KIDNEY

I. Kidney Introduction

A. The kidney should be considered a controlling organ rather than an excretory organ.

B. The kidney has an excellent blood supply.

1. It is located very near the heart and the renal artery is the first major branch from the descending aorta. Thus, there is a substantial blood pressure here, which is needed for the portal system and the filtration process.

2. 1/4 th to 1/5 th of the blood from the heart passes through the kidneys which amounts to about 1.2 liters per minute! (That corresponds to 72 liters per hour from which 7.8 liters of filtrate per hour are made.)

II. Gross Anatomy of kidney: Cortex, medulla, calyx, ureter, renal vein, renal artery.

III. Kidney and nephron structure (1 million per kidney):

IV. Kidney (nephron) function

A. Filtration from glomeruli capillary bed into Bowman's capsule (Malpighian Body).

1. The force from the filtration process ultimately comes from the heart.

2. Filtration is non-selective except for size ( <70,000 MW; 7 nm in diameter). Blood cells and proteins do not pass through the filter. Therefore, the fluid in the Bowman's capsule appears very much like plasma without the proteins.(If proteins, or red blood cells, appear in the urine, this suggests damaged Bowman's capsules and glomeruli which suggests a very serious problem. Very high blood pressure or certain kinds of infections can cause this to happen).

3. The glomeruli capillaries are substantially more (100 to 1000X) leaky than regular capillaries and have 2-3 times more pressure than regular capillaries (This pressure is somewhat counteracted by the Bowman's capsule pressure and the osmotic pressure remaining in the glomerulus).

4. Filtration rate is, for the most part, constant. (130 ml/min or 7.8 l/hr (show the volume)).

5. Thus all waste material is removed from the blood, as well as good material. Through tubular reabsorption, the nephron will reuptake what it deems as good and therefore, by definition, everything else is waste. Thus, the kidney does not have to work by a process of identifying what is bad; rather it works by identifying those things that are good for the body. Thus, much of the urea is lost in this way simply because the kidney chooses not to recover it after it has been filtered. Thus, any small foreign molecule that has entered our blood, even if it has not existed in human evolutionary history (drugs, new pollutants, etc) can be removed by the kidney.

6. Glomeruli capillaries reform into arterioles and then split off into capillaries again around the other parts of the nephron (renal portal system).

B. Tubular reabsorption. A process where much of the water and all that is deemed 'good' is reabsorbed from the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.

1. The glomeruli capillaries had reformed into arterioles. Now this blood passes into a second capillary bed that starts by surrounding the proximal convoluted tubule.

2. Having gone through the glomeruli, this blood now has a

a. low hydrostatic pressure

b. high solute concentration from the proteins that were left behind in the filtered blood (i.e. high osmotic pressure.)

3. Water readily reenters the proximal convoluted tubule because of the osmotic gradient caused by the proteins in the remaining blood (and reabsorbed solutes) and because there is little hydrostatic pressure to oppose the movement. Some of this passive type of water reabsorption continues into the loop of Henle. About 80% of the water that was filtered is reabsorbed passively in the proximal tubule. The other 20% now becomes more difficult to recover.

4. Those items that are good for the body are reabsorbed by a combination of free diffusion, facilitated diffusion, and active transport. The proximal convoluted tubule is the primary structure that does this, but the loop of Henle, distal convoluted tubule, and collecting duct get heavily involved too, depending upon what is being reabsorbed.

a. Glucose and all monosaccharides are tenaciously recovered. No monosaccharides should appear in the urine. If they do, the active transport systems were probably overloaded which suggests that there was to much glucose in the blood to start with, which suggests diabetes.

b. Amino acids are tenaciously reabsorbed with strong active transport mechanisms.

c. Ions are reabsorbed as needed (homeostasis). Hormones help decide what ions are too high or too low and the ions are reabsorbed accordingly (E.g H+, Ca++, Mg++, HPO4-- , Cl-, HCO3- , etc.) . Note that the reabsorption of some of these ions (E.g H+, HPO4-- , HCO3-, etc.) is involved in pH control too.

d. The increased osmolarity from the reabsorption of ions will cause more water to flow back osmotically.

