Class Notes (806,449)
Canada (492,253)
BIOC34H3 (114)
Lecture 16

Lecture 16 Notes.pdf

10 Pages
Unlock Document

University of Toronto Scarborough
Biological Sciences
Stephen Reid

1    Lecture 16: The Renal System and Kidney Function 1. Pressures Driving Fluid In and Out of Capillaries At a standard capillary bed, there are several pressure differentials that force fluid from the capillaries into the interstitial (extracellular) fluid, and in reverse (from the ECF back into the capillary). There are two important pressure gradients, the hydrostatic pressure gradient and the oncotic pressure gradient. Hydrostatic pressure is the pressure exerted by the presence of a fluid in a system. Within the capillary itself, the hydrostatic pressure is equal to the blood pressure within that capillary. Within the ECF, the hydrostatic pressure is the pressure exerted by the presence of fluid in this space. 1a. Hydrostatic Pressure Gradients The overall hydrostatic pressure gradient across a capillary can be calculated by subtracting the hydrostatic pressure of the interstitial fluid from the hydrostatic pressure within a capillary. The hydrostatic pressure in the interstitial fluid is relatively constant throughout the body, at about 1 mmHg. The hydrostatic pressure in capillaries, however, changes from the arterial side of the capillary to the venous side. We know that capillary beds are sources of resistance to blood flow and this leads to the pressure drop from one side (the arterial side) of a capillary to the other (the venous side). If the pressure at the arterial side of a capillary bed is 38 mmHg then we subtract the hydrostatic pressure in the ECF (1 mmHg) to get an overall hydrostatic pressure gradient of 37 mmHg. However, on the venous side, pressure in the capillary is far lower – approximately 16 mmHg. The hydrostatic pressure differential across the venous side of the capillary is therefore 15 mmHg; 16 mmHg minus 1 mmHg). These pressure gradients, on both the arterial and venous side of the capillary, tend to force fluid out of the capillary and into the extracellular fluid. They are both positive pressures with the driving force directed out of the capillary. 2    1b. Oncotic Pressure Gradients There is another pressure gradient across capillaries; the oncotic pressure gradient. Oncotic pressure is the osmotic pressure exerted by the presence of proteins. All dissolved solutes cause the existence of an osmotic pressure. The osmotic pressure exerted by the presence of plasma proteins is the oncotic pressure. The interstitial fluid has essentially no proteins in it, so the oncotic pressure of the ECF is approximately 0 mmHg. However, there are proteins in the blood plasma and the oncotic pressure within the capillaries is approximately 25 mmHg. Proteins are too large to diffuse out of the capillary walls, meaning that oncotic pressure is equal throughout the capillary bed. 1c. Net Filtration Pressure Whereas hydrostatic pressures “push” fluid from an area of hydrostatic pressure to an area of low hydrostatic pressure, oncotic pressures “pull” fluid. Fluid will tend to move from an area of low oncotic pressure to an area of high oncotic pressure. The movement of fluid across capillaries results from the balance between the two hydrostatic pressures (in the capillary and in the ECF) and the two oncotic pressures (within the capillaries and in the ECF).At the arterial end of a capillary, the hydrostatic pressure is higher than the oncotic pressure, whilst on the venous side, hydrostatic pressure is less that oncotic pressure (which has remained the same on both sides of the capillary). 3    We can quantify the net filtration pressure which will ultimately lead to fluid movement across the capillary. This is calculated by subtracting the oncotic pressure gradient from the hydrostatic pressure gradient. A positive NFP will cause filtration, whilst a negative NFP will cause absorption. So if we go back to our example capillary, on the arterial side the NFP will be 37 mmHg - 25 mmHg = 12 mmHg. On the venous side, the NFP will be 15 mmHg - 25 mmHg = -10 mmHg. Thus, on the arterial side of the capillary, fluid tends to filter out, into the interstitial fluid. On the venous side, fluid tends to absorb back into the capillary. Throughout the length of the capillary fluids are flowing in and out of it, but the balance on the arterial side is in favour of filtering out, and on the venous side in favour of absorbing in. From a whole-body perspective, approximately 20 L a day of fluid is filtered across the capillaries into the ECF but only 17 litres is absorbed back into the capillaries from the ECF. The remaining 3 litres is returned to the circulation by the lymphatic system. We don't go into great detail on the lymphatic system in this course, but essentially the lymphatic system consists of separate lymphatic capillaries that have open ends. They absorb excess fluid from the extracellular space and return it to the vena cava and back into the circulation. These same pressures and pressure gradients drive the filtration of fluid (plasma) at the kidneys to form the fluid that ultimately becomes pre-urine. 4    2. Kidney Functions The kidneys perform a number of functions. They filter the entire plasma volume every 22 minutes. They regulate the plasma ionic composition (that is, the amounts of Na +, K , Ca , Mg , Cl , HCO , 3- and H PO in the blood), the plasma osmolarity, and the plasma volume. They regulate the arterial pH 2 4 (i.e., blood acid-base balance). They are involved in removing metabolic water and foreign substances (for example, urea, drugs, etc.), and produce hormones such as erythropoietin and enzymes such as renin. They also activate vitamin D3. Though they make up a little more than 1% of body weight, they receive 20% of cardiac output. 3. Kidney Anatomy 3a. Gross Anatomy The kidneys are positioned just below the adrenal glands, just below and between the ribs. Blood is supplied by the renal artery and removed by the renal vein. Urine leaves the kidney via the ureter and enters the bladder. Urine leaves the kidneys fully formed, and is held in the bladder until urination. 5    If we look at a cross-section of a kidney, we see that it is made up of sections called renal pyramids. The outer section of the kidney is referred to as the renal cortex and the inner region is the renal medulla. In between the renal pyramids, within the inner region of the kidney, is space called a minor calyx, and each of these connects to the hollow core of the kidney, called the renal pelvis. Each minor calyx acts a collecting area, conveying fluid (at that point, urine) to the renal pelvis where it collects, before draining down the ureter into the bladder. 6    3b. The Nephron The functional unit of the kidney is the nephron. It consists of a number of regions, each specializing in handling ions or water in a different manner. A nephron begins in a capillary bed called the glomerulus. Each glomerulus sits in a glove-like structure called Bowman's Capsule. Fluid moves from the glomerulus into Bowman's capsule, and then moves into the proximal tubule. Each proximal tubule has two sections -the convoluted section and straight section. Bulk absorption of substances, as well as sodium handling, occurs in the proximal tubule. From the proximal tubule, fluid (pre-urine at this stage) moves into the Loop of Henle, which c
More Less

Related notes for BIOC34H3

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.