PGY renal homework

What are the characteristics of substances that can be filtered by the kidney? What special adaptations exist in this organ?

Large molecules and negatively charged molecules do not pass through the membrane formed by the glomerular capillary endothelium cell, the basement membrane and the nephron cell. Therefore these substances are not filtered into Bowman's capsule. Water, small ions (mostly positively charged ones) and small molecules such as glucose and amino acids all pass readily into the ultrafiltrate.

Secretion

Movement of substances from the blood into the tubule. Many of these are actively transported.

Reabsorption

Movement of substances from the tubule into the blood. Many of these are actively transported.

Excretion

the loss of substances from the body. In the renal system, many substances are excreted in the urine

Alveolar Oxygen Tension (PAO2)

(Patm - PH2O) FiO2 - PaCO2/RQ Patm is the atmospheric pressure (at sea level 760 mm Hg), PH2O is partial pressure of water (approximately 45 mm Hg). FiO2 is the fraction of inspired oxygen. PaCO2 is partial pressure of carbon dioxide in alveoli (in normal physiological conditions around 40 to 45 mmHg). RQ is the respiratory quotient. The value of the RQ can vary depending upon the type of diet and metabolic state. RQ is different for carbohydrates, fats, and proteins (average value is around 0.82 for the human diet). Indirect calorimetry can provide better measurements of RQ by measuring the VO2 (oxygen uptake) and VCo2 (carbon dioxide production).

Describe the effects on renal blood flow, local pressures and GFR of vasoconstriction of afferent arteriole

Afferent arterioles deliver blood into the glomeruli. GFR is the rate at which filtrate is formed by both kidneys per min. Vasoconstriction or dilation of afferent arterioles affects the local pressure in the glomerular capillary and the rate of blood flow to the glomerulus and this in turn affects the GFR. Sympathetic activity stimulates constriction of the afferent arterioles. This increase in renal arterial resistance causes RBF to decrease. Vasoconstriction of the afferent arteriole causes an increase in the upstream pressure and a decrease in glomerular pressure which decreases GFR, resulting in a decreased rate of urine formation. When mean arterial pressure drops to 70 mmHg, afferent arterioles dilate. When MAP increases, afferent arterioles vasoconstrict.

What adaptations of the kidney would you expect in the desert rat which produces a very concentrated urine?

Because the Loop of Henle is responsible for creating the countercurrent multiplier and the gradient, it follows that the more efficient - the longer - the loop is, the greater gradient and the more concentrated the urine can be. Long LOH are typically found in juxtamedullary nephrons. So the desert rat probably has a greater percentage of juxtamedullary nephrons and they have longer LOH.

arterial oxygen pressure

CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)

Describe the concept of a transport maximum (Tm)

Carrier-mediated transport (whether it is active transport or facilitated diffusion) requires a protein molecule embedded in the cell membrane to transport or carry the molecule. The number of these carriers will influence the rate at which molecules can be moved. If all the carriers are saturated, the rate of transport will not be increased if the concentration of the substrate molecule increases. Glucose is transported (reabsorbed) from the nephron in this fashion and the typical kidney can move glucose at a rate of about 375 mg/minute. Since blood (plasma) normally contains about 100mg glucose/100 ml and we normally filter about 125 ml/min, the amount of glucose entering the nephron is (100mg/100ml) times (125 ml/min) = 125 mg glucose/minute - All the filtered glucose should be reabsorbed! If the GFR increases or the plasma concentration of glcose increases, the filtered load of glucose will increase. (More filtration or less reabsorption can cause glucose EXCRETION.)If the number of glucose carriers decreases - or if the transport of Na+ is decreased (remember it's secondary active transport), the Tm will decrease.

venous pressure of oxygen

CvO2 = Hgb * 13.4 * O2vSat / 100 + PvO2 * 0.031

Baroreceptor reflex hypertension

If blood pressure is too low increase SNS activity, and decrease PNS activity. increases heart rate, and cardiac output. Vasoconstriction of arterioles is increased, which increases Total Peripheral Resistance which increases blood pressure If blood pressure is too high. It will decrease heart rate, and increase vasodilation.

Glomerular filtration rate (GFR) is regulated by a process called tubuloglomerular feedback (TGF). What is meant by the term "autoregulation of GFR"? How does TGF operate to achieve this?

If systemic blood pressure is low, the afferent arteriole dilates to increase glomerular capillary pressure and bring GFR back up toward normal. Conversely, with hypertension, the afferent arteriole constricts to return glomerular capillary pressure toward normal. TGF is a second regulatory mechanism. Sensors in the macula densa are part of the JGA (juxtaglomerular apparatus). If they sense an increase in the filtered load of sodium (that is, there are more molecules of Na+ delivered to the distal tubule), the JGA sends a signal which constricts the afferent arteriole and decreases the GFR - and the amount of Na+ entering the nephron.

