PSIO 241 CH 14

general renal structures

1. Kidneys supplied by **single renal artery** & **vein**
2. Body roughly divided into **cortex and medulla**
3. Human kidneys are partially segmented with the medulla  consisting of __pyramids__ 
4. The urine is collected by minor and major calices; drains into **pelvis** and to  **ureter**
5. Abundant **sympathetic innervation** of vasculature and tubules

renal artery

carries unprocessed blood

renal vein

carries processed blood

20%

kidney gets _ % of cardiac output approx 1200 ml/min

nephron

functional unit of kidney consists of filtering glomerulus and tubule system surrounded by capillaries

vascular system

– afferent arteriole, glomerulus (capillary), efferent arteriole

– peritubular capillaries, vasa recta

tubular component

– Bowman’s Capsule

– Proximal Tubules

– Loop of Henle

– Distal Tubules

– Collecting ducts

afferent arteriole

brings blood into the glomerulus

efferent arteriole

takes blood out of the glomerulus

filtrate

fluid in Bowman’s Capsule (filtered blood)

• modified by **reabsorption and secretion** and what’s left behind in the tubules is excreted as urine. 

1 million

about _ nephrons per kidney

**Superficial (cortical)** nephrons  

have short Loops of Henle

Juxtamedullary nephrons 

have long Loops of Henle that dip into the deeper medulla 

Vertical osmotic gradient 

exists in the interstitial fluid from cortex to medulla. **Important in water conservation.**

300; 1200

cortex mOSm is *_* and the medulla mOSm is _

filtration of blood

afferent arteriole, glomerulus and efferent arteriole-function 

cortical nephrons 

surrounded by **peritubular capillaries, function is reabsorption**

juxtamedullary nephrons 

surrounded by peritubular capillaries AND **vasa recta** that surround their Loops of Henle, **function is reabsorption**

Glomerular Filtration Rate (GFR)

approx 100-120 ml/min

urine flow rate

approx 1 ml/min

urine production

1\.0-1.5 L/day; range 0.5-15 L/day

filtration

occurs at specialized capillary beds in the kidney called the **glomerulus**

tubular reabsorption

• the selective transfer of specific substances in the filtrate back into the blood of the peritubular capillaries.

• Reabsorption rates vary for different substances.

•On average, of the 180 liters of plasma filtered per day, 178.5 liters are reabsorbed; remaining 1.5 L excreted as urine

tubular secretion and excretion

• the selective transfer of substances from the peritubular capillary blood into the tubular lumen. 

• Process is opposite of reabsorption.

• **Provides additional route** for substances to enter the renal tubules from the blood.

• **Urine excretion** is the elimination of substances from the body in the urine.

sodium

almost all of the _ in bowman’s space gets reabsorbed to keep water

hydrogen and potassium

body wants to get rid of _

micturition

Urine is temporarily stored in the bladder and emptied by _

bladder

• Urine move from ureters to the urinary bladder my peristalsis. 

• can accommodate up to 250 to 400 ml of urine before stretch receptors initiate the micturition reflex. 

• **Ureter**- carries urine to bladder

bladder sphincters

• **Internal sphincter**-smooth muscle

• **External sphincter**- skeletal muscle, part of the pelvic diaphragm

in males

in _ prostate gland sits between the bladder and external sphincter

bladder wall

smooth muscle or **detrusor muscle**

micturition reflex

• Urine moves from ureters to the urinary bladder by peristalsis. 

• This reflex causes involuntary emptying of the bladder. 

• Involves bladder contraction and opening of both the **internal and external urethral sphincters**.

• Micturition can be voluntarily prevented by deliberately tightening the external sphincter and pelvic diaphragm. 

external

* urine is only expelled if we voluntarily **relax the _** sphincter via activity of **somatic motor system.** 


* We can choose to keep the sphincter closed.

sympathetic; parasympathetic

_ **activity** relaxes the bladder allowing it to fill*;* stretch receptors trigger _ **reflex** that causes the smooth muscle in **bladder wall to contract** then i**nternal sphincter opens**

glomerular filtration

• the formation of protein-free filtrate at the glomerulus

• 100-125 ml of glomerular filtrate is formed each minute or 180 liters each day (GFR)

• 20-25% of the plasma that enters the glomerulus is filtered

increase GFR

dialating afferent arteriole

 capillary endothelial wall with fenestrations 

have a magnitude greater than proteins; in addition, the wall is covered with negatively charged compounds

glomerular basement membrane 

made up of a matrix of extracellular negatively charged proteins and other compounds

epithelial cell layer of podocytes 

next to Bowman’s space; the podocytes have foot processes bridged by filtration slit diaphragms

glomerular filtration

• First step in the process of making urine.

• Specialized capillary exchange, but general rules of bulk flow across capillary bed apply.

• Greater rate of exchange compared to other capillaries because:

• **Higher permeability**

**• Higher capillary hydrostatic pressure**

**• BP constant across glomerulus** 

high permeability

1. **Fenestration**s-pores in capillary wall
2. Glomerular blood pressure **higher** than that found in other beds
3. Blood pressure **does not fall** across the glomerulus
4. Important to note that size of afferent and efferent arteriole can be regulated
5. **This impacts GFR**

physical forces involved in filtration

–glomerular capillary blood pressure

**- (major force that causes glomerular filtration)**

–plasma-colloid osmotic pressure

–Bowman’s capsule hydrostatic pressure

net filtration pressure 

the net difference in these forces favoring filtration

filtration forces

• __Glomerular Pressure__= **55 mm Hg**

• __Plasma Colloid Osmotic Pressure__= **30 mm Hg**; exists because of plasma proteins

• __Bowman’s Capsule Hydrostatic Pressure__= **15 mm Hg**; fluid accumulates in capsule and opposes filtration

• Assuming NFP of 10 mm Hg is normal, this produces **a normal GFR**

glomerular exchange pathway

results in protein-free filtrate containing water, electrolytes, nutrients and wastes

process for substance to be filtered

1. the pores between and then fenestrations within the endothelial cells of the glomerular capillary
2. an acellular basement membrane
3. the filtration slits between the foot processes of the podocytes in the inner layer of Bowman’s capsule

nephrotic syndrome (minimal change disease)

• marked disruption of the filtering membrane; results in loss of negative  charges from filtration  barrier

• Plasma __**proteins now pass**__ through the membrane and are eliminated in urine 

• Associated with a __non-inflammatory injury__ to glomerular membrane system

nephrotic syndrome signs

• **Marked proteinuria > 3.5 gm/day** 

• Edema (loss of plasma oncotic pressure)

• Hypoalbuminemia (albumin lost in urine)

80-100 mmHg

Renal blood flow remains relatively constant from about *_* to almost _ mmHg

autoregulation

prevents spontaneous GFR changes; kidney regulates itself via intrinsic controls that keeps GFR constant