High blood pressure is almost always present at the time of diagnosing Chronic Kidney Disease. And as time passes during your CKD journey, controlling your BP becomes an essential part of your treatment plan to delay kidney failure.
But why do you get high BP in chronic kidney disease in the first place?!
For that let us first take you through all the essential facts that you must grasp about the whole concept of Blood Pressure.
Knowing this will help you master your understanding of what goes wrong with your BP regulation systems and to what extent, when CKD hits.
So, let’s begin then, shall we?
BLOOD PRESSURE
In good health, all the blood in our body flows within tubes called blood vessels and passes through the “pump-system” i.e. our Heart to continue to circulate throughout our body. This is much like the water pipelines and the pump-set of a house.
The circulating blood exerts some force on the inner lining of blood vessels while coursing through. This is what is called, your Blood Pressure.
These blood vessels are mainly of three types:
• Arteries
Thick-walled vessels that carry oxygen-rich blood from the Heart to other parts of the body.
• Veins
Less thick-walled than arteries, these vessels bring back blood to the heart from the rest of the body.
• Capillaries
Numerous tiny and delicate vessel branches that connect arteries and veins in necessary places.
NORMAL BLOOD PRESSURE
Your BP is normally recorded as two numbers separated by a slash mark (/). This is followed by the unit “mm Hg”. For instance, 120/96 mm Hg.
These numbers depend on the “relationship” between the blood, blood vessel and heart activity.
Let us explain.
I. In the example above (120/96 mm Hg), the upper, higher number (120) corresponds with the pressure that blood exerts on the “inner walls of Arteries” when heart actively pumps blood out to the rest of the body.
Pumping of the heart is called “Systole” (sis-toll). So, the technical name for this upper number in your BP reading is “Systolic Blood Pressure” or Systolic BP.
As per the criteria by the American Heart Association (AHA), in good health, the normal systolic BP for adults at rest should not exceed 120 mm Hg.
II. Similarly, from the example (120/96 mm Hg), the lower number after the slash (96) is the pressure that blood exerts on the “inner walls of Veins” at the time of returning to the “relaxed” Heart for an oxygen re-fill from Lungs.
This pressure is associated with “relaxed heart muscles” which is technically called “Diastole” (daai-a-stole). So, this lower number in your BP reading is called “Diastolic Blood Pressure”.
As per the criteria by the American Heart Association (AHA), in healthy adults, the normal diastolic BP at rest should not exceed 80 mm Hg.
III. As you can see, the pressure on the walls of Arteries is a lot higher than the pressure on the walls of the veins. Capillaries or tiny blood vessel branches that connect Arteries and Veins actually help in “gradual transition” from higher pressure in arteries to lower pressure in veins during blood circulation in our body. Else, sudden movement of high pressure blood from arteries to veins could permanently damage walls of the vein, normally built to handle relatively lower pressures.
THE CORRECT WAY TO MEASURE BLOOD PRESSURE
Whenever you get your blood pressure measured with the average BP machine, there is this “cuff” wrapped around your arm, fastened snugly in place with Velcro® straps. Then your doctor/nurse pumps up the cuff with air to make it increasingly tight around your arm only to release the air pressure gradually. If it is a digital BP machine, the cuff inflates and relaxes simply at the touch of a button, thanks to in-built pressure sensors!
In the midst of all this, your doctor / nurse somehow gets to “understand” your blood pressure or the digital machine automatically comes up with your BP.
How does that even work?!
Fret not. We are here to explain you this procedure step by step.
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1. When you doctor or nurse wraps that cuff around your arm, they don’t just do it randomly. This cuff is placed in a manner that the part that inflates with air is directly above an important “Artery” that supplies blood to your forearm and hand from the heart. This is the “Brachial Artery”.
S
You might be wondering as to why specifically choose the brachial artery in the arm? What’s so special about it after all?
