Blood Pressure

Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.

Blood pressure in the arteries.

The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.

Blood pressure is expressed as two numbers, e.g., 120/80.

Blood pressure in the capillaries

The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.

Blood pressure in the veins

When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins

Exchanges Between Blood and Cells

With rare exceptions, our blood does not come into direct contact with the cells it nourishes. As blood enters the capillaries surrounding a tissue space, a large fraction of it is filtered into the tissue space. It is this interstitial or extracellular fluid (ECF) that brings to cells all of their requirements and takes away their products. The number and distribution of capillaries is such that probably no cell is ever farther away than 50 µm from a capillary.

When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space.

Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins. (It has the same composition and is formed in the same way as the nephric filtrate in kidneys.)

Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.

Near the venous end of a capillary, the blood pressure is greatly reduced .Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.

Control of the Capillary Beds

An adult human has been estimated to have some 60,000 miles of capillaries with a total surface area of some 800–1000 m². The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example, capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse occurs after a heavy meal.

The walls of arterioles are encased in smooth muscle. Constriction of arterioles decreases blood flow into the capillary beds they supply while dilation has the opposite effect. In time of danger or other stress, for example, the arterioles supplying the skeletal muscles will be dilated while the bore of those supplying the digestive organs will decrease. These actions are carried out by

• the autonomic nervous system.
• local controls in the capillary beds

Neural Substrates of Breathing

A.    Medulla Respiratory Centers

Inspiratory Center (Dorsal Resp Group - rhythmic breathing) → phrenic nerve→ intercostal nerves→ diaphragm + external intercostals

Expiratory Center (Ventral Resp Group - forced expiration) → phrenic nerve → intercostal nerves → internal intercostals + abdominals (expiration)

1.    eupnea - normal resting breath rate (12/minute)
2.    drug overdose - causes suppression of Inspiratory Center

B.    Pons Respiratory Centers

1.    pneumotaxic center - slightly inhibits medulla, causes shorter, shallower, quicker breaths
2.    apneustic center - stimulates the medulla, causes longer, deeper, slower breaths

C.    Control of Breathing Rate & Depth

1.    breathing rate - stimulation/inhibition of medulla
2.    breathing depth - activation of inspiration muscles
3.    Hering-Breuer Reflex - stretch of visceral pleura that lungs have expanded (vagal nerve)

D.    Hypothalamic Control - emotion + pain to the medulla

E.    Cortex Controls (Voluntary Breathing) - can override medulla as during singing and talking

Glomerular filtration

Kidneys receive about 20% of cardiac output , this is called Renal Blood Flow (RBF) which is approximatley 1.1 L of blood. Plasma in this flow is about 625 ml . It is called Renal Plasma Flow (RPF) .
About 20 % of Plasma entering the glomerular capillaries is filtered into the Bowman`s capsule .
Glomerular filtration rate is about 125 ml/min ( which means 7.5 L/hr and thus 180 L/day) This means that the kidney filters about 180 liters of plasma every day.

The urine flow is about 1ml/min ( about 1.5 liter /day) This means that kidney reabsorbs about 178.5 liters every day .

Filtration occurs through the filtration unit , which includes :

1- endothelial cells of glomerular capillaries , which are fenestrated . Fenestrae are quite small so they prevent filtration of blood cells and most of plasma proteins .

2- Glomerular basement membrane : contains proteoglycan that is negatively charged and repels the negatively charged plasma proteins that may pass the fenestrae due to their small molecular weight like albumin . so the membrane plays an important role in impairing filtration of albumin .

3- Epithelial cells of Bowman`s capsule that have podocytes , which interdigitate to form slits .

Many forces drive the glomerular filtration , which are :

1- Hydrostatic pressure of the capillary blood , which favours filtration . It is about 55 mmHg .

2- Oncotic pressure of the plasma proteins in the glomerular capillary ( opposes filtration ) . It is about 30 mm Hg .

3- Hydrostatic pressure of the Bowman`s capsule , which also opposes filtration. It is about 15 mmHg .

The net pressure is as follows :

Hydrostatic pressure of glomerular capillaries - ( Oncotic pressure of glomerular capillaries + Hydrostatic pressure of the Bowman capsule):
55-(35+10)
=55-45
=10 mmHg .

Te glomerular filtration rate does not depend only on the net pressure , but also on an other value , known as filtration coefficient ( Kf) . The later depends on the surface area of the glomerular capillaries and the hydraulic conductivity of the glomerular capillaries.
 

• Partial Pressures of O₂ and CO₂ in the body (normal, resting conditions):

• Alveoli
  • PO₂ = 100 mm Hg
  • PCO₂ = 40 mm Hg
• Alveolar capillaries
  • Entering the alveolar capillaries
    • PO₂ = 40 mm Hg (relatively low because this blood has just returned from the systemic circulation & has lost much of its oxygen)
    • PCO₂ = 45 mm Hg (relatively high because the blood returning from the systemic circulation has picked up carbon dioxide) 

• While in the alveolar capillaries, the diffusion of gasses occurs: oxygen diffuses from the alveoli into the blood & carbon dioxide from the blood into the alveoli.

