Ch. 19 Physiology: The Kidneys (nephron)

3 processes performed by the nephron

Filtration, Reabsorption, and Secretion

Explain why more plasma is filtered (20% of the plasma volume, or 180 liters/day) out of the glomeruli than other capillaries in other organs of the body.

To filter out blood and excrete wastes, maintain Osm, and pH. Filtration of all the plasma would leave behind a sludge of blood cells and proteins that could not flow out of the glomerulus. Only 20% filters into the nephrons. Only 1% is excreted. Remaining plasma, plasma proteins and blood cells flow into the peritubular capillaries.

Basal lamina

separates the capillary endothelium from the epithelium of Bowman's capsule. Consists of negatively charged glycoproteins, collagen, and other proteins. Acts like a coarse sieve, excluding most plasma proteins from the fluid that filters through it.

Secretion

selectively removes molecules from the blood and adds them to the filtrate in the tubule lumen. more selective process than filtration that uses membrane proteins to move molecules across the tubule epithelium - In PT, DT, CD

Functions of the renal system

1) Regulation of extracellular fluid volume and blood pressure 2) Regulation of osmolarity 3) Maintenance of ion balance - Na+ in regulation of extracellular fluid volume and osmolarity. K+ and Ca++. 4) Homeostatic regulation of pH - extracellular fluid - acidic. Kidneys remove H+ and conserve HCO3 (bicarbonate) acts as a buffer. Alkaline - remove HCO3 and conserve H+. 5) Excretion of wastes 6) Production of hormones - Kidney cells synthesize erythroprotein, cytokine hormone that regulates rbc synthesis - release renin enzyme that regulates the production of hormones involved in sodium balance and blood pressure homeostasis. - renal enzymes help convert Vitamin D3 into a hormone that regulates Ca++ balance

Describe the forces involved in Glomerular filtration. Which forces favor filtration and which oppose filtration?

1. capillary blood pressure (PH) - favors filtration into Bowman's capsule 2. capillary colloid osmotic pressure (π) - favors fluid movement back into capillaries - opposes filtration 3. capsule fluid pressure (Pfluid) - opposes filtration

Explain how GFR (glomerular filtration rate) remains fairly constant even though systemic arterial blood pressure may range between 80 & 180 mm Hg

Autoregulation of glomerular filtration rate takes place over a wide range of blood pressures. GFR is controlled primarily by regulation of blood flow through the renal arterioles. If resistance of arterioles increases, renal blood flow decreases, and blood is diverted to other organs.

Describe the location and function of the podocytes in the nephron

Bowman's capsule. consists podocytes: podocytes: have long cytoplasmic extensions called foot processes that exten from the main cell body. Foot processes wrap around the glomerular capillaries and interlace leaving narrow filtration slits. a. epithelium around glomerular capillaries is modified into podocytes b. Podocytes leave slits through which filtration takes place. Mesangial cells between the capillaries contract to alter blood flow.

Cortical vs Juxtamedullary nephrons

Cortical Nephron is mostly in the cortex. More common in humans - 80%. Juxtamedullary dips deep into the medulla. Has a Vasa recta. Greater ability for reabsorption because of longer LOH. Such as H2O reabsorption. Common in desert environment animals - Camel

The effect of increased resistance on GFR depends on where the resistance takes place.

If resistance increases in afferent arteriole by vasoconstriction, it decreases renal blood flow, capillary blood pressure (PH) and GFR. - most regulation occurs at the afferent arteriole Increased resistance of efferent arteriole decreases renal blood flow but increases PH and GFR.

List 2 ions that can be secreted from the epithelial cells that line the nephron tubule

K+ and H+ from the extracellular fluid into the lumen of nephron.

Autoregulation (local control) of GFR

Myogenic response - the intrinsic ability of vascular smooth muscle to respond to pressure changes Tubuloglomerular feedback - paracrine signaling mechanism where changes in fluid flow through the loop of Henle influence GFR

In each region of the nephron tubule, indicate which solutes (organic, Na+, Cl-, K+, urea) are reabsorbed or secreted.

Na+, Cl-, K+, urea PT - Na reabsorbed 65-70%. Glucose, AA's. H secreted DLOH - H2O reabsorbed ALOH - Na, Cl, K reabsorbed DT - Na, Cl reabsorbed. H and K secreted (NAKATPase). site of diuretics CD - Na, H2O reabsorbed, K secreted

Be able to calculate the net filtration pressure if given values for each of the 3 forces.

PH - π - Pfluid = + (filtration) ....if... - (no filtration) PH (Hydrostatic pressure (blood pressure) ) π (coloid osmotic pressure gradient due to proteins in plasma, not in Bowman's Capsule) Pfluid (fluid pressure created by fluid in Bowman's capsule)

Tubuloglomerular feedback

Paracrine signaling between the macula densa and afferent arteriole afferent arteriole contain specialized muscle cells called granular cells which secrete renin - an enzyme involved in Na and H2O balance. When NaCl delivery past the macula densa increases due to increased GFR, macula densa cells send a paracrine message to neighboring afferent arteriole. Making the afferent arteriole to constrict, increasing resistance and decreaseing GFR. This protects the filtration barriers from damage. ​ Changes in the diameter of the afferent and efferent arterioles will affect the flow rate and volume of blood entering and exiting the glomerulus. ​

Explain why peritubular capillary pressures favor reabsorption

The hydrostatic pressure that exists along the entire length of the peritubular capillaries is less than the colloid osmotic pressure. net pressure gradient favors reabsorption. Peritubular capillaries have an avg hydrostatic pressure of 10mm Hg. Colloid osmotic pressure is 30 mm Hg. Result is 20 mm Hg, favoring absorption of fluid into the capillaries to the venous circulation back to the heart.