Well, fact is, the ideal artery to record your BP would be the ones in our neck, because they arise straight out of our heart. But wrapping a cuff around our neck and inflating it would basically suffocate us. That is simply not an option, right? So, “prominent”, wide arteries in our limbs (arms and thighs) make for a good choice. Arms make for a more convenient spot than thighs, both for the patient and the doctor / nurse.
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2. Inflating the cuff exerts pressure on this artery up until the point that the artery is completely squeezed momentarily shuts-off blood flow to the forearm and hand. Then gradually releasing the pressure starts to open up the squeezed artery and restores the blood flow.
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3. This exercise helps the machine “read” the point at which the artery relaxes adequately for the blood flow to restore. In other words, this point gives us the pressure that the heart applies for the blood to start flowing. This is the upper number i.e. the Systolic Blood Pressure.
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4. If the heart is able to “patiently wait” for the cuff to open relax so as to start blood flow below 120 mm Hg at rest, it is a normal Systolic BP for a healthy adult.
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5. Conversely, when the cuff completely relaxes, the heart again begins to receive used blood from tissues, back for an oxygen re-fill via veins. This is the point that gives us the blood pressure corresponding to a relaxed heart. This is the lower number, i.e. the Diastolic Blood Pressure.
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6. If the heart relaxes sufficiently while receiving blood back from tissues via veins, it should not require the veins to “pump blood back” at pressures above 80 mm Hg at rest. So, the lower number of the BP recording for a healthy adult should ideally remain below 80 mm Hg at rest. The image above shows high Diastolic BP of 96 mm Hg at rest.
NORMAL BP VARIES WITH ACTIVITY
Normal Blood Pressure is “dynamic”. It varies with activity levels. This is because our body needs more oxygen whenever we do something physically or emotionally demanding. Such activities include but are not limited to intense exercise (lift weights / run / climb / squat etc.), dance, intercourse or feeling stressed-out, angry or even suddenly ecstatic. To meet with this demand, our heart “temporarily” starts to beat faster and harder.
Blood vessels are like an elasticated hair band. They can also widen up to a certain limit whenever there is such increased demand for blood flow. All in all, automatically increases blood circulation & blood pressure.
However, in healthy adults, such increase in heart rate and blood pressure is temporary. The blood pressure is efficiently brought back to below 120 / 80 mm Hg once the person rests.
NORMAL BP VARIES WITH AGE (1)
Our blood vessels tend to become increasingly stiff and less pliable with age, especially beyond 40 years. They fail to widen adequately when there is a need for increased blood flow to any part of the body or relax back afterwards. As a result, our body loses the ability to restore normal blood pressure after a period of physical or emotional activity, even after taking rest.
This causes the normal, “physiological” blood pressure range to increase with age. This is especially true for the upper, higher number, i.e. the Systolic BP.
WHEN CAN YOU SAY YOU HAVE HIGH BLOOD PRESSURE?
High blood pressure or Hypertension is a major health problem worldwide. Not just in itself, but because of all the myriad complications that it causes, ranging from Heart attacks, Strokes to Kidney damage & failure, it has dominated research interests and clinical conferences for decades now.
Among such clinical focus groups, two major ones namely the American Heart Association (AHA) and the European Society of Hypertension (ESH) have come up with their definitions for High blood pressure. These definitions and criteria inform day-to-day allopathic clinical practice worldwide.
We are presenting the BP definition basics by AHA and ESH for you below. We recommend you learn about these to be able to monitor your own blood pressure accurately.