C. Counter -current exchange to reabsorb water and make a concentrated urine.

1. As the fluid passes up the ascending loop of Henle, Na+ ions are actively transported into the medullary tissue of the kidney. Cl- passively follows because of facilitated carriers and the charge gradient created by the Na+.

2. Many of these Na+ ions get reabsorbed into the adjacent descending loop of Henle; only to be pumped out again as the fluid goes back up the ascending loop! This counterflow of fluids acts as a trap for NaCl such that the NaCl gets more and more concentrated as you get toward the base of the loop of Henle. (Thus the medulla of the kidney is very 'salty').

3. The high salt concentration creates an abnormally high high osmotic gradient which will osmotically pull water from the forming urine.

4. By this mechanism (which requires energy because of the active transport of Na+ , the urine can be made more osmotically concentrated than blood.

5. Much of the water reabsorbed by this created osmotic gradient actually occurs from the collecting tubule which also projects into the medulla. This water reabsorption is under distinct control and the collecting duct is especially adapted for this (see below).

D. Secretion- Some substances are in fact identified as being 'bad' and are actively transported from the blood of the capillaries flowing around the tubules.

1. Ammonia that remains in the blood even after filtration is actively transported into the nephronal fluid.

2. Some ions are actively transported out of the blood.

V. Control of the kidney

A. It is so efficient in homeostatic control of the blood, that many malfunctions of the body can be determined as a result of its efforts (by urine examination).

B. Ion homeostasis often occurs at the level of the kidney ( osmotic regulation controlled by ionic regulation).

1. Na+, K+, Cl- controlled by Na+-K+ ATPase.

a. Aldosterone from the adrenal cortex increases the rate of reabsorption of Na+ and Cl- . The adrenal cortex does the ion detection directly. Note that this also affects water uptake. Less aldosterone can mean more dilute urine. Note that this can also affect blood pressure. Less aldosterone can mean more water water loss and and less salty medulla which reduces the pressure of water reentering the kidney capillary system.

b. K+ is ultimately controlled by renin, a hormone made by the kidney. Renin induces the formation of angiotensin II in the blood that depresses K+ uptake from the nephron back into the blood. Angiotensin II stimulates aldosterone secretion.

2. Other ions also regulated at the kidney level (i.e. H+, Ca++, Mg++, HPO4--)

C. Water homeostasis

1. The filtration rate is constant, therefore there is no control elicited here.

2. Passive reabsorption due to osmotic pressure from proteins in the blood.

3. Reabsorption in collecting duct. Collecting duct membranes are naturally water impermeable, but permeability to water is induced by vasopressin (antidiuretic hormone =>ADH = vasopressin). Counter current exchange makes it possible to make a highly concentrated urine. The level of ADH (via the hypothalmus) actually is the determining factor if the urine is to be concentrated or dilute. The more ADH the more permeable the collecting duct to water and the more concentrated the resulting urine. In a way, ADH says "this is the body talking, I need more water". ADH also affects and controls blood pressure- the more ADH the higher the higher the blood pressure.

VI. Micturition-the removal of urine from the body.

A. The fluid leaving the collecting duct collects in the calyx and renal pelvis.

B. It flows into the ureter which is a tube that carries the urine to the bladder. The ureters have smooth muscle and can propel the urine by peristalsis.

C. The bladder is made largely of smooth muscle and has good expandability. It is simply designed to store urine. There are stretch receptors in the wall of the bladder that send signals to the spinal column and brain as it stretches.

D. There is an internal (smooth) muscle sphincter and an external (skeletal) muscle sphincter that hold the urine in the bladder. Once the bladder reaches a certain size, a reflex relaxes the internal smooth muscle sphincter. The external sphincter is under voluntary control, and allows for voluntary release (usually) of urine. The latter does not develop until about the age of 2-3.

E. During micturition not only do the sphincters relax, but the bladder and abdominal muscles contract to expel the urine.