• Leaving the alveolar capillaries
  • PO₂ = 100 mm Hg
  • PCO₂ = 40 mm Hg
• Blood leaving the alveolar capillaries returns to the left atrium & is pumped by the left ventricle into the systemic circulation. This blood travels through arteries & arterioles and into the systemic, or body, capillaries. As blood travels through arteries & arterioles, no gas exchange occurs.
• • Entering the systemic capillaries
    • PO₂ = 100 mm Hg
    • PCO₂ = 40 mm Hg
  • Body cells (resting conditions)
    • PO₂ = 40 mm Hg
    • PCO₂ = 45 mm Hg
• Because of the differences in partial pressures of oxygen & carbon dioxide in the systemic capillaries & the body cells, oxygen diffuses from the blood & into the cells, while carbon dioxide diffuses from the cells into the blood.
• • Leaving the systemic capillaries
    • PO₂ = 40 mm Hg
    • PCO₂ = 45 mm Hg
• Blood leaving the systemic capillaries returns to the heart (right atrium) via venules & veins (and no gas exchange occurs while blood is in venules & veins). This blood is then pumped to the lungs (and the alveolar capillaries) by the right ventricle.

White Blood Cells (leukocytes)

White blood cells

• are much less numerous than red (the ratio between the two is around 1:700),
• have nuclei,
• participate in protecting the body from infection,
• consist of lymphocytes and monocytes with relatively clear cytoplasm, and three types of granulocytes, whose cytoplasm is filled with granules.

Lymphocytes: There are several kinds of lymphocytes, each with different functions to perform , 25% of wbc The most common types of lymphocytes are

• B lymphocytes ("B cells"). These are responsible for making antibodies.
• T lymphocytes ("T cells"). There are several subsets of these:
  • inflammatory T cells that recruit macrophages and neutrophils to the site of infection or other tissue damage
  • cytotoxic T lymphocytes (CTLs) that kill virus-infected and, perhaps, tumor cells
  • helper T cells that enhance the production of antibodies by B cells

Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T cells migrate from the bone marrow to the thymus where they mature. Both B cells and T cells also take up residence in lymph nodes, the spleen and other tissues where they

• encounter antigens;
• continue to divide by mitosis;
• mature into fully functional cells.

Monocytes : also originate in marrow, spend up to 20 days in the circulation, then travel to the tissues where they become macrophages. Macrophages are the most important phagocyte outside the circulation. Monocytes are about 9% of normal wbc count

Macrophages are large, phagocytic cells that engulf

• foreign material (antigens) that enter the body
• dead and dying cells of the body.

Neutrophils

The most abundant of the WBCs. about 65% of normal white count  These cells spend 8 to 10 days in the circulation making their way to sites of infection etc  Neutrophils squeeze through the capillary walls and into infected tissue where they kill the invaders (e.g., bacteria) and then engulf the remnants by phagocytosis. They have two types of granules: the most numerous are specific granules which contain bactericidal agents such as lysozyme; the azurophilic granules are lysosomes containing peroxidase and other enzymes

Eosinophils : The number of eosinophils in the blood is normally quite low (0–450/µl). However, their numbers increase sharply in certain diseases, especially infections by parasitic worms. Eosinophils are cytotoxic, releasing the contents of their granules on the invader.

Basophils : rare except during infections where these cells mediate inflammation by secreting histamine and heparan sulfate (related to the anticoagulant heparin). Histamine makes blood vessels permeable and heparin inhibits blood clotting. Basophils are functionally related to mast cells.  . The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.

Thrombocytes (platelets):

Thrombocytes are cellular derivatives from megakaryocytes which contain factors responsible for the intrinsic clotting mechanism. They represent fragmented cells  which contain residual organelles including rough endoplasmic reticulum and Golgi apparati. They are only 2-microns in diameter, are seen in peripheral blood either singly or, often, in clusters, and have a lifespan of 10 days.

Biological Functions are Extremely Sensitive to pH

• H+ and OH- ions get special attention because they are very reactive
• Substance which donates H+ ions to solution = acid
• Substance which donates OH- ions to solution = base
• Because we deal with H ions over a very wide range of concentration, physiologists have devised a logarithmic unit, pH, to deal with it
  • pH = - log [H⁺]
  • [H⁺] is the H ion concentration in moles/liter
  • Because of the way it is defined a high pH indicates low H ion and a low pH indicates high H ion- it takes a while to get used to the strange definition
  • Also because of the way it is defined, a change of 1 pH unit means a 10X change in the concentration of H ions
    • If pH changes by 2 units the H+ concentration changes by 10 X 10 = 100 times

• Human blood pH is 7.4
  • Blood pH above 7.4 = alkalosis
  • Blood pH below 7.4 = acidosis
• Body must get rid of ~15 moles of potential acid/day (mostly CO₂)
  • CO₂ reacts with water to form carbonic acid (H₂CO₃)
  • Done mostly by lungs & kidney
• In neutralization H⁺ and OH^- react to form water
• If the pH changes charges on molecules also change, especially charges on proteins
  • This changes the reactivity of proteins such as enzymes
• Large pH changes occur as food passes through the intestines.