As per the ACC-AHA criteria
| CATEGORY OF BLOOD PRESSURE | SYSTOLIC BLOOD PRESSURE (Upper number) | DIASTOLIC BLOOD PRESSURE (Lower number) |
|---|---|---|
| NORMAL BP | Less than 120 | Less than 80 |
| ELEVATED BP | 120-129 | Less than 80 |
| STAGE 1 HYPERTENSION | 130-139 | 80-89 |
| STAGE 2 HYPERTENSION | 140-159 | 90-99 |
| STAGE 3 HYPERTENSION | Greater than 160 | Greater than 100 |
As per the ESH criteria:
| CATEGORY OF BLOOD PRESSURE | SYSTOLIC BLOOD PRESSURE (Upper Number) | DIASTOLIC BLOOD PRESSURE (Lower Number) |
|---|---|---|
| OPTIMAL BP | Less than 120 | Less than 80 |
| NORMAL BP | 120-129 | 80-84 |
| HIGH NORMAL BP | 130-139 | 85-89 |
| GRADE 1 HYPERTENSION | 140-159 | 90-99 |
| GRADE 2 HYPERTENSION | 169-179 | 100-109 |
| GRADE 3 HYPERTENSION | Greater than 180 | Greater than 110 |
| ISOLATED SYSTOLIC HYPERTENSION | Greater than 140 | Less than 80 |
HOW DOES YOUR BODY REGULATE YOUR BLOOD PRESSURE?
Amidst all this talk of heart and blood vessels in terms of blood pressure, you must be wondering as to where your kidneys come in all this.
Fact is, as simple as the phrase blood pressure sounds, its effective regulation is equally complex and multi-factorial, if not more.
There are four systems within our body that co-ordinate beautifully with each other to keep our blood pressure in control.
These are:
I. HEART & BLOOD VESSELS (CVS)
Your Heart & Blood vessels (fancy term: Cardio-vascular system) regulate blood pressure by:
1. How fast and hard your heart beats and how well your blood vessels widen up in response to this, to handle an increase in blood flow as needed.
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2. How efficiently your heartbeat slows down upon rest after a period of activity and blood vessels adjust their width to restore only enough blood to other organs. This is to prevent an “angry, shoving and pushing” tsunami of blood flow to organs without need.
II. BRAIN
What is the chief purpose of our heart constantly pumping blood to different organs? It is to ensure regular delivery of nourishment to cells and clear out their metabolic wastes simultaneously so that our body functions efficiently.
An essential part of such nourishment is Oxygen. So, when oxygen levels in the body reduce, for whatever reason, blood pressure MUST rise immediately to maintain adequate oxygen delivery to tissues & prevent them from “feeling suffocated”.
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1. In such a scenario, the instant response of the body comes from “oxygen sensing centres” located in the part of our brain at the back of our head.
2. These centres contact a very useful bunch of nerves called the “sympathetic nervous system” for help.
3. These nerves coax the Adrenal gland to release Adrenalin hormone.
4. Adrenaline literally “orders” our heart to beat faster & our blood vessels coming out of our heart to tighten-up so that the heart needs to exert more force to pump blood out.
5. This increases the blood pressure and restores normal oxygen delivery to all tissues of the body in times of need.
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Now, our blood pressure simply cannot continue to rise forever, right? There has to be that “goldilocks point” that’s “just right” to meet fuel demands for our cells in times of intense emotional or physical activity.
To help with this situation, there is a “supervisor section” in the brain that tracks the rise in the blood pressure & keeps the much-needed leash on the activity of the “oxygen sensing area”. This allows for adequate blood pressure regulation as and when needed.
III. KIDNEYS (RAAS)
Among other functions, our Kidneys regulate the balance of “water-attracting” minerals like Sodium in our blood by “suitably adjusting” their levels in our Urine. They do this by either sending Sodium back to bloodstream or removing excess Sodium from the blood via Urine.
Kidneys “direct this Sodium movie” to regulate our blood pressure with the help of a special hormone system comprising of:
Renin from Kidneys
Angiotensin from Liver
Aldosterone from our Adrenal glands
This hormone system is quite naturally called the RAAS i.e. the Renin-Angiotensin-Aldosterone System.
Whenever our blood pressure falls below normal, the pressure at which blood travels to kidneys for filtration & purification also drops down. This is a problem because you need your blood to reach your kidney filters with just enough pressure so that impurities and excessive components are properly filtered-off via urine. Inadequate pressure causes ineffective blood filtration.