Describe the effects on renal blood flow, local pressures and GFR of vasoconstriction of a proportional increase in vasoconstriction of both.

If there is a proportional increase in both afferent and efferent arteriolar vasoconstriction, the renal blood flow will be greatly decreased, but the glomerular capillary pressure will be relatively unchanged. Therefore GFR will be preserved, despite the decrease in renal blood flow.

Renal diseases are often accompanied by proteinuria or by blood in the urine. How does this occur?

Proteinuria is described as a condition in which urine contains an abnormal amount of protein. Normally there is no protein in the urine and only a few amino acids. As blood passes through healthy kidneys, they filter the waste products out and leave in the things the body needs, like proteins. Most proteins and RBCs are too big to pass through the kidney filters into the urine and they do not appear in the urine unless the kidneys are damaged. Renal diseases are often accompanied by proteinuria because the basement membrane, endothelial cells and nephron cells become "leaky".

Renal Autoregulation hypertension

The kidneys also regulate themselves in the absence of any SNS control. They work to maintain a constant rate of GFR under all circumstances. When systemic arterial pressure decreases, they dilate the afferent arterioles by local, chemical action. When systemic arterial pressure increases, they constrict afferent arterioles. These actions serve to regulate blood flow in order to maintain the constant GFR.

The condition of metabolic acidosis is quite common (e.g., you become acidotic with moderate to severe exercise). A) Describe the mechanisms by which the kidney excretes excess acid and restores plasma pH. B) The urine of an acidotic person may have a strong ammonia smell. Explain why.

The kidneys regulate the balance of the electrolytes or ions in the body. During exercise, metabolic acidosis is prone to occur with the increase in lactic acid build up in the body over a long period of time which in turns causes that soreness in the muscles, but it also causes a drop in the pH of the arteries. The kidneys consequently help by excreting H+ and reabsorbing the HCO3- as the blood passes through the kidneys. When the kidneys reabsorb the HCO3-, the H+ is being excreted through the urine, which often causes urine to be slightly acidic but not less than a pH of 4.5. The H+ excreted normally creates a bond with other ions such as HPO4-2 or NH3. The NH3 would thus cause the ammonia smell in urine.

What is the countercurrent multiplier system of the kidney? How does it operate to concentrate urine?

The loop of Henle is typically referred to as the countercurrent multiplier. Because of the selective permeabilities of the descending LOH (only water) and the ascending limb (only NaCl), an osmotic gradient is created with a hypertonic medulla. The hairpin loop is necessary for this to function properly. It establishes a gradient as you move toward the medulla. This gradient in turn is responsible for the reabsorption of water by osmosis. The more hypertonic the medlla, the greater the urine can be concentrated.

Filtration

The movement of water from the blood into the tubule using the processes of diffusion and bulk flow. Small molecules, ions and water are filtered from the plasma into Bowman's space.

describe how the plasma concentration of any substance can be decreased.

To decrease the plasma concentration the substance must be excreted. To accomplish excretion substances could be secreted - or they could be filtered and not reabsorbed. If they are freely filtered, but they return into the plasma as water is reabsorbed, there may be very little excretion.

Describe the effects on renal blood flow, local pressures and GFR of vasoconstriction of the Efferent arteriole.

With vasoconstriction of the efferent arteriole there may also be an increase in renal resistance - therefore renal blood flow would decrease because not as much blood would be able to pass through the renal circulation. The pressure upstream in the glomerular capillary would then increase and the filtration rate would increase.

Would you expect the GFR (Glomerular Filtration Rate) to be increased in a person with hypertension?

Without any other adaptations, one would expect an increase in pressure to result in an increased filtration (GFR). However, in a healthy individual, hypertension is usually accompanied by an increase in AFFERENT arteriolar constriction, tending to autoregulate the pressure in the glomerular capillary and maintain a normal GFR. There are at least two main mechanisms of GFR regulation: the sympathetic nervous system (SNS), and Renal Autoregulation. (There is also the baroreceptor reflex using SNS & PNS to control systemic blood pressure). SNS: Stimulation of the SNS results in vasoconstriction of the afferent arterioles of the kidneys. This causes less blood to be filtered, thus the GFR is decreased and urine output is decreased. SNS activity is more pronounced during fight or flight situations, and during exercise.

Primary Hypertension

almost always involves increased total peripheral resistance. Many times there is also an increased heart rate and cardiac output. Renin secretion, which one would expect to be lower with high blood pressure, is actually normal or higher than average. Eventually, arterial walls become thicker in order to cope with the higher pressure, and this too will increase hypertension. Prolonged stress, high salt intake, and genetics seem to be important factors in the development of hypertension. Treatment of hypertension may involve the use of diuretics, sympathoadrenal inhibitors: beta and alpha adrenergic blockers, vasodilators, calcium channel blockers, and Angiotensin 2 inhibitors.