EXCRETION: Control of Blood Volume and Pressure.

Hello, dear hivers. This post is sequel to my previous post, where I explained the structure and function of the mammalian kidney. In that post, I also explained the role of the kidney in homeostasis and in water balance. Today, I shall be explaining the control of blood volume and pressure, the control of salt balance, osmoregulation and finally on kidney transplants.

So, how can the blood volume and pressure be controlled?

Since Anti-diuretic Hormone (ADH) regulates water reabsorption, it also regulates blood volume. A drop in blood volume leads to a drop in blood pressure that is detected by stretch receptors in the walls of the aorta and carotid arteries. Impulse from these detectors pass to the hypothalamus, which then triggers the secretion of more ADH. This acts on the kidneys and causes them to retain more water, so increasing blood pressure. To control blood volume and pressure, then the body needs to maintain the acid/base balance, buffers and the secretion of acid.

^{A healthcare professional performing blood pressure monitoring on a patient. rawpixel.com, CC0}

Acid/base balance

The body must maintain a relatively constant pH. Enzymes are very sensitive to changes in pH, and even a small change can inhibit their activity and so have serious consequences for an organism. Under normal circumstances, the human body is engaged in a constant battle against accumulating acid, although this depends on several factors, including diet. All of the following activities tend to lower blood pH:

Cell respiration – this produces carbon dioxide, which dissolves in the plasma to form carbonic acid.
Strenuous exercise – this produces lactic acid.
Digestion – breakdown of certain foods, such as meat, eggs and cheese, result in the formation of acid products.
Generally, the body is able to regulate the pH of blood plasma and tissue fluid in the range of 7.3 to 7.45 by using buffers and by secreting acid into the urine.

Buffers

A buffer is a chemical, or combination of chemicals, that can resist change in pH by mopping up any excess acid or base. The main buffers important in living organisms are: blood proteins, such as haemoglobin and albumin. Then we have the inorganic buffer systems in body fluids, such as the hydrogencarbonate (bicarbonate) system:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^-

This system acts as a buffer because any extra acid tends to shift the equilibrium to the left, so the extra hydrogen ions are neutralised by the hydrogencarbonate ions.

Also the sodium hydrogen phosphate system in plasma:

Na₂PO₄ + H⁺ → NaHPO₄ + Na⁺

NaHPO₄ + H⁺ → H₂PO₄ + Na⁺

This shows that, in the presence of extra hydrogen ions, the equilibrium changes so that hydrogen ions are swapped for sodium ions that do not affect pH.

Secretion of acid

Most people produce slightly acidic urine of between pH 5 and 6. Active transport mechanisms in the kidney enable the body to excrete excess acid but still retain the vital buffer chemicals mentioned above. If the blood becomes too alkaline, the kidney can reabsorb hydrogen ions and secrete basic ions such as ammonia.

SALT BALANCE: THE CONTROL OF SODIUM CHLORIDE LEVELS

The mechanism by which salt balance is regulated is shown in the figure below. The concentration of sodium ions in body fluids is controlled by the hormone aldosterone, which is secreted by the adrenal cortex of the adrenal glands. When sodium ions actively transported, a negative ion, usually chloride, automatically follows to maintain electrolytic balance (balance of positively and negatively charged ions).

When the body loses sodium ions, it also loses water by osmosis, and blood volume and blood pressure fall. This is detected by a group of receptor cells, the juxtaglomerular complex, situated next to the renal capsule. These cells respond to a fall in blood pressure by releasing an enzyme called renin (not to be confused with the digestive enzyme rennin). Renin converts a plasma protein into an active hormone called angiotensin, This stimulates the adrenal cortex to secrete aldosterone.

Aldosterone increases the reabsorption of sodium ions from the intestines and kidney. The increased salt concentration in the blood leads to a greater retention of water, so bringing blood volume and pressure back to normal.