To prevent this, a special hormone called Renin secreted from Kidneys themselves, “report” the kidney blood vessels about this fall in pressure.
1. Renin
This hormone co-ordinates with another hormone called Angiotensin from Liver. It acts on Angiotensin to produce a more active version called Angiotensin-II.
2. Angiotensin-II:
Angiotensin refers to “Angion = Blood vessels + Tensin = Protein that “tenses things up” i.e. the hormone that tones up blood vessels
Angiotensin-II tenses-up the tiny blood vessels in kidneys
It “informs” the Adrenal gland located right above each kidney to release one of its hormones called Aldosterone.
3. Aldosterone
Adrenal glands located right above the kidneys secrete Aldosterone. This hormone coaxes kidney filters to send back some of its Sodium in the Urine forming within, back to the bloodstream.
When blood sodium levels increase, it holds onto more water in the blood than necessary. Sometimes it even continues to “attract” more water from urine back into the blood.
This increases the overall blood volume in our body. As a result, the walls of our blood vessels feel stretched. This directly reflects as an increase in our blood pressure.
Once the blood pressure adequately rises & efficient kidney filtration resumes, any more Renin released by kidneys becomes unnecessary. Such “excess” Renin is ultimately “jobless”. So, kidneys reduce & gradually stop releasing any more of Renin hormone.
As a result, the RAAS hormone system increasing your BP comes to a halt. This prevents unchecked rise in BP which could otherwise damage delicate kidney filters beyond repair.
IV. ENDOCRINE (other hormones)
Emotional excitement or prolonged stress literally tires out our cells and makes them demand more Oxygen for survival. This would mean a necessity for increase in blood flow and hence, blood pressure.
Similarly, physical cues such as dehydration imply a decrease in water levels in our blood. This reduces our overall blood volume & the corresponding blood flow to all cells in the body. Ultimately there is a drop in our blood pressure as well, which if not corrected could spell serious health trouble. So, such a situation would again demand a temporary increase in blood pressure to “feed our cells” adequately up until they are “satisfied”.
The heart, blood vessels, kidneys obviously work in tandem to combat such situations. But there are important Hormones that help these organs to initiate BP regulation or prevent potential damage to cells due to an abnormal BP.
These hormones either act on blood vessels directly or on kidney filters in response to emotional & physical cues such as those mentioned above, to suitably alter blood pressure. These hormones include but are not limited to:
Cortisol – “Stress hormone” from Adrenal glands
Vasopressin – “Thirst hormone” from Pituitary gland in the brain
Endothelin-1 & Nitric oxide – “blood vessel relaxing hormones” from cells in blood vessel walls
Dopamine & Natriuretic Peptides
A glitch in any one of the organ systems above clinically presents as a rise in blood pressure. Chronic kidney disease affects all these BP regulation systems over time to varied extent.
On the one hand, this may sound a tad demotivating, while on the other, please understand just how important and complicated your kidney function is, as opposed to the common opinion about them “simply producing urine”.
Please click here to read a point-wise explanation of how Chronic Kidney Disease affects your Blood Pressure regulation.
1. This will help you better understand all those BP meds that kidney patients like us are, unfortunately, prescribed to literally gulp / munch as if they were a snack!
2. You will be able to stick to your treatment better, with less negativity (and trust us that is extremely important in the course of any chronic condition).
Bookmark www. allthingskidney.com to stay informed, updated and in sync with your doctor’s clinical decisions about your kidney health.
Your kidney queries are welcome at the All Things Kidney — Official Contact Page
REFERENCES
1. Normal Blood Pressure limits change with Age
2. Chopra S., Baby C., Jacob J.J., Neuroendocrine regulation of blood pressure, Indian J of Endocr Metab 2011; 15:281-8 DOI: 10.4103/2230-8210.86860
3. Neural and Cardiovascular regulation of blood pressure American Heart Association
4. Hormonal regulation of blood pressure