Question

How does filtrate form? What pressures are involved?  What affect does each pressure have on formation? Trace a drop of filtrate from its formation to the point where it turns into urine.  Explain ALL of the micro anatomy; and the physiology that occurs at each microanatomical structure along the way.

Now trace the drop of urine from the point it becomes urine to the point it is eliminated from the body.   Name all of the structures and substructures in order.

Answer 1

The process of filtrate formation occurs at the filtration membrane which is located at the boundary between the glomerulus and Bowman's capsule.

The rate at which kidneys filter blood is called the Glomerular Filtration Rate. The key driving force for the filtration process is the blood pressure as it enters the glomerulus. This is offset to some extent by inward pressure due to the hydrostatic pressure of the fluid within the urinary space, and the pressure generated by the proteins left in the capillaries that tend to pull water back into the circulatory system called as colloidal osmotic pressure.

Glomerular Filtration Rate is influenced by the hydrostatic pressure and colloid osmotic pressure on either side of the capillary membrane of the glomerulus. As the filtration membrane limits the size of particles crossing the membrane, the osmotic pressure inside the glomerular capillary is higher than the osmotic pressure in Bowman’s capsule. Hydrostatic (fluid) pressure is sufficient to push water through the membrane despite the osmotic pressure working against it.

Net Filtration Pressure = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

A slight change in osmolarity of the blood or changes in capillary blood pressure result in major changes in the amount of filtrate formed at any given point in time.

When blood pressure goes up, smooth muscle in the afferent capillaries contracts to limit any increase in blood flow and filtration rate.

When blood pressure drops, the same capillaries relax to maintain blood flow and filtration rate.

The net result is a relatively steady flow of blood into the glomerulus and a relatively steady filtration rate.

The path of urine formation

Renal artery → glomerulus → bowman's capsule → renal tubules → calyx → renal pelvis →ureter → bladder →urethra.

Renal artery	The renal arteries are a pair of lateral branches arising from the abdominal aorta below the level of superior mesenteric artery at the upper lumbar level.
Glomerulus	The glomerulus is a tuft of small blood vessels called capillaries located within Bowman's capsule within the kidney. Glomerular mesangial cells structurally support the tufts. Blood enters the capillaries of the glomerulus by a single arteriole called an afferent arteriole and leaves by an efferent arteriole.
Bowman's capsule	It is a cup-like sack at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac.
Renal tubules	The renal tubule is the portion of the nephron containing the tubular fluid filtered through the glomerulus. After passing through the renal tubule, the filtrate continues to the collecting duct system.
Calyx	The minor calyces surround the apex of the renal pyramids. Urine formed in the kidney passes through a renal papilla at the apex into the minor calyx; two or three minor calyces converge to form a major calyx, through which urine passes before continuing through the renal pelvis into the ureter.
Renal pelvis	The renal pelvis is the point where the two or three major calyces join together. It has a mucous membrane and is covered with transitional epithelium and an underlying lamina propria of loose-to-dense connective tissue. The renal pelvis functions as a funnel for urine flowing to the ureter.
Ureter	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. It ends at its junction on the dorsolateral surface of the bladder within the lateral ligament.
Bladder	The bladder is pear shaped, becoming more oval as it fills with urine. The bladder opens into the urethra at the neck of the bladder.
Urethra	The urethra is the vessel through which urine passes after leaving the bladder. Urethral smooth muscle cells are mechanically coupled to each other to coordinate mechanical force and electrical signaling in an organized, unitary fashion.