CHAPTER 26 URINARY SYSTEM
The kidneys do the major work of the urinary system. Other parts are mainly passageways and storage areas.
Parts:
2 kidneys
2 ureters
1 urinary bladder
1 urethra
Nephrology and urology are terms for the branch of medicine concerned with the urinary system.
Functions of the kidneys:
1. Regulation of ion levels in blood
2. Regulation of blood pH
3. Regulation of blood volume
4. Regulation of blood pressure
5. Maintaining blood osmolarity (proper ratio of water to solutes)
6. Release of hormones
Calcitriol—active form of Vit. D
Erythropoietin—increased production of RBC
7. Regulation of blood glucose level—kidney cells can produce glucose and release it into the blood when needed
8. Excretion of wastes and foreign substances
Paired reddish kidney-bean shaped organs, located retroperitoneal, just above the waist with the right kidney slightly lower than the left.
On the average, about 4-5" long and 2-3" wide with a medial concave border, the hilum, where the ureter, blood vessels, lymphatics, and nerves enter and leave. Each kidney is surrounded by 3 layers:
1. Renal capsule (inner layer)--smooth transparent fibrous membrane--barrier against trauma and spread of infection
2. Adipose capsule--mass of fatty tissue for protection
3. Renal fascia (outer layer)--thin layer of dense irregular CT that anchors kidney in place
A section through the kidney shows an outer reddish cortex and an inner, reddish-brown medulla. Within the medulla are 8-18 cone-shaped renal pyramids. Their bases face the cortex and the apexes, called renal papillae, face the hilum. Portions of the cortex called renal columns extend between the pyramids. Inside the kidney is a large cavity, the renal sinus. It contains the renal pelvis, which becomes continuous medially with the ureter. The lateral edge of the pelvis contains cuplike extensions called major and minor calyces, which bring urine in to the pelvis.
Kidneys receive through the renal arteries 20-25% of resting cardiac output. As the renal artery enters each kidney, it gives off 5 segmental arteries, which continue to branch and eventually form the afferent arterioles to the glomeruli. Efferent arterioles emerge from the glomeruli. Afferents are larger than efferents, which is what creates the pressure to force fluids across the filtration membrane.
After leaving the glomerulus, the efferent arteriole forms a network of peritubular capillaries around the PCT and the DCT and also another capillary network, the vasa recta, which surrounds the loops of Henle of juxtaglomerular nephrons.
Blood leaves the kidney through the renal vein.
Nerve supply to the kidneys is from the renal plexus--these are sympathetic vasomotor nerves which regulate the diameter of arterioles within the kidney. Most of the control of activities in the kidney is hormonal.
The nephron is the functional unit of the kidney. 3 basic functions:
1. Filtration
2. Reabsorption
3. Secretion
These processes together result in urine formation.
A nephron contains 2 portions:
1. Renal corpuscle--filtration
2. Renal tubule--processing of filtrate--reabsorption & secretion
1. Renal corpuscle--consists of a mass of capillaries called the glomerulus surrounded by a double-walled epithelial cup, the glomerular capsule (Bowman's capsule). Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. (The capillaries are arterial capillaries--different from others in the body). As blood flows through the glomerular capillaries, water and most solutes are filtered across the visceral layer into the capsular space and then into the renal tubule.
Contractile cells called mesangial cells are located among the glomerular capillaries. They help regulate the rate of filtration—when they contract there is less capillary surface area available so filtration slows. When they relax, more area is available and filtration increases.
2. Renal tubule
a. Proximal convoluted tubule (PCT)
b. Loop of Henle
c. Distal convoluted tubule (DCT)
The renal corpuscle and both convoluted tubules lie in the cortex and the loop of Henle extends down into the medulla.
The DCTs of several nephrons join to form a collecting duct--these merge as follows:
Renal corpuscle
PCT
Descending limb of loop of Henle
Loop of Henle
Ascending limb of loop of Henle
DCT
Collecting tubule (formed by DCTs from several nephrons)
Papillary duct (formed as collecting tubules merge)
Minor calyx (formed as papillary ducts merge)
Major calyx (formed as minor calyces merge)
Renal pelvis
Ureter
Urinary bladder
Urethra
Some nephrons (cortical nephrons) have their glomeruli located in the outer portion of the cortex and short loops of Henle which barely penetrate into the medulla. Others (juxtamedullary nephrons) have their glomeruli located deep in the cortex next to the medulla and long loops of Henle which penetrate deeply into the medulla. The number of nephrons is constant from birth, but existing nephrons may enlarge if necessary.
1. Renal corpuscle--the filter. The filtration membrane is the filter in the nephron. Layers:
a. Endothelium of the glomerulus--single layer with fenestrations (pores) of a size that allows almost everything to pass except blood cells
b. Basal lamina—layer of fibrils in a glycoprotein matrix—restricts passage of larger plasma proteins
c. Visceral layer of Bowman's capsule--specialized epithelial cells called podocytes which have footlike extensions called pedicels that cover the basal lamina except for little spaces called filtration slits. A thin membrane, the slit membrane, extends across the filtration slits and restricts the passage of medium-sized plasma proteins.
