder

The ureter enters the bladder obliquely near the neck of the bladder and runs between the muscular layers and mucosa. They open through 2 slits on a raised hillock.

Movement of Urine

The movement of urine along the ureters is achieved by peristalsis which is powered by locally regulated smooth muscle. This maintains a low pressure in the renal pelvis.

Vascular Supply

The renal pelvis and proximal ureter is supplied by the renal artery. The distal ureter is supplied by the cranial vesicular artery and the vaginal artery (in the female) and prostatic artery (in the male).

Lymphatic Drainage

• Lumbar lymph nodes
• Medial iliac lymph nodes

Innervation

The ureter receivs parasympathetic and sympathetic innervation.

Bladder

The bladder is where urine is stored before being expelled by the body through the micturition reflex. Without a bladder urinary continence would be impossible.

The bladder is a hollow, muscular organ. It is divided for descriptive purposes into three parts:

• Cranial Pole
• Intermediate body
• Caudal neck

Its wall comprises a muscle layer covered in transitional epithelium. Its size and posistion are determined by how full it is. When empty the mucosa of the bladder wall is wrinkled and thicker. The bladder rests on the pubic bones, entirely within the pelvis and is largely retroperitoneal. When full and distended, the mucosal folds disappear and the wall appears thinner, it then becomes intraperitoneal. There are two folds (plicae uretericae) that do not disappear, even when the bladder is distended; these extend from the ureteral opening to the neck of the bladder before folding to become the urethral crest.

The trigone of the bladder gets its name as it looks like a triangle without a base. The trigone is bounded by the plicae uretericae. It is believed to have increased sensitivty and is of different embryological origin to the rest of tissue. More details can be found here.

Muscles of the Bladder

A schematic overview of the lower urinary tract showing the innervation and muscles of the bladder

The three muscular components of the bladder described below play a pivotal part in the micturition reflex.

Detrusor Muscle

This network of smooth muscle fibres lie in three sheets within the bladder wall and are supplied by both parasympathetic and sympathetic nerves. It is responsible for storage and expression of urine from the bladder.

Vascular Supply

The main blood supply to the bladder is via the caudal vesical arteries. These are branches of the vaginal artery (in the female) and the prostatic artery (in the male). A minor supply to the cranial bladder is provided by the reduced umbillical arteries.

Lymphatics

Lymphatics of the bladder drain into the iliosacral lymph nodes

Innervation

The bladder receives sympathetic and parasympathetic nerve supply.

• Parasympathetic supply: This comes from S1-S3, synapses within the pelvic plexus, continues as the pudendal nerve and is excitatory to the destrusor muscle via release of Acetylcholine which binds muscarinic acetylcholine receptors. Parasympathetic dominance allows emptying of the bladder.
• Sympathetic Supply: This comes from L1-L4, synapses within the caudal mesenteric ganglion before entering the pelvic plexus. Postganglionic fibres continue as the hypogastric nerves, which terminate on beta 2 receptors within the detrusor muscle. It has inhibitory action on muscular contraction, therefore enabling bladder filling.

Urethra

This muscular tube is the connection between the bladder and the external environment and plays a vital role in conscious urinary continence.

The Layers of the Urethra

The layers of the walls of the urethra are largely similar to those of the bladder apart from one significant difference; in both the male and the female the urethral submucosa has a network of veins which may contribute to continence by forming a kind of erectile tissue.

Female

The urethra empties at the external urethral orifice on the ventral wall of the vagina. This is often at the vestibulo-vaginal junction. Only urine passes through the female urethra. In the horse, the urethra is short and wide.

Male

A schematic overview of the path of the male urethra

A schematic overview of the blood supply to the bladder and urethra

The urethra empties at tip of penis. The male urethra carries urine, semen and seminal secretions from the accessory sex glands. The uretha is divided into 3 parts:

• Pre-prostatic - bladder neck to seminal hillock
• Prostatic portion - openings of deferant, vesicular and prostatic ducts
• Penile portion - ischial arch to penile tip

The combination of the pre-prostatic and prostatic portions are called the pelvic portion.

Muscles of the Urethra

The urethralis muscle runs the entire length of the urethra and forms the external urethral sphincter. Unlike the internal sphincter, the external sphincter is composed of striated muscle fibres which are under the voluntary control of the somatic nervous system.

Innervation of the External Urethral Sphincter

The urethralis muscle receives somatic innervation originating from S1-S3 and continuing via the pudendal nerve. This releases the neurotransmitter Acetylcholine, which binds to nicotinic Acetylcholine receptors and mediates skeletal muscle contraction of the external urethral sphincter. This is important in urine retention.

Vascular Supply

Blood Supply comes from the Urethral Artery, which is a branch of the vaginal artery which in turn is a branch of the internal pudendal which is a branch of the internal iliac.