Explain how a transport maximum (renal threshold) may be reached during reabsorption in the nephron. Describe the process of glucose handling in this respect.

The transport rate of a substance is proportional to the plasma concentration of the substance, up to the point where transporters become saturated. Normal plasma glucose concentrations, all glucose that enters the nephron is reabsorbed before it reches the end of PT. The tubule epithelum is well supplied with carriers to capture glucose. Excessive glucose - glucose is filtered faster than the carriers can reabsorb it. Carriers become saturated. As a result some glucose escapes reabsorption and is excreted in the urine. Renal threshold - plasma concentration at which glucose first appears in the urine

Glomerular capillary epithelium

a. epithelium around glomerular capillaries is modified into podocytes b. Podocytes leave slits through which filtration takes place. Mesangial cells between the capillaries contract to alter blood flow. c. Glomerular capillary endothelium, basal lamina, and Bowman's capsule epithelium create a three layer filtation barrier. Filtered substances pass through endothelial pores and filtration slits.

Why is the process of urea reabsorption from the proximal tubule is considered to be passive

active transport of Na+ and other solutes in the PT creates a urea concentration gradient. Na+ and solutes are reabsorbed from the PT and makes the extracellular fluid more concentrated than the filtering lumen. Creating an osmotic gradient. In response of the osmotic gradient, water moves by osmosis across the epithelium. When water is reabsorbed, the concentration of urea in the lumen increases, creating a concentration gradient for urea. Urea moves out of the lumen through passive diffusion.

Glomerulus capillaries

are fenestrated with large pores that allow most components of plasma to filter through endothelium. Pores allow rapid fluid filtration into tubules. Pores are small enough to prevent blood cells from leaving capillary. Also negatively charged proteins on pore surfaces repel negatively charged plasma proteins.

Bowman's capsule

consists podocytes: have long cytoplasmic extensions called foot processes that extend from the main cell body. Foot processes wrap around the glomerular capillaries and interlace leaving narrow filtration slits.

Compare the chemical composition of glomerular filtrate in Bowman's capsule with that of plasma

filtrate = plasma minus the proteins Most proteins are too big to past through Some smaller proteins and peptides can pass through but are too big to be reabsorbed. Most enter PT by receptor-mediated endocytosis - where they're digested by lysosomes and become amino acids where they can be reabsorbed into the blood.

Juxtaglomerular apparatus

formed by the DT and afferent arteriole in regions where they come in contact. Main function is to regulate blood pressure and the filtration rate of the glomerulus. Where Tubuloglomerular feedback happens consists of macula densa and granular cells. Paracrine signaling between the nephron and afferent arteriole influences GFR Regulate blood flow into the glomerulus. - GFR increases - flow through tubule increases. - Flow past macula densa increases. - Paracrine diffuses from macula densa to afferent arterioles. - Afferent arteriole constricts. - Resistance in afferent arterial increases. - Hydrostatic pressure in glomerulus decreases. - GFR decreases.

Mesangial cells

lie between and around the glomerular capillaries. They have actin-filaments that can contract and alter blood flow. release cytokines associated with immune and inflammatory processes.

Reabsorption

process of moving substances in the filtrate from the lumen of the tubule back into the blood flowing through peritubular capillaries - In PT, LOH, DT, CD

Excretion

removal in the urine. anything that filters in the neuron and not reabsorbed. - In CD

The direction that blood flows through the kidneys

renal artery → afferent arteriole → 1st capillary (glomerulus) → efferent arteriole → peritubular capillaries..."vasa recta" in juxtamedullary nephrons → venules → veins -Renal arteries take blood to the cortex. -receives 20-25% of the cardiac output - Afferent arterioles and glomeruli are all found in the cortex - 80% of nephrons are in cortex, 20%; juxtamedullary nephrons dip down to medulla - blood vessels forms a portal system: two capillary beds in series (one after another)

Myogenic response

similar to autoregulation of systemic arterioles increased blood pressure stretches arteriole walls and stretch-sensitive ion channels open depolarizing muscle cells. Depolarization opens voltage gated Ca++, and vascular smooth muscle contracts. Vasoconstriction increases resistance to flow, decreasing blood flow in the arteriole. Which decreases filtration pressure in the glomerulus. If blood pressure decreases, contraction disappears and arteriole becomes maximally dilated. However, Vasodilation is not as effective at maintaining GFR because afferent arteriole is fairly relaxed. When mean blood pressure drops to 80mm Hg, GFR decreases. - a decrease in GFR helps the body conserve blood volume

3 filtration barriers before plasma enters the tubule lumen

the glomerular capillary endothelium a basal lamina (basement membrance) the epithelium of Bowman's capsule

Filtration

the movement of fluid from blood into the lumen of the nephron takes place only in the renal corpuscle, where the walls of glomerular capillaries and Bowman's capsule are modified to allow bulk flow of fluid. - In Bowman's capsule

Renal portal system function

to filter fluid out of the blood and into the lumen of the nephron at the glomerular capillaries, then to reabsorb fluid from the tubule lumen back into the blood at the peritubular capillaries.