2. Renal tubule--walls consist of a single layer of epithelial cells resting on a basement membrane. Cells vary from simple cuboidal to simple squamous (in loop of Henle) back to simple cuboidal.
In the DCT and collecting tubules, most cells are principal cells, whose permeability to water varies according to ADH level. A few cells are intercalated cells, which can excrete H+ ions.
In the final portion of the ascending limb of the loop of Henle and the afferent arteriole, which make contact, is the juxtaglomerular apparatus. It consists of modified tubule cells (macula densa) and modified smooth muscle fibers (juxtamedullary cells). It helps regulate BP by secreting renin when needed.
PHYSIOLOGY
OF URINE FORMATION
The nephrons do the work--other parts are passageways and storage areas. There are 3 steps in urine formation:
I. Glomerular filtration
II. Tubular reabsorption
III. Tubular secretion
I. GLOMERULAR FILTRATION--filtration is forcing of fluids and dissolved substances across a membrane by pressure. In the kidneys it occurs across the filtration membrane due to BP. The fluid that leaves the blood, enters the capsular space and travels on through the tubules is called the filtrate.
Large volumes of fluid are filtered in the renal corpuscle because:
1. Glomerular capillaries provide a large area for filtration
2. Glomerular capillaries are leaky and the filtration membrane is thin.
3. Glomerular blood pressure is high due to larger afferent and smaller efferent arteries.
48 gallons of filtrate leaves the blood each day (60X total blood volume) and all but 1-2 liters are reabsorbed. The filtrate contains all materials present in plasma except:
a. Formed elements (blood cells)
b. Most proteins (only the smallest slip through)
Glomerular capillaries are thin, porous and much more permeable than other body capillaries, so all these pass easily into the capsular space:
Water Small proteins
Glucose Nitrogenous waste
Vitamins Ions
Amino acids
Filtration depends on the following forces:
1. Glomerular blood hydrostatic pressure (GBHP) due to BP--pushes OUT
2. Capsular hydrostatic pressure (CHP) due to fluid already in tubule
--pushes IN against GBHP
3. Blood colloid osmotic pressure (BCOP) due to proteins in plasma--pulls IN
Net filtration pressure = GBHP - CHP - BCOP
The arrangement of larger afferent and smaller efferent arterioles keeps the GBHP high enough in glomerular capillaries that filtration occurs. Blood pressure in regular capillaries is only 30mm Hg or less.
Glomerular filtration rate (GFR) is the amount of filtrate that forms in both kidneys each minute--125 ml/minute--180 liters/day
If systemic BP falls too low, filtration stops. If the GBHP is below 45 mmHg no urine is formed (anuria).
GFR depends on the flow of blood into the glomerular capillaries. Two main factors determine this:
· Systemic blood pressure
· Diameter of afferent and efferent arterioles
Regulation:
1. Renal autoregulation--operates within the kidneys in 2 ways. This is most important in dealing with everyday small fluctuations in blood pressure.
a. Myogenic mechanism—when BP rises, smooth muscle fibers in the walls of afferent arterioles are stretched. They respond by contracting, which narrows the lumen and reduces blood flow. A drop in BP reduces stretch and causes smooth muscle fibers to relax.
b. Tubuloglomerular feedback—macula densa monitors the level of Na+, Cl-, and water in the filtrate. When this indicates that the glomerular filtration rate is above normal, the macula densa cells inhibit release of nitric oxide, a vasodilator, from cells in the juxtaglomerular apparatus. Without, nitric oxide, afferent arterioles constrict and GFR decreases. This mechanism works to a lesser degree when GFR decreases—this causes some increase in release of nitric oxide and dilation of afferent arterioles.
2. Neural regulation--sympathetic fibers (only kind the kidney has) can cause several effects
a. Little or no sympathetic stimulation (at rest)—dilat ion of all renal vessels to the same degree
b. Increased sympathetic stimulation leads to greater constriction of afferent arterioles than efferent, so blood flow into the glomerular capillaries decreases which:
1) Reduces urine output
2) Permits greater blood flow to other tissues
3. Hormonal regulation
a. Angiotensin II-decreased blood flow through the kidney causes juxtaglomerular cells to secrete the enzyme renin, which activates the following pathway:
Angiotensinogen (a plasma protein)
Renin
Angiotensin I
ACE (angiotensin converting enzyme) in the lungs
Angiotensin II (active hormone)
Effects of angiotensin II:
1) Vasoconstriction of both afferent and efferent arterioles—reduces GFR
2) Generalized vasoconstriction bodywide
3) Secretion of aldosterone, which enhances reabsorption of Na+ and water
4) Release of ADH, resulting in retention of water
5) Activates thirst center
All of these tend to increase blood volume, which increases BP and GFR.
b. Atrial natriuretic peptide--secreted by cells in heart atria and promotes excretion of Na+ and water. This hormone is released when stretching of the atria indicates an increased blood volume. It causes relaxation of the mesangial cells, which increases the capillary surface area available for filtration.