Many western diets contain too much salt and this can pose a real problem for our salt balance. A lot of extra salt in food increases salt levels in our blood and so we retain more water to dilute it. As a result, our blood pressure increases. Many people suffering from high blood pressure, or hypertension, need a low-salt diet.

^{RAAS schematic. Soupvector, CC BY-SA 4.0}

Osmoregulation in desert animals

Studies of kangaroo rats kept in captivity have shown that they can survive on dried food and not drinking water for long periods. Their ability to control their water loss is remarkable and is a classic example of how animals have adapted to a harsh environment.

This remarkable rodent is well suited to life in the desert. Not only does it avoid dehydration, but it avoids overheating without the cooling effects of evaporation, which it could not ‘afford’ in terms of water loss. The kangaroo rat shows the following adaptations to its dry environment:

Its kidneys consist mainly of juxtamedullary nephrons, which contain extra-long loops of Henle. These allow the animal to produce very concentrated urine and so it can lose its metabolic waste without losing significant amounts of water.
It spends long periods underground, where the air is cooler and more humid. This reduces its water loss by evaporation.
Its nasal passages cool the air before it is exhaled, so that much of the water vapour condenses within its nose instead of being breathed out
The kangaroo rat is able to survive on dry plant material without drinking water at all. In a study described whereby the rats ate nothing but 100 g of barley and were kept at a constant temperature of 25 ˚C and a humidity of 20 per cent. The kangaroo rat ‘creates’ most of its water (metabolic water) by metabolic reactions, notably cell respiration. The rest is absorbed from its food. So all the kangaroo rat’s water comes either directly or indirectly from its apparently dry food. Metabolism of the protein in barley produces a substantial amount of urea which must be excreted in the urine. Faeces also contain some water, but the most wasteful activity is breathing out.

Finally on kidney transplants……..!

When a donor kidney becomes available, it is a relatively simple operation to transplant it into another body. Surprisingly, the old kidneys are left in place: they are rather inaccessible and so are dificult to remove, but they do no harm. The new kidney is placed in the lower abdomen. Surgeons choose this site because the new kidney can be attached easily to a large artery (the femoral artery supplying the leg) and is usefully right next to the bladder.

Finding a suitable donor for an organ transplant is difficult. Although several hundred thousand people die each year in the UK, only a tiny fraction can provide organs for transplant. For example, accident victims can become organ donors only if their injuries have not affected the organ itself. Owing to road safety improvements, the number of serious accidents is decreasing. This is good news, but it also means that the number of organs available for transplant is getting ever smaller. A further complication is that only a minority of people carry donor cards, and permission to use body parts from recently deceased people has to be given by distressed relatives, who often say no.

CONCLUSION

The kidneys remove metabolic waste and control water and solute levels in the body. As a result of these functions, the kidney also plays a vital role in the control of blood volume and pressure. Each kidney is made from around one million nephrons, narrow tubules closely entwined with blood vessels. At one end of the nephron, the renal capsule, the kidney filtrate is formed by ultrafiltration (pressure filtration) of the blood.

The first filtrate has the same composition as tissue fluid. As it passes along the nephron, the composition of the filtrate is altered by various active transport mechanisms that reabsorb some substances while allowing others to pass into the urine. A large amount of filtrate is formed, but over 99 per cent of it is reabsorbed in the first convoluted tubule, mainly by active transport mechanisms and osmosis. Usually, all glucose and amino acids are reabsorbed into the blood.

The movement of solutes from the loop of Henle creates a region of high solute concentration in the medulla, through which the collecting ducts must pass. As filtrate (now urine) flows along the collecting ducts, water leaves it by osmosis. The resulting urine is hypertonic (more concentrated than body fluids). The second convoluted tubule is involved in several homeostatic mechanisms including the regulation of salt, water and pH levels.

When the water potential of the blood drops (e.g when we dehydrate) it is detected by the hypothalamus which stimulates ADH release from the pituitary. ADH makes the second convoluted tubule and the collecting duct more permeable to water, so water leaves the filtrate and enters the blood.

Thank you for reading.