Consists of two kidneys; two ureters, which carry urine from the kidneys to the urinary
bladder; urinary bladder; urethra, which carries urine from the bladder to the outside of the
body. Functions of the urinary system
Excretion: The kidneys filter blood and produce a large volume of filtrate. Large
molecules (proteins and blood cells) stay in the blood. Smaller ones (ions) enter the
filtrate. The filtrate flows through the kidneys and is modified until it’s converted to
urine. Waste products enter the filtrate and useful molecules are reabsorbed.
Regulation of blood volume and pressure: They control the extracellular fluid volume in
the body by producing either a large volume of dilute urine or a small volume of
concentrated urine. Therefore, the kidneys regulate blood volume and hence blood
Regulation of the concentration of solutes in the blood: They help regulate the
concentration of the major ions.
Regulation of extracellular fluid pH: They secrete variable amounts of H+ to help
regulate the extra cellular fluid pH.
Regulation of red blood cell synthesis: they secrete the hormone erythropoietin, which
regulates the synthesis of RBCs in bone marrow.
Regulation of vitamin D synthesis: they help control blood levels of Ca by regulating the
synthesis of vitamin D.
Location and external atatomy of the kidneys
Location: behind the peritoneum on the posterior abdominal wall on each side of the vertebral
column, extending from the last thoracic to the third lumbar vertebrae, partially protected by
the ribcage. The liver is superior to the right kidney therefore the right kidney is slightly lower
than the left.
Renal capsule- layer of fibrous connective tissue surrounding each kidney surrounded
itself by a dense layer of adipose tissue which cushions the kidneys.
Renal facia- and a thinner layer of connective tissue surrounding the adipose tissue
which helps anchor the kidneys and tissue to the abdominal wall
Hilum- is a small area where the renal artery and nerves enter, and the renal vein and
ureter exit. It opens into the renal sinus.
Renal sinus- a cavity filled with adipose tissue and connective tissue which is occupied
by the major and minor calix. Internal anatomy and histology of the kidneys
Inner medulla: composed of renal pyramids, the bases of which form the boundary between
the cortex and medulla. The tips of the pyramids are called the renal papillae which point
toward the sinus, and the minor calyces are chambers into which the papillae extend draining
urine out of the pyramid and into the calyces. The minor calyces merge to form the major
calyces which then converge to for the renal pelvis, which is surrounded by the sinus. The renal
pelvis narrows into the ureter which exits via the hilum and connects to the urinary bladder.
Structure of Nephron:
Functional unit of the kidney
Renal corpuscle- a proximal convoluted tubule, a loop of Henle, and a distal convoluted tubule.
The distal convoluted tubule empties into a collecting duct, which carries urine from the cortex
of the kidney towards the renal papilla. Near the tip is where several collecting ducts merge
into the papillary duct.
Papillary duct- empties into the minor calyx.
Corpuscles and tubules are located in the cortex. Collecting ducts, parts of loop of Henle, and
papillary ducts are in the renal medulla.
Bowman Capsule and Glomerulus
The wall of the bowman capsule is indented to for a double-walled chamber; within this
chamber is the glomerulus, which is a network of capillaries
Fluid is filtered from the glomerulus into the Bowman capsule which then flows to the
proximal convoluted tubule and is carried away.
Bowman capsule has an outer layer, the parietal layer, made up of simple squamous
epithelium which become cube shaped at the beginning of the proximal convoluted
And an inner layer called the visceral layer which is constructed of podocytes, which
wrap around the glomerular capillaries
In the endothelial cells of the glomerular capillaries are fenestrae
Gaps called filtration slits are between cell processes of the podocytes which make up
the visceral layer of the Bowman capsule
A cell membrane is sandwiched between the endothelial cells of the capillaries and
visceral layer of the Bowman. These three things form the kidneys filtration membrane. The slits in the podocytes regulate what will be allowed to be filtered and what will stay
in the blood, to regulate the function for pressure filtration out of the apparatus.
A basement membrane selectively blocks plasma proteins & other molecules from
leaving capillary. cells, proteins are too big nutrients, waste products and electrolytes
The renal tubules:
Proximal Convoluted Tubule-
Simple cuboidal epithelium which lies upon a basement membrane which forms the
outer layer of the tubule.
Many microvilli line the inner layer of the tubule
Loops of Henle-
Continuations of the PCT, each loop has two limbs: the descending limb and the
the beginning of the descending limb is simple cuboidal epithelium like that of the PCT
and then it thins and becomes simple squamous epithelium
the beginning of the ascending limb, therefore, begins with simple squamous and
thickens into simple cuboidal epithelium, ending by giving rise to the DCT.
Distal convoluted tubule-
shorter than the PCT, and the cells are smaller and do not have many microvilli
The DCT in many neurons connects to the collecting ducts which extend through the
medulla towards the tips of the renal pyramids. Blood Supply:
An afferent arterial supplies blood to the glomerulus and an efferent arteriole drains it.
At the point where the afferent arteriole enters the glomerulus, the smooth muscle cells
form a cufflike arrangement around the arteriole of juxtaglomerular cells.
Specialized cells called the macular densa and the juxtaglomerular cells, found in part of
the distal convoluted tubule adjacent to the glomerular apparatus, make up the
The juxtaglomerular apparatus helps regulate the flow of blood into the Bowman and
therefore the amount of filtrate produced.
• Paired Renal arteries from the abdominal aorta- branching, arcuate artery (cruved
trajectory between cortex and medulla), smaller inter lobular arteries, to afferent
arteriole, glomerular capillaries (filtrate ), efferent arteriole, peritubular capillaries
(reabsorption process), vasa recta or inter lobular veins, arcuate veins, renal veins,
inferior vena cava, heart
1)Renal artery – Red
2) Segmental Artery – socks
3) Interlobular artery – Ingrid
4) Arcuate Artery – And 5) Interlobular Artery – Ivan
6) Afferent arteriole – Are
7) Glomerulus – Going
8) Efferent Arteriole – Every
9)Peritubular Capillaries – Place
10) Vasa recta – (on) Vacation
11) interlobular vein – Inside
12) Arcuate vein – An
13) Interlobular Vein – Indigo
14) Renal vein – Rucksack
15) Inferior vena cava – Very
16) Heart - happily
There are three processes that determine the volume and composition of urine
1. Filtration in the glomerulus, the more pressure the more filtrate (increase the blood
pressure in and you will produce more filtrate). Blood pressure non-selectively forces
water and the other small molecules out of glomerular capillaries into the bowman
2. Reabsorption happens in the medulla, where we retrieve molecules we want to be
reabsorbed into the blood
3. Tubular Secretion- when the nephron cells transport solutes from the blood into the
filtrate Filtration Pressure
There are three different types of pressure in the glomerular capillary:
GCP- blood pressure (50 mm Hg) within the glomerulus, outward pressure of blood pressin on
the capillary walls. Moves fluid and solutes from the blood into the Bowman capsule to add to
filtrate. The efferent arteriole is smaller in diameter that the afferent one so the pressure builds
here as resistance against the walls increases.
CHP- an inward pressure from the pressure of filtrate accumulation inside the Bowman capsule,
(10 mm Hg) which moves fluid from the Bowman capsule into the blood. CHP is comparable to
blood pressure in that, as blood pressing on the walls of the capillary creates blood pressure,
filtrate pressing on the walls of the Bowman capsule creates CHP.
BCOP- because there are a lot of large proteins staying in the capillaries they create an osmotic
pressure, the more materials in the blood the higher the cop. Fluid moves from the Bowman
capsule to the blood by osmosis. A condition where the cop would go up is when you are
dehydrated and blood is concentrated.
Net filtration pressure (10 mm Hg) = GCP-CHP-BCOP if we minus the two forces pushing fluid
from the Bowman capsule into the blood from the force pushing fluid into the Bowman from
the blood then we can calculate the net movement. Tubular Reabsorption:
The return of filtered water and solutes to the blood, nearly 99 % of water and solutes
are returned to the blood via the kidney with toxins removed. Reabsorption insures we
do not dehydrate.
The filtrate leaves the Bowman capsule and flows through the PCT, the loop of Henle,
the DCT and then into the collecting ducts. As it passes through these structures many
of the substances in the filtrate are reabsorbed by one or more of several processes.
Reabsorption in the PCT
- PCT is responsible for the majority or reabsorption and cilia within the tubule
increases surface area for this reabsorption.
- The cells are what contribute to this; they have an apical surface which makes up the
inside surface of the nephron (touches urine); a basal surface which